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Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
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Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
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Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 5
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 6
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 7
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 8
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 9
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 10
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 11
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 12
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 13
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 14
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 15
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 16
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 17
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 18
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 19
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 20
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 21
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 22
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 23
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 24
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 25
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 26
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 27
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 28
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 29
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 30
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 31
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 32
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 33
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 34
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 35
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 36
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 37
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 38
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 39
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 40
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 41
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 42
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 43
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 44
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 45
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 46
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 47
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 48
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 49
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 50
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 51
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 52
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 53
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 54
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 55
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 56
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 57
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 58
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 59
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 60
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 61
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 62
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 63
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 64
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 65
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 66
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 67
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 68
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 69
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 70
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 71
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 72
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 73
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 74
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 75
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 76
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 77
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 78
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 79
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 80
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 81
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 82
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 83
Suggested Citation:"VISION IN SPACE TRAVEL." National Research Council. 1968. Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson. Washington, DC: The National Academies Press. doi: 10.17226/18636.
×
Page 84

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

CRITERIA FOR LABORATORY EXPERIMENTS USEFUL IN FIELD SITUATIONS1 Wilson P. Tanner, Jr. Sensory Intelligence Laboratory The University of Michigan It is of the utmost importance at the present time to design labo- ratory experiments useful in describing human behavior so that data relevant to the problems of U.S. space and military efforts may be obtained. If scientific laboratories are to continue to look to the government as a major source of financial support, the problem is critical to scientific progress. The solution of the problem does not exist in leaning more heavily toward field studies. Neither does it exist in increasing the rate of laboratory experiments as now conducted, for these tend to ignore many of the significant variables. The solution requires a more careful study of the problems, leading to a more nearly precise state- ment of the interacting variables which need careful examination. Laboratory experiments should, then, be designed to study these variables along with the interactions. The design of laboratory experiments to accomplish these desired objectives may require the development of new experimental techniques and new meth- ods of analysis. DESCRIPTION OF THE EXPERIMENTAL PROCESS One might begin by defining the purpose of an experiment as the reduction of uncertainty about a particular phenomenon or set of phenomena. The experiment itself is like an optical instrument designed to look at the phenomenon. The observations of the scientist using the instrument constitute the data. The scientist 1. This work was supported by the U.S. Air Force, Office of Scientific Research, Grant No. AF-AFOSR-367-63.

is the counterpart of an observer in a visual experiment, and his interpretation of the experimental results is the observer's re- sponses. In other words, when one performs a visual experiment, he is studying processes very similar to those he is performing in conducting the experiment. The same theoretical framework can be applied to the task of evaluating the performance of ex- perimenters as is applied to the evaluation of observers in psy- chophysical experiments. Both problems can be illustrated by the same block diagram (Fig. 1). In the simple psychophysical experiment, the message Message ensemble Transmitter Perturbance i er + < Observer FIG. 1. Block diagram. consists of a set of signals. In a vision experiment, one such set consists of two signals: a light flash with finite energy greater than zero, and a light flash of zero energy. Ideally, a random selection of the members of the set is made, and the selected signal is then transmitted through the channel. As the signal traverses the channel, it is perturbed. In the case of light sig- nals, the energy spreads and random or irrelevant photons from the environment are added. The observer's input, then, is some combination of the transmitted signal and of the perturbances. It is the task of the observer in responding to indicate which of the signals of the ensemble was responsible for that particular input. The data for such an experiment are summarized in terms of a measure indicative of the average reduction in uncertainty that can be attributed to the observer's responses. In other words, if the observer's response is known, can the selected signal be better stated than when such knowledge is not available ? In infor- mation theoretic terms, the entropy of the source, minus the con- ditional entropy of the response, gives the desired information content for the experiment. The analysis of an experimenter's behavior in terms of the same block diagram leads to surprisingly parallel statements. The message ensemble is a set of hypotheses, each with an

associated probability. The entropy of this set is the uncertainty of knowledge, prior to the experiment regarding which hypothe- sis is "true." One of the hypotheses in the set is presumed to be selected by nature for transmission. The transmitter and the channel constitute the experimental design and conduct. The data combine to constitute the input to the observer who, in this case, is the scientist. His response is a scientific publication which, hopefully, leads to a different set of probabilities associated with the hypotheses in the ensemble. The entropy of the latter set is the uncertainty associated with one's knowledge posterior to the experiment. The difference between the a priori entropy and the a posteriori entropy is the information content of the experiment. In other words, how much more is known about the "truth" of the hypotheses after the experiment than before? Describing the experimental process in terms of the block diagram is essentially a statement of the problem of the design, the execution, and the interpretation of the experiments. The value of a problem statement is determined by its contribution to the solution: How does this statement lead toward a solution ? First of all, the experiment is defined as an instrument to convert probabilities associated with hypotheses from one value to another. The conversion indicates a Bayesian procedure. Letting X(E) be a function of the experiment, and P(H .) be the a priori probability that Hj is the true hypothesis, Bayes' theo- rem states the a posteriori probability associated with Hj as P(H)P [X(E)J 1 H Pw [X(E)]+P(H)P- [X(E)J ' tl~ 1 n.. Examination of this equation leads to certain obvious state- ments. 1. Any hypothesis with an associated a priori probability equal to zero will have an a posteriori probability equal to zero. 2. Any hypothesis with an associated a priori probability equal to unity will have an associated a posteriori probability equal to unity. 3. The statement of associated a posteriori probabilities is a function of the statement of the associated a priori probabilities. If the probabilities are interpreted as degrees of belief (a reasonable interpretation from the information theoretic point of view), examination of the above statements suggests ways in which a scientist introduces his biases into the design, the exe-

cution, and the interpretation of experiments. At the outset, the first statement indicates that complete disbelief in an hypothe- sis eliminates that hypothesis from consideration. For example, early experiments conducted within the framework of the theory of signal detectability were not considered by this author in terms of extrasensory perception, although they were by another scien- tist. Fortunately, he did not have complete belief in his hypothe- sis and a posteriori, as an explanation for the results, he associ- ated a small probability (p < e) to extrasensory perception. Another example is that of the experimenter who determines a threshold by having an observer turn a knob until he sees or hears a signal. Built into his design is a credibility of unity as- sociated with the threshold concept. His results are unlikely to question the validity of the concept. The third statement illustrates the most serious controversy involving the use of Bayes' theorem. How can a set of hypotheses have associated probabilities in the face of a complete lack of knowledge ? Perhaps the possible hypotheses cannot even be enu- merated. The answer to this dilemma exists in a philosophy of science. As long as one is concerned with a finite set of data, there is an infinite set of possible hypotheses. The probability of identifying that which is true is zero. Thus, the scientist must be content with the knowledge that the probability of proposing an incorrect hypothesis or theory is unity. Once this attitude is accepted, it is again possible to proceed. Watanabe (1960) has demonstrated that if the set of hypotheses has erroneous associated probabilities, repeated experiments with the application of Bayes' theorem will nevertheless lead to a convergence on the most likely hypothesis of the set. This theorem is a fortunate result, for without it experiments would be useless. If a correct statement were required a priori, this statement would have the same information content as that usu- ally sought as the result of an experiment. If the result could be obtained a priori, there would be no need either for the experi- ment or for concern with Bayes' theorem. Watanabe's theorem states that the convergence is to the most likely hypothesis of the set. The set may or may not contain the "true" hypothesis. What, then, is meant by the "most likely" hypothesis? It is that member of the set that is most likely to describe the data. From this point on, the task of an experimen- ter will be considered that of finding the most likely hypothesis of a set. He is not worried about truth since he knows that this is a fruitless attack. The usefulness of his work, either with 6

regard to scientific or practical application, depends on the choice of a useful set of hypotheses with which to work. Further examination indicates the required content of the data. The term on the left of the equation is the a posteriori proba- bility. Contained in the expression on the right are the a priori probabilities and some conditional probabilities. The theorem can be rewritten to express the additional terms as a single operator. P(H ) {f [X(E)J/f 1 1 Hl P •X(E) v I7 P(HJ If [X(E)]/f 1 Hj Hj where f(X) is a probability density, and the ratio is described as a likelihood ratio. Thus, the information content of an experi- mental result X(E) is contained in a set of numbers which are functions of the hypotheses being tested. If a particular result is equally probable under two hypotheses, it furnishes no infor- mation on which to base a choice between the hypotheses. Care- ful and precise statements of the hypotheses to be tested are essential to efficient experimental design and will point the way to experiments not likely to lead to results equally probable under the various hypotheses. THE SIZE OF THE EXPERIMENT In an attempt to determine how incorporation of a priori knowl- edge influences the size of the experimental task, some calcula- tions have been performed. The following assumptions are in- volved in the computations. 1. The hypotheses are each orthogonal to the others. 2. The hypotheses are a priori equally likely. 3. Each hypothesis, if true, leads to an observation contain- ing equal energy. 4. One of the hypotheses is "true." The amount of energy required to lead to a particular level of confidence was determined as a function of the number of alter- natives in the set. For the two cases studied (confidence of 0.75 and 0.90), the energy was found to be linear with the logarithm of the number of alternatives. Since, under the assumptions, the total energy contained in an experimental result is the energy per observation times the number of observations, the size of the experiment required to lead to a particular level of confidence

16 32 NO. OF ALTERNATIVES FIG. 2. Number of trials required to achieve a given level of confidence as a function of the number of alter- natives, d' = 1 is assumed. is linear with the logarithm of the number of hypotheses to be tested (see Fig. 2). The fact that the size of the experiment needed to develop a particular level of confidence is linear with the logarithm of the number of hypotheses to be tested leads to a consideration of the problem of the statement of the hypotheses to be included in the set. One can begin the process by describing as carefully as possible those hypotheses that appear a priori likely. Ideally, the next step is to define a mathematical space that includes each member of the set as well as all linear combinations of elements of the set. Given such a mathematical space, one should then attempt to describe a new set of basic hypotheses which span the space and are orthogonal to each other. The new set is the basis for the experimental design. At this point, it should be observed that the problem is gradu- ally being shifted. It is no longer that of choosing one of a finite set of hypotheses. It is rather that of searching for a set of co- efficients applying to the orthogonal axes of the space. The coef- ficients are used to describe a point in a continuous space, this point representing the "most likely" hypotheses of an infinite set. The coefficients are similar to the factor loadings of factor analysis. The dimensionality of the space, however, is deter- mined a priori rather than a posteriori. The coefficients thus are more like those of the Fourier analysis of electrical waveforms.

Consider the possibility of computing the number of observa- tions necessary to reduce the entropy of a parameter coefficient from one value to another. The terms are defined as follows: a j = initial variance, ff2Q = variance associated with an observation, cj2 = variance of estimate following the experiment. Now, treating these variances as representing Gaussian noise and letting No equal the number of observations in the experi- ment, the entropy can be written as H(I) = log ireffj = initial entropy, H(E) = log we (a^0/NQ) = entropy of observation, H(P) = log TeOp = posterior entropy. Then the reduction of entropy as a result of the experiment is R(E) = H(I) - H(P). By writing CT^J = CT^J/N as the initial variance representing a number of previous observations, and by writing 0% - <3^Q/^^^ as the a posteriori variance in terms of a variance dependent on both the observations prior to and during the experiment, then R(E) = logffefc/N) - log ire \ = iog (i+ [No(ff2/«r2o)]}. Solving for N 2 r 2 2 , 2 2 No =CTo[(3I-%)/ffI%l = (T2oL(l/a2p) - d/a2:) I- The last equation indicates clearly that the number of obser- vations required of an experiment, if the a posteriori result is intended to be within a previously specified level of confidence, depends on the incorporation of prior knowledge. The greater the prior knowledge, the smaller the experiment required. 9

The discussion of size of experiment to this point has been entirely in terms of estimating a single coefficient. If one tries to extend this to a set of W orthogonal coefficients, then a band- width term is introduced, and each of the entropy terms must then be multiplied by W, as must the size of the experiment. ILLUSTRATIVE EXPERIMENTS The Imperfect Memory Problem An example of an experiment in which the suggested technique was used is one performed by the author (Tanner, 1961). In attempting to explain the shape of the psychometric function for the detection of acoustic sinusoid segments in noise, the human observer was conceived as having an imperfect memory. The problem thus became one of establishing an hypothetical space for describing imperfect memories. The first step was an examination of the knowledge necessary to a perfect memory. This led to the identification of a set of parameters that would describe a segment of sinusoid in its en- tirety: amplitude, starting time, duration, frequency, and phase. If the memory is not perfect, it seems reasonable to assume that the imperfection will lead to an error in the recorded values for these five parameters. The error has the same effect on per- formance as an uncertainty in the specification of the signal. For example, if there is no phase memory, but all other memories are perfect, then the performance is expected to be that of an ideal receiver detecting a signal specified except for phase. An imperfect memory has the same effect on performance as a signal with uncertain specification. Thus, the measure of an imperfect memory is the degree of uncertainty necessary to ac- count for an observed level of performance. In the experiments, frequency and phase were grouped as a single parameter, starting time and duration as a second parame- ter, amplitude as a third, and internal noise as a fourth parameter. These four dimensions were assumed to be independent and to be spanning the space in which the likely hypothesis can be described. Experiments were then performed in order to estimate numbers describing an uncertainty introduced by memory imperfection for each of these parameters. There was actually a sequence of experiments involved in esti- mating the parameters. The first of these, being a study of ampli- tude memory as a function of time, provided both an estimate of 10

the internal noise and uncertainty in amplitude memory. The memory requirements for frequency, phase, starting time, and duration were removed by superimposing the signal to be de- tected on a pedestal—a segment of sinusoid of the same frequency, phase, starting time, and duration as the signal. This pedestal occurred in each of two intervals in time, with the signal imposed on the pedestal in one of the two intervals. It was the observer's task to state whether the signal was superimposed on the first or second pedestal. In the experiment described in the preceding paragraph, the variable was the time between the ending times of the two pedes- tals. It was assumed that the amplitude of the first pulse was measured and then stored until the second pulse for comparison with the measure of that pulse. With the assumption that the ped- estals provided the observer with the frequency, phase, starting time, and duration knowledge, then the only uncertainty in this experiment consisted of that introduced by the internal noise to the measures of the amplitude, and a variance added to the first measure as it was stored in the memory. After the data were analyzed for an increase in variance, as the time between the intervals was increased, the curve was extrapolated to zero delay to estimate the internal noise. A second experiment was then performed in which the knowl- edge of starting time and duration was removed, although that of frequency and of phase was still provided. The signal to be de- tected was superimposed on a steady sine wave component added to the noise. The component was of the same frequency and phase as the signal. In order to estimate the uncertainty introduced by the inability to store starting time and duration, it was assumed that the values of the parameters for amplitude and internal noise from the first experiment for each observer still applied. In a third experiment, the continuous wave was removed from the noise, and a requirement for memory of frequency and phase was introduced. Thus, all of the memory requirements are now demanded of the observer. A parameter estimating frequency and phase uncertainty was determined from this experiment, again on the assumption that the parameters estimated from the previous experiments still applied. The uncertainties estimated were these: the internal noise reduced efficiency by about 0.3; the variance enlarged by the amplitude memory added one unit of noise every 400 msec: the 50-msec signal appeared to be fixed within a 75-msec interval: 11

and the frequency could be described as being defined within an interval of 80 to 100 cycles (Tanner, 1961). The Problem of Vigilance Behavior Another example of the proposed design criteria may be found in an analysis of the subject of vigilance behavior. Briefly, the vigilance situation involves tasks in which small, infrequent sig- nals occur at random intervals over long periods of time. There have been a number of experimental studies dealing with varied situations of this general type; several writers have also ad- vanced theoretical formulations with the hope of describing vigi- lance behavior within a general framework. The kind of effect traditionally observed was that a rapid deterioration in correct signal detections appeared to occur during the task period. An early experimental observation was that these detection rates in- creased as a function of the input signal rate and decreased with the variability of the intersignal interval. Indeed, the addition of "artificial" signals mixed in with the actual ones appeared to be helpful. The following hypotheses have been suggested in explain- ing these effects: 1. lowered arousal or alertness level due to task monotony, 2. fatigue or accumulation of inhibition over time, 3. low expectancy, and 4. distraction or attention shifts away from the task. Two recent theoretical approaches are those of Broadbent (1964), and Jerison and Pickett (1963). Broadbent argues from the point of view of signal detectability theory that perhaps many of the observed vigilance effects are due to criterial shifts over time rather than sensitivity shifts. He noted recently, however, that the data bearing on this question are ambiguous. Jerison and Pickett introduced the concept of "value of observing" in the vigilance experiment. Their construct controls the probability of observing. They suggest that detection failures are attributable to the fact that the observer was not observing at the time the signal occurred. It seems evident from the data presented that these writers are not theorizing about the same phenomena. The difference in explanations suggests that the experiments involved hypotheses existing in different spaces. The practical problem is that it is highly desirable to describe behavior which might exist in certain field situations. Laboratory experiments designed to meet this goal should, then, fall in the same descriptive space as the practical situations. For example, an explanation requiring the concept of "value of observing" is clearly not in the same 12

space as some tasks of practical interest where the cost of not observing is prohibitively expensive. Indeed, if it were not, there would be no interest in describing behavior in such situations. Thus, defining characteristics of vigilance tasks are the occur- rence of signals, the worthiness of observing these signals, and the uncertainty of the arrival time of the signals. In some of the recent studies investigating the decision theory type of explana- tions of vigilance decrements, such characteristics of the tasks were absent. Other constraints may be crucial for several rea- sons. First, the signals, when they occur, must not be completely discernible to the observer. Second, the nature of the decision rule employed by the observer may be an important function of the expected time distribution of the signals. An important ex- perimental parameter of what should be considered "vigilance" situations is the degree of uncertainty of the observer concern- ing the starting time and duration of the possible signals. Realiz- ing that though the early studies by Mackworth (1950) on vigilance did employ clear signals, i.e., clock-pointer double jumps, it seems obvious that again the interest is not in the analogous field situation. It seems evident that such tasks could be easily automated; hence, one could safely avoid the possibly hypnotic effects of clock-pointer watching. The general problem of memory in such tasks may be an ex- tremely important one and may serve as a possible descriptive dimension. For example, having available noiseless stored reference parameters of the expected signals conceivably can improve detection performance and, indeed, may serve as a partial explanation of the facilitative effect of artificial signal insertion. The decay of such a memory may explain performance decrements over time. An example of an hypothetical vigilance situation, as defined here, illustrates the advantages of the cri- teria proposed. Consider a detection experiment in which an observer must participate for some period of time. The input is noisy, and occasionally, although infrequently, a signal will be in the noise. The observer is allowed only a fixed number of detection responses; his task is to remain solvent until the task time period is over. He may lose his solvency either by spending all of his detection responses before the time period is over, or by failing to detect and thereby turning off an incoming signal within some short time interval after its arrival. A considerable monetary reward is the payoff for remaining solvent throughout the task time period. Admittedly, such a situation has some dif- ficulties in theoretical analysis. Certain bounds on efficient per- 13

formance can be established, however, and suitable performance measures conceivably may be developed for describing behavior in such a task. It is clear, for example, that when the a priori probability of a signal at any point in time during the observation period is low, one should adopt a variable false alarm rate such that, on the average, the allowable responses will be exactly used up at the end of the experimental period. In laboratory experiments, it is probably true, as Jerison and Pickett (1963) suggest, that the observer is guilty of failure to observe, and that this failure may increase as the experiment progresses. The occurrence of this phenomenon may be due to the trivial nature of the experiments, particularly from the point of view of the observer; watching a clock face for a deflection of a needle which may be a low probability event is a task which is unlikely to keep most observers interested. Their thoughts will obviously stray to other things, and even though they may be co- operative observers, they may still fail to respond from time to time. Rather than try to recover.data from these trivial ex- periments, it seems more profitable to look in the direction of an improved experimental design yielding data which can be utilized in terms of some a priori satisfying theory, rather than one which merely attempts to describe the data after it has been collected. The importance of the problem of vigilance is attested by the investment of large sums of money in methods of improv- ing performance in vigilance tasks. In any laboratory experiment in which failure to observe because of the worthlessness of the task is reported, the wrong problem has been studied. The vigi- lance problem that concerns a worthwhile task is the type to be used as the basis of a laboratory experiment. SUMMARY An explanation of the establishment of criteria for the design of laboratory experiments useful to field situations has been pre- sented. The first step, that of describing the current state of knowledge, is accomplished by stating a set of possible hypothe- ses to which an associated probability or degree of belief is assigned. The space in which these hypotheses exist is then described in terms of a set of orthogonal dimensions spanning the spaces. The size of the experiment necessary to assign co- efficients to the orthogonal dimensions within a predetermined level of confidence is then determined. The procedure was 14

illustrated by an experiment on memory in psychophysical tasks, and the problem of the interpretation of data obtained from ex- periments on vigilance was discussed. In conclusion, it is again emphasized that the most important factor is a precise statement of the problem to be studied. REFERENCES Broadbent, D. C. Vigilance. Brit, med. Bull., 1964, 17-20. Jerison, H. J., & Pickett, R. M., Vigilance: a review and re-evaluation Hum. Factors. 1963, 5, 211-238. Mackworth, N. H., Researches on the measurement on human peform- an ance. Med. Res. Council Spec. Rep. Series, H. M. Stationery Office (London), 1950, No. 268. Tanner, W. P., Jr., Physiological implications of psychophysical data. Ann. N.Y. Acad. Sci.. 1961, 8j>, 752-765. Watanabe, S., Information—theoretical aspects of inductive and deductive inference. IBM J. Res. Develop., 1960 (April), 4, 2, 208-231. 15

I VISUAL FITNESS FOR SPACE TRAVEL Arthur Jampolsky, Ailene Morris, and Carter Collins Eye Research Institute Presbyterian Medical Center Determination of visual fitness for an unknown environment is the challenge currently presented. What visual capabilities are required for space travel ? How does one provide for unpredict- able stresses on an already overloaded man confronted with a mortal dilemma ? How can one evaluate and select the man who can meet novel and unknown crises which may involve critical visual performance ? To answer these questions new tests adapted to dynamic stressful situations must be developed. What should be done rather than what can be done is the measure to be considered. Although evaluation and selection on the basis of static tests have been satisfactory for simple tasks, clearly they have not been suitable for complex ones. Static visual tests do not ade- quately reflect operational needs. Visual parameters presently tested do not necessarily have a bearing on the visual functions required and, indeed, the visual parameters may change during the dynamic or stressful performance situation. For example, in the area of auto driving static visual tests fail completely in adequately selecting night vision capability. An older person with decreased dark adaptability, a slight myopia which will increase at night, a senile pigmentary degeneration or an intraocular scattering of light due to a partial cataract may well pass all of the static daylight tests and yet be a distinct hazard on the road at night. Indeed, he has an almost specific disability for night driving. The few existing dynamic tests have been more successful for the selection of candidates able to meet complex operational situ- ations; for example, driving qualification tests now include 16

measurement of glare recovery, dynamic visual acuity, etc. More sophisticated qualification tests involve measurement of visual performance while the subject is made to endure disturb- ing stresses. Visual fitness can be established best during a totally simulated situation. However, in the interest of economy, critical components of the whole task can be simulated to pro- vide suitable qualification tests. Physicians have long used the technique of measuring an organ or system before and during known stresses, or under increased performance demands. Stress upon the heart is induced by the subject exercising, and upon the kidney by increasing demands to excrete physiological products or certain drugs. Similarly, the visual system might be evaluated under stresses such as anoxia, vibration, or acceleration. How does one evaluate the effects of psychological factors such as fear, anxiety, and so forth on visual performance ? Many stresses may occur in high-altitude, suborbital, and orbital environments. The critical factor is tolerance to stress rather than possession of basic visual perfection. A person with a "perfect" visual apparatus may fall easy prey to a complete upset of particular visual parameters which will virtually render him visually incapacitated. On the other hand, certain anomalies currently considered disqualifying may actually enhance visual performance under stress. For example, anoxia or alcohol pro- duces an esophoric shift which may result in diplopia if the pre- existing status was normal; and especially so if somewhat eso- phoric. A person with exophoria actually possesses more latitude for resisting such stress than a "visually perfect" individual. Further, a person with a well-adapted strabismus, such as equal vision alternating exotropia, will not suffer diplopia due to stress of the muscle imbalance; indeed, his distance judgment is good if he is allowed to use his eyes in his own alternate monocular way. He may see in two directions at almost the same time or with rapid alternation because his eyes are already well adjusted to dissociation. Also, one eye may be dark-adapted for cockpit visual tasks while the other eye could be light-adapted for visual search of the sky. The authors are not so naive as to believe that this notion will be accepted as practical, but it does serve to emphasize the prin- ciple that perfection in the different visual parameters, as mea- sured under static conditions, is not to be equated to visual un- stressability nor to visual fitness. Relatively unstressable visual apparatus or comfortable visual efficiency may be preferred 17

to stressable visual perfection. Tests of tolerance to visual stress should be further developed. One must test what one should rather than what one can. One of the more obvious results of the stress of space travel might be "blackout." The "threshold" tolerance to blackout is a variable thing. A present bias exists that there is a firm basis for it being primarily, if not solely, retinal in origin. This con- cept is based, among other things, on the fact that the retinal arterioles are seen to collapse at the time of the visual blackout. If one could but see the cerebral vessels one wonders whether these would not similarly collapse. In fact, there are parallels between the watershed of vascular supply to the visual cortex and to the peripheral retina that make one feel that the origin of blackout is perhaps not so firmly understood as may have been supposed. It should be pointed out that the dynamic tests of performance under stress will necessarily involve reliable means of objec- tively recording and evaluating visual efficiency. Psychophysical measurements depending on subjective report from a man under stress may be contaminated by the stress, or may be impossible to achieve. Sophisticated electrophysiological techniques in- volving cross-correlation of stimulus and evoked retinal and cortical potentials may be the means of achieving the above re- quirements. Further, these objective measurements might be employed without interfering with the primary simulated or actual operational task. It is hoped that those responsible for visual fitness criteria and selection standards will recognize the lessons from the past: static tests are insufficient to evaluate the dynamic stressful performance situation. Finally, the importance of measuring what one should rather than what one can is again emphasized. 18

THE EFFECT OF FLASH DISTRIBUTION AND ILLUMINANCE LEVEL UPON THE DETECTION OF LOW INTENSITY LIGHT STIMULI Richard E. Wienkel Lockheed Missiles and Space Company Research in the area of flashing lights as navigational signals has had a primary emphasis on the value of the conspicuity, or brightness equivalence, of a flashing light to a steady-state light. The classical work in the field was done by Blondel and Ray (1912). It has been confirmed and extended by the studies of Toulmin-Smith and Green (1933) and Schuil (1940). Schuil was interested in and determined conspicuity as a function of flash rate. Those studies have established the functional relationship of intensity in flashing and steady-state lights. However, they do not provide any information about the probability of detection of either light flashes or steady-state lights by subjects who must search a large solid angle. Toulmin-Smith and Green found that a flashing light in the order of 0.149 kmc (0.425 mile-candles) is adequate for visibility against a dark surround. Langmuir and Westendorp (1931) recommended, on the basis of experimental work, that a flashing light should have 10 times the illuminance of a threshold flashing light to ensure detection by the second or third flash. If the nominal of 0.0083 kmc is taken as the thresh- old of vision, then Langmuir and Westendorp are recommending an illuminance of about 0.083 kmc for signal lights. This is somewhat lower than the 0.149 kmc recommended by Toulmin- Smith and Green, but it is still in fair agreement. Both illumi- nance values are centered about the 0.13-kmc value produced by a third-magnitude star. 1. Now at the 6570th Aerospace Medical Research Laboratories, Wright- Patterson Air Force Base, Ohio. 19

Since search has not been a variable in the majority of the reported work, little is known about search and detection of flash- ing lights. This study was undertaken to determine the capability of naive subjects to detect flashing lights in a large visual field and visually to track dynamic flashing stimuli. APPARATUS The stellar surround was provided by a planetarium in the Mor- rison Planetarium, San Francisco, California. The projector is similar in design to the Zeiss instrument. The light flashes were produced by a telescopic projection system that rotated about its axis at the rate of T of arc/sec. The light flash was produced by a cam-actuated switch which pulsed a tungsten source. The light pulse had a width of 180 msec when measured one-third peak to one-third peak. The intensity of the projected area may be determined by the equation: I = BA/144, (1) where I = the candle power, B = the luminance of the projected patch, A = the area of the patch in inches and 144 = the constant to convert from square feet to square inch. The projector produces a collimated beam, and, within the limitations of the projection distance used, its illuminance is in- dependent of distance. Calibration of the beam yielded a value of 0.70 ft-c. The relationship between illuminance and luminance is: B = ER, (2) where B = luminance, E = illuminance in foot-candles, and R = the reflectance of the surface. Since R = 0.68, the luminance was equal to 0.48 ft-c. Substituting this value in Equation (1) gives one known. Therefore, it was necessary to measure the cross section of the light beam. This was 0.192 in.2 This was also placed in Equation (1). The inten- sity of the projected spot was 2.02 10~4 candles. However, the effective intensity when the lamp is pulsed is known to decrease. This was corrected by the Blondel-Ray equation: 20

I = I0 (T + 0.21).'T, (3) where I - the intensity of the flashing source, IQ = the intensity of the steady-state source, T = the flash duration in seconds, and 0.21 = an empirically derived constant. The light source therefore had an apparent intensity of 9.35x 10"^ candles. Since the target was matte and the light patch sub- tended a visual angle of between 2- and 3- of arc, the inverse square law of illumination is applicable and is accurate to about 2 per cent. E - I/D2, (4) where E = the illuminance, I = the intensity of the source, and D = the distance. The distance between the observer's eye and the patch was, on the average, 10 m and atmospheric attenuation was negligible. Consequently, the illuminance at the observer's eye was equiva- lent to 0.935 kmc of steady-state light. Similar calculations indicated that a steady-state illuminance of 0.29 kmc would be required to produce a flash with a con- spicuity of 0.13 kmc. This illuminance is that of a third-magni- tude star. Equation (1) was solved for A to produce the required illuminance and the area was found to be 0.45 in.2 The altera- tion of area is permissible under Ricco's law which states: AI = C, (5) where A = the area, providing it is a spot whose diameter is not more than 10-m of arc, I = the intensity, and C = a constant. Therefore, either area or intensity can be altered to produce the desired change in illuminance. When the projected patch was compared with the third-magni- tude stars in the surround, it was found that the surround stars were too bright. This was corrected by reducing the voltage on the planetarium projector until several of the simulated stars which should have an illuminance of 0.30 kmc appeared to have the same brightness as the steady-state 0.29-kmc calibrated source. This equality was obtained by the method of limits. 21

Three trained observers were used. The data were recorded on a 20-channel event recorder. One channel was used to record the occurrence of the stimulus; the remaining 19 were activated by the subjects. The subject was instructed to activate the switch when he saw a light flash. EXPERIMENTAL CONDITIONS A total of 130 experimentally naive subjects were utilized. Nor- mally, the subjects were run in subgroups of 19, which were serially labeled 1 through 7. In general, each subject was run only once so that the results would not be confounded by practice effects. However, an exception was made to this rule in sub- groups 1, 4, and 7. In the case of these three subgroups, the order of presentation of the conditions was alternated, and a total of four data runs was made. The temporal and spatial characteristics of the stimulus were produced by the flash generator. The track of the flash was iden- tical in all conditions, but since the stimulus did not pass through zenith, and since the timing was generated at the projector, the number of degrees in the specification of the conditions was less than 180°. In the first or massed condition, 0.13 kmc flashes were pre- sented in two groups of six flashes each.^ The first and last flashes were, respectively, about 30° of arc above the southern and northern horizon. Each flash was separated from the next by 1 sec of time and about 1° of arc. The total of 57 subjects was run under this condition. The second or distributed condition consisted of 12 flashes with a conspicuity of 0.13 kmc. These flashes were presented at the rate of 1 flash each 10 sec, and each was separated by about 10° of arc. A total of 54 subjects was run under this con- dition. However, the fourth subgroup was inadvertently given an auditory cue in the form of a switch activation about 5 sec before the first flash. The presence of this inadvertent cue made the data of this subgroup suspect; therefore, they were subjected to additional analysis. The third condition is distributed as was the second. In this case, the illuminance was increased to 0.935 kmc with a sample of 19 subjects. 2. Only 11 flashes were presented to one subgroup of 19 subjects because of equipment failure. 22

The subjects were given instructions that stated that the first flash would occur in the south and the path of the flashes would be in a northerly direction. They were instructed to search the south constantly, but occasionally to search off toward the north since they might possibly miss the first flash. Immediately after the instructions were given, the planetarium lights were dimmed and the flash projector was started. Since the flash projector was stopped immediately after it delivered the last flash, there was a period of approximately 4 min before the first flash in the new sequence was presented. Therefore, there were about 4 min of dark adaptation for all subjects in the initial condition. There was a 4 min interval between trials for the three subgroups that were presented multiple trials. RESULTS Table 1 summarizes the number of subjects reporting flashes and the number of flashes reported. It was anticipated that sub- group 4, which had been presented the auditory cue, would be superior to the other subgroups in that condition. Examination of the data in Table 1 tends to sustain that opinion. Therefore, two hypotheses were advanced: first, that the null hypothesis would be sustained between this subgroup and the other subgroups in condition II; and, second, that the null hypothesis would be sustained between this subgroup in condition II and the subgroup run in condition III. They were tested by means of continuity corrected \^ for proportions, and the results are given in Table 2. The first hypothesis advanced was rejected, and the second was sustained. Since this group was atypical it was removed from the remainder of the analysis. The hypothesis was then advanced that the same proportion of subjects in the three experimental conditions made detections. This was tested by means of standard large sample tests of pro- portions. The results are given in Table 3. The null hypothesis was sustained between conditions I and II, but rejected between conditions I and III, and between conditions II and III. It was hypothesized that there was no difference between the proportion of flashes seen under the three experimental condi- tions. The large-sample proportion statistic was again used and the results are given in Table 4. The null hypothesis was re- jected between conditions I and II, conditions I and III, and con- ditions II and III. 23

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TABLE 2. Comparison of Detection of Flashes with and without an Auditory Cue With auditory cue Without auditory cue % Experimental condition Massed Illuminance of flash 0.130 kmc 24.47* Distributed 0.130 kmc 12.06* Distributed 0.935 kmc 0.51 *Probability less than 0.01. TABLE 3. Significance of Difference of Number of Subjects Reporting Flashes 0.13-kmc distributed flashes 0.935-kmc flashes 0.13 kmc massed -0.593 -3.40** 0.13 kmc distributed -2.62* *Probability less than 0.005. **Probability less than 0.00034. TABLE 4. Test of Significance of Differences of Number of Flashes Reported 0.13-kmc distributed flashes 0.935-kmc distributed flashes 0.13-Magnitude massed flashes 0.13-Magnitude distributed flashes -20.3* -12.1* *Probability less than 0.00003. -8.7* The effect of practice is shown in Table 5, where the number of subjects that detected flashes is given. The significance of difference was tested by means of correlated x that was cor- rected for continuity. The null hypothesis was sustained between trials 1 and 2 and between trials 2 and 4, but it was rejected for trials 1 and 3, and trials 1 and 4. The effect of practice on the number of flashes detected is shown in Table 6. It was hypothesized that practice had no effect 25

TABLE 5. Summary and Analysis of Number of Subjects Detecting- Flashes for the First Four Trials Number of Subjects Detecting Flashes 0.13 kmc 0.935 kmc Trial 1 Massed 1 Distributed 1 Reported 4 12 Not reported 15 7 Total 19 19 Trial 2 Distributed 1 Massed2 Reported 7 15 Not reported 12 4 Total 19 19 Trial 3 Massed 1 Distributed2 Reported 11 18 Not reported 8 1 Total 19 19 Trial 4 Distributed 2 Massed Reported 11 17 Not reported 8 2 Total 19 19 Statistical Comparison of Trial 1 with Trials 2, 3, and 4 and Trial 2 with Trial 4 Trial 1 0.130 kmc Trial 4 Trial 2 2.00 3.12 Trial 3 7.11* Trial 4 7.11* 0.935 kmc Trial 1 Trial 4 Trial 2 3.12 2.25 Trial 3 9.14* Trial 4 6.12* *Significant at the 0.01 level of confidence. 26

TABLE 6. Summary and Analysis of Number of Flashes Reported for the First Four Trials Number of Flashes Reported 0.13 kmc 0.935 kmc Trial 1 Reported 6 Massed 109 Distributed Not reported 203 119 Total 209 228 Trial 2 Reported 43 Distributed 94 Massed Not reported 185 115 Total 228 209 Trial 3 Reported 36 Massed 162 Distributed Not reported 173 66 Total 209 228 Trial 4 Reported 80 Distributed 141 Massed Not reported 148 68 Total 228 209 Statistical Comparison of Trial 1 with Trials 2,3, and 4 and Trial 2 with Trial 4 0.130 kmc Trial 1 Trial 4 Trial 2 -5.28* -4.00* Trial 3 -4.40* Trial 4 -8.48* 0.935 kmc Trial 1 Trial 4 Trial 2 0.501 -3.72* Trial 3 -5.15** Trial 4 -4.24** *Null hypothesis rejected at 0.0001 level of confidence. **Null hypothesis rejected at 0.00003 level of confidence. 27

on the number of flashes reported, and this hypothesis was tested by means of a statistic for the testing of proportions obtained from large populations of subjects. It may be seen, for the 0.13- kmc flash, that the null hypothesis was rejected in the case of trials 1 and 2, trials 1 and 3, trials 1 and 4, and trials 2 and 4. It was sustained, for the 0.935-kmc flash, for trials 1 and 2, but rejected for trials 1 and 3, trials 1 and 4, and trials 2 and 4. Figure 1 shows the cumulative percentage of first detections as a function of serial position. It was found that 67 per cent of the massed 0.13-kmc subjects, 88 per cent of the distributed 0.13-kmc subjects, and 74.9 per cent of the 0.935-kmc subjects made first detections on or before the fourth flash. OISTRIBUTEO FLASH 0.935 KMC OISTRIBUTE0 FLASH 0.13 KMC MASSEO FLASH 0.13 KMC 4567 SERIAL POSITION OF FLASH FIG. 1. DISCUSSION The data indicate that the same proportion of subjects made de- tections when illuminance level was held constant, irrespective of flash configuration. However, a larger proportion of flashes was seen under the distributed condition than under the massed condition. It is hypothesized that this resulted because it was necessary to detect two bursts under the massed condition, whereas tracking of the patch was relatively simple once the initial detection had been made in the distributed condition. These conclusions would have to have been altered if the fourth subgroup had been included in the analysis. The inclusion of an auditory cue in the experimental condition made the data from this subgroup suspect. Analysis indicated that the subjects in this subgroup made significantly more detections than any 28

other subgroup which was presented stimuli of the same intensity. It was also indicated that the null hypothesis was sustained be- tween the fourth subgroup and the subgroup which was presented stimuli with an illuminance of 0.935 kmc. It was concluded that inclusion of those data into the general analysis was unwarranted. The error suggests that proper use of auditory cues should be investigated for use in energy-restricted systems. The analysis suggests that learning is fairly rapid. However, since the design was both incomplete and counterbalanced, all conclusions are tentative. In general, it appears that 50 per cent of the subjects in the 0.13-kmc illuminance condition can make detections after a small amount of training. The subjects given 0.935-kmc stimuli were superior in ability to detect; however, even in their case no single run was made in which all subjects made detections. The ability to learn to detect flashes deserves more study. Examination of the number of flashes reported again shows the superiority of the distributed over the massed condition. In addition, it is shown that the number of flashes detected generally is a function of the number of trials presented. The evidence is suggestive rather than definitive. The present study indicates that only one-half to two-thirds of the subjects made their first detection by the third flash. This is inferior to the performance predicted by Langmuir and Westen- dorp when they stated that one could assure detection by the second or third flash, particularly, when it is considered that between 40 and 80 per cent of the subjects were incapable of making a detection. There are many obvious differences between the Langmuir and Westendorp study and this study. First, Langmuir and Westendorp used highly trained subjects. They stated that they had corrected for training, but gave no explanation as to the basis of that correction. Second, they searched a relatively small, well-defined solid angle of space. The smaller and better . defined the solid angle, the greater the probability of detection becomes. Third, the flash was always static in space. Detection is probably easier when the stimulus does not move. The 0.13-kmc illuminance used in this study is very near the lower limit of illuminance allowable if detection of the stimulus is critical. This was implicitly recognized by Toulmin-Smith and Green who specified their value of 0.149 kmc for visibility and not for detection. The value of 0.935 kmc should be used for semitrained subjects who have large and ill-defined solid angles 29

to search. Consequently, present data suggest that the range of illuminance required for successful search and detection of flash- ing point source stimuli lies between 0.13 kmc and 0.935 kmc. SUMMARY The proportion of light flashes detected by naive subjects was determined as a function of two flash groupings and two levels of flash intensity. One flash grouping, the "massed" condition, consisted of two groups of six flashes. The flashes were pre- sented at the rate of 1 flash/sec with about 1° of arc separations. The groups were separated by approximately 90 sec of time and 90° of arc. The second, or "distributed" condition, consisted of 12 flashes presented at the rate of 1 flash/each 10° of arc and 10 sec of time. The illuminance-level of these two conditions was equivalent to 0.13/kmc. One group of subjects was run under the distributed condition when the illuminance was in- creased to 0.935 kmc. There was no significant difference in the proportion of subjects detecting flashes where flash distri- bution was the independent variable. A greater proportion of flashes was seen under the distributed condition than under the massed condition. More subjects made detections when the stimulus was 0.935 kmc than when it was 0.13 kmc. REFERENCES Blondel, A., & Ray, J. The perception of lights of short duration at their range limits. Trans, nium. Engrg. Soc. (Amer.), 1912, 7, 625-658. Langmuir, I., & Westendorp, W. F. A study of light signals in aviation and navigation. Physics, 1931, 1, 273-317. Schuil, A. E. The effect of flash frequency on the apparent intensity of flashing lights having constant flash duration. Trans. Ilium. Engrg. Soc. (London), 1940, 5, 117-121. Toulmin-Smith, A. K., & Green, H. N. The fixed light equivalent of flash- ing lights. Ilium. Engrg., 1933, 26, 304-306. 30

LANGLEY RESEARCH CENTER SIMULATORS AND STUDIES RELATED TO SPACE RENDEZVOUS AND DOCKING Jack E. Pennington NASA Langley Research Center The best way of investigating many piloting tasks is through the use of simulators that duplicate the mission as closely as pos- sible. National Aeronautics and Space Administration (NASA) Research Centers use such simulators extensively because: (a) all flight parameters can be continuously recorded, (b) parame- ters can be varied from flight to flight, and (c) simulated flights can be repeated as many times as desired. Much of the simula- tion work done at the NASA Langley Research Center is devoted to investigating techniques that make maximum use of man's capabilities, thereby tending to minimize system requirements and to increase the probability of mission success. This paper summarizes the simulation work at Langley Re- search Center which relates to the rendezvous and docking of two vehicles in space. Current simulators, studies conducted, and visual problems encountered are discussed. Rendezvous can generally be defined as bringing two vehicles together in space. The visual rendezvous technique, illustrated in Fig. 1, and described in Lineberry, Brissenden, and Kurbjun (1961), utilizes the pilot's capabilities not only to control the vehicle, but also to sense and to process the required informa- tion. In the visual rendezvous, the pilot must first visually ac- quire (or detect) the target. A study of these visual aspects is described in Brissenden (1962), and in Pennington and Brissenden (1963). Directly after acquisition, an interception course is attained by arresting the angular motion of the line of sight seen as the motion of the target against the star background, which is as an inertial reference. Once the intercept course has been estab- 31

DOCKING ACQUISITION FIG. 1. Phases of visual rendezvous. lished, the braking operation is begun and continues until the range is a few hundred feet, or less, the point where the docking operation begins. The acquisition phase of the rendezvous has come to mean detecting a flashing light mounted on the target, at night. Two high-intensity flashing lights mounted on the Agena will enable it to be detected by the Gemini pilots at ranges up to 20 miles. However, such a flashing light can be used only at night, and the power requirements are relatively high. Another technique, cur- rently being studied at Langley, uses optical filtering for detec- tion of a sunlit target. By successively viewing the search area first through a filter that transmits both the background and tar- get, and then through a complementary color filter that trans- mits the background but reflects the target luminance, the target appears to blink against a steady background, which greatly en- hances the target. Experimental results showed that subjects could detect the target when it was as bright as a fourth- to a fifth-magnitude star. This means that the filtering technique does not change the threshold of detection, but with the use of solar illumination the target could be detected at considerably greater ranges than would be possible with the use of artificial lighting. Research is under way to find suitable coatings and filter combinations which could be used on a manned space vehicle. Coplanar rendezvous closure control was investigated as early as 1960 (Brissenden, Burton, Foudriat, & Whitten, 1961), assum- ing a generalized spacecraft configuration and a simple visual display. Non-coplanar simulations of visual and instrumented displays are described in Lineberry, ^t al. (1961), and Wolowicz, Drake, and Videan (1960), respectively. The results of this 32

simulation work were important in defining man's part in the Gemini rendezvous, and also strongly influenced the adoption of the Lunar Orbit Rendezvous technique for the Apollo mission. Studies of rendezvous with low thrust levels, such as reported in Beasley (1963), as well as effects of display resolution (Pen- nington, 1963), also provided design information important to Gemini. A new simulation using Gemini control parameters is cur- rently under way. The simulator is located inside an inflatable radome which has a diameter of 53 feet (ft) (Fig. 2), and which serves as a planetarium. A star background, target reference, and earth horizon are projected on the walls of the radome. FIG. 2. Gemini simulation equipment. The simulator (Fig. 2) consists of a static cockpit linked through an analog computer to a modified Nike antenna drive unit, which contains star background, target, and horizon pro- jectors driven dynamically to produce the Gemini's visual en- vironment. The simulator drives the star background in response to a Gemini rotation, superimposes the target against the star background, and drives the target against the background with proper line-of-sight rate. The pilot's ability to detect the target's motion against the star background, which is very small in the Gemini program, is an important factor in completing a success- ful visual rendezvous. One problem was encountered in this simulation. When the bright target spot moved near a dim star the star sometimes 33

COMPUTER SIGNALS TO PROJECTION SYSTEM AND PILOTS INSTRUMENTS PROJECTION SYSTEM PILOT CONTROL SIGNAL TO COMPUTER FIG. 3. Visual simulator for remote docking. disappeared. The pilot would then lose his reference for deter- mining line-of-sight rate. This effect is currently being in- vestigated further. The docking phase of the mission takes place from a few hundred feet in to zero range. One of the first simulators to study general pilot docking (Fig. 3) utilized two circular light spots projected on a cylindrical screen to simulate remote as- sembly of two objects, such as fuel tanks, controlled from a spacecraft a short distance away. Riley and Suit (1964) describe this study. An analog computer commanded the images to grow in size or to move relative to each other in response to the pilot's control inputs. This simulation showed that pilots could accurately control the docking or latching using only visual in- formation, and with a wide range of control levels. Because this early work showed that the pilot could serve as a sensor with sufficient accuracy for visual docking control, two more elaborate simulators have been constructed at Langley to simulate the Gemini-Agena docking with high fidelity. The first simulator, shown in Fig. 4, is called the Visual Docking Simulator (VDS). It can simulate the docking from ranges up to 300 ft. A closed-circuit television (TV) system and an ana- log computer are employed. In this system a small-scale model of the target vehicle having three degrees of freedom is mounted in front of a TV camera. The model translates along the camera axis and rotates in response to the pilot's control inputs and the analog computer. The image of the target is transmitted by the TV system to a 2-axis mirror above the Gemini pilot's head and is projected on the inside surface of a 20-ft-diameter spherical screen. Through the added action of this mirror system, all six 34

-SMM cute Moea urn met <UKMM FIG. 4. Visual docking simulator. degrees of freedom are simulated. The pilot and crewman are seated in a full-scale wooden mock-up of the Gemini vehicle. A moving star field responsive to the Gemini vehicle's angular rates gives an impression of angular motion. Two of the studies made using the YDS are now discussed. The first investigated the effects of control modes (direct com- mand and rate command) on the pilot's control of docking. The second was a series of flights made under daytime and nighttime lighting conditions to determine any docking problems arising from the target lighting. The results of the first study showed that it was easier to control the docking in the rate command mode than in the direct mode. This was expected because, in the rate command mode when the controller is returned to zero, unwanted angular rates are automatically damped out, while in the direct command mode the pilot must provide his own damping by applying a manual con- trol input to bring the attitude rates to zero. Somewhat surpris- ingly, the study showed that the reason the direct mode was more difficult to control was not because the pilot could not make pre- cise corrections, but rather because the pilot could not distinguish between the attitude rates and the translational rates. The pilot determined the capsule's attitude in the YDS by look- ing at the nose position relative to the target. Translation cues were obtained from the aspect of the target. The second study, which compared the docking under daytime and nighttime lighting conditions, showed that it was difficult to determine precisely the Gemini's attitude and translation errors during the day, but it was considerably more difficult at night for two reasons. First, 35

only the cone was illuminated, rather than the entire body of the target. Second, the nose of the Gemini was not lit, so the pilot saw the indexing bar only when it was silhouetted against the illuminated target cone. Thus, the pilot had to use the cone it- self, rather than the body of the target for the orientation cues, and the lack of aspect made the problem, in effect, one of dock- ing with a two-dimensional rather than three-dimensional target. Since the pilots could not determine the vehicle alignment, they then concentrated on just flying the indexing bar into the docking slot. As a result, the pilots positioned the indexing bars slightly (about an inch) more accurately at night, but only with a sacrifice in vehicle alignment. The next logical step was to look for a visual-aid technique that could be added to the Gemini-Agena without a major modifi- cation, and that could reduce the inaccuracies and increase the pilot's confidence, particularly in the darkside (night) docking. Several visual aids were tested, using both the VDS and the Rendezvous Docking Simulator (RDS). The RDS (Fig. 5) involves a full-size model of the cabin and nose sections of the Gemini spacecraft, associated drive systems, a general-purpose analog computer, and a full-size lightweight model of the Agena target. The Gemini capsule is mounted in a hydraulically driven gimbal system which provides three de- grees attitude freedom. The entire capsule and gimbal system is, in turn, mounted on a horseshoe-shaped box frame, which is suspended by eight cables from an overhead bridge-crane system. FIG. 5. Full-scale rendezvous docking simulator. 36

The electrically driven bridge crane provides three degrees of translational freedom. The analog computer commands the drive systems to move the capsule in response to the pilot's control inputs, just as though the capsule were the Gemini vehicle in space. The RDS can simulate the docking from ranges up to 150 ft and permits studies using the actual Gemini and Agena hardware. Two of the studies made using the RDS are now discussed. The first was an evaluation of the suitability of the Agena Target Docking Adapter, (TDA). The second was an investigation of the effect of thruster failure on the pilot's control of docking. For the first study, McDonnell Aircraft Company supplied the hardware mock-up of the Agena TDA for use in an investigation of possible problems in docking, using the TDA and an optimiza- tion of the Agena's visual aids. The TDA is shown in Fig. 6. In addition to the docking cone and latching mechanism, it contains two high-intensity flashing lights mounted at about 11 o'clock and 5 o'clock on the Adapter. These lights enable the astronauts to detect the Agena at ranges up to 20 miles. The lights are turned off at 500 ft in order not to distract or blind the pilot. Pilots made part of the simulated flights with these lights on to determine to what extent the dock- ing would be degraded if the lights did not turn off. Pilots agreed that the lights were distracting and reduced the pilot's confi- dence, but they felt that they could dock successfully, particularly if the lights could be repositioned on the target. If the lights FIG. 6. Production target docking adaptor (TDA). 37

were placed at 9 o'clock and 3 o'clock they would not be seen by either astronaut when docked. As mentioned earlier, the night flights had shown a need for a visual-aid technique that could increase the docking accuracy. Two types of aids were indicated. The first aid would be a light to illuminate the nose of the Gemini so the pilot could determine the vehicle's attitude. A floodlight mounted on the capsule to illuminate the nose was tried and found to be satisfactory. The second aid was to be mounted on the target to provide a reference for aligning the axes of the capsule and target. Three aids were tested on the TDS. The first was a probe projecting out of the TDA along the pilot's line of sight. The second aid was a 30-inch square with lights at three corners, mounted near the rear of the target. A light near the front of the target completed the square when the vehicles were aligned. The third aid tested consisted of illuminated vertical and horizontal bars mounted front and back of the target. All the pilots who flew the simulator, in- cluding four astronauts, agreed that the bar aids were better. Another study using the RDS investigated the effects of jet failure on the pilot's ability to complete the docking. The case in which a control jet failed to fire was simulated. If a jet were to fail open (not turn off), the astronaut could cut off the fuel to that particular jet. The situation would then be the same as that simulated. Vertical and lateral jet failures were the most diffi- cult to control because these jets fire singly. All other jets fire in pairs, so if, for example, a braking jet failed to fire it would cut only the control power in half. If a vertical jet failed to fire, however, the capsule just could not move unless the pilot either rolled and fired a lateral jet, or pitched and fired a longitudinal jet. Only these most critical malfunctions were studied, and techniques were developed for overcoming them successfully. An example of some of the simulation work at Langley related to rendezvous and docking has been presented. Other studies made with the simulators include: (a) technique for manually determining range and range rate during rendezvous, (b) evalua- tion of the Gemini cockpit instruments and controllers, (c) tech- niques for reducing control cross-coupling by canting the trans- lation jets, and (d) remote-controlled docking using closed-circuit television. The VDS and the RDS are excellent examples of closed-circuit TV and dynamic simulators. Each has inherent advantages and disadvantages. Closed-circuit TV permits simulating relatively high velocities and longer ranges, and it is relatively easy to 38

vary the lighting conditions, but the picture loses fidelity at close ranges, and the minimum range is determined by the dis- tance from the observer to the projection screen. The dynamic simulator gives the pilot the same view he would have from the spacecraft including target aspect, and permits closure to ve- hicle contact, but it is difficult to eliminate visual cues. Flat black curtains to keep ambient light out of the darkened hangar are used with filters over the capsule windows. Thus, it is neces- sary to consider not only the pilot's visual capabilities, but also the simulator's visual characteristics. All of the simulators that have been discussed are used for research rather than for training, so they are designed to be versatile. This permits investigating many problems with one piece of equipment. For instance, the rendezvous simulator will also be used to study the lunar take-off phase of the Apollo mis- sion, the VDS will be used to study space station docking, and the RDS will be used for lunar landing studies. REFERENCES Beasley, G. P. Pilot-controlled simulation of rendezvous between a space- craft and a commanded module having low thrust. Langley Field (Hampton, Va.): NASA tech. Note, 1963, No. D-1613. Brissenden, R. F. A study of human pilot's ability to detect angular mo- tion with application to control of space rendezvous. Langley Field (Hampton, Va.): NASA tech. Note, 1962, No. D-1498. Brissenden, R. F., Burton, B. G., Foudriat, E. C., & Whitten, J. B. Ana- log simulation of a pilot-controlled rendezvous. Langley Field (Hamp- ton, Va.): NASA tech. Note, 1961, No. D-747. Lineberry, E. C., Brissenden, R. F., & Kurbjun, M. C. Analytical and preliminary simulation study of a pilot's ability to control the terminal phase of rendezvous with simple optical devices and a timer. Langley Field, (Hampton, Va.): NASA tech. Note, 1961, No. D-965. Pennington, J. E. Effects of display noise on pilot control of the terminal phase of space rendezvous. Langley Field (Hampton, Va.): NASA tech Note, 1963, No. 1619. Pennington, J. E. & Brissenden, R. F. Visual capability of pilots as ap- plied to rendezvous operations. Amer. Inst. Aeronautics & Astronau- tics (Meeting, Jan. 21-23, 1963) 1963, No. 63-15. Riley, D. R., & Suit, W. T. A fixed-base visual simulator study of pilot control of orbital docking of attitude-stabilized vehicles. Langley Field (Hampton, Va.): NASA tech. Note, 1964, No. D-2036. Wolowicz, C. H., Drake, H. M., & Videan, E. N. Simulator investigation of controls and display required for terminal phase of coplanar orbital rendezvous. Langley Field (Hampton, Va.): NASA tech. Note, I960, No. D-511. 39

SOME LANGLEY RESEARCH CENTER PLANS IN THE AREA OF VISUAL DISPLAYS FOR LUNAR MISSION SIMULATION Donald R. Riley and Byron M. Jaquet NASA Langley Research Center As presently planned, Project Apollo will have automatic capa- bility for most phases of the lunar mission. The design over-all probability of success of the Apollo mission is currently speci- fied as 0.90. In order to meet this criterion, the various subsystems must have very high reliability figures and to improve this number would presumably require some form of man-machine integration. Consequently, the National Aeronautics and Space Administration (NASA) has considerable interest in the utilization of the astro- naut to increase systems reliability. The ability of the astronauts to perform many tasks was demonstrated in planned maneuvers in Project Mercury and, probably more important, was clearly demonstrated in the case of failures of automatic systems. That experience and a wealth of previous experience with man-machine combinations has shown that the reliability of systems can be increased through the proper integration of man and machine. A basic requirement in such a combination is that procedures be available for the man to follow. Preferably, guidance for the application of these pro- cedures should be independent of complex automatic equipment. Many task areas of Project Apollo exist for which simple manual procedures have not been developed, for example, midcourse navigation, orbit establishment, lunar landing, etc. In such situ- ations the first step is to develop piloting procedures for the various tasks, and the next step is to demonstrate the pilot's proficiency in performing the tasks. Inherent in the development of piloting procedures is the use of man's visual sense. At present, not much visual experience 40

exists in the space environment. In addition, it is not easy to acquire. At the present stage of development several years of preparation are invested in each manned space program. Each mission is planned in detail and practiced in order to acquire proficiency and to assure success. This procedure dictates the use of simulators and inherently specifies that sophistication will be required if realism is to be obtained. The present paper reviews the work at the Langley Research Center in this area, stressing simple piloting procedures which are based on visual cues, and describes in some detail the use of visual cues and how these cues will be generated in the simulators. DEVELOPMENT OF PROCEDURE The areas being examined at the Research Center are listed on Fig. 1. They include earth entry, rendezvous, docking, midcourse navigation, orbit ephemeris determination, lunar orbit establish- ment, powered lunar descent, hover and translation, and lunar launch. A first step is the development of a simple piloting pro- cedure. Consider, for example, a powered mission phase; in this situation the primary task of an automatic or manual guidance system is simply to point the thrust vector in the proper direc- tion. Therefore, in looking for a simple manual guidance tech- nique visible references are sought which an astronaut can use for orienting the thrust vector. Solutions to this problem take the steps shown in Fig. 2. First, compute a fuel-optimum maneuver for the task. Then, examine the results to see how the thrust vector orientation changes relative to references external to the spacecraft. In most cases, some reference exists so that the angle between the thrust vector and the line of sight to that reference remains about constant during the maneuver. The EARTH ENTRY RENDEZVOUS DOCKING MIDCOURSE NAVIGATION ORBIT EPHEMERIS DETERMINATION LUNAR ORBIT ESTABLISHMENT POWERED LUNAR DESCENT HOVER AND TRANSLATION LUNAR LAUNCH FIG. 1. Task areas under study. 41

1. COMPUTE OPTIMUM MANEUVER 2. EXAMINE THRUST VECTOR ORIENTATION 3. RECOMPUTE MANEUVER USING VISUAL REFERENCE 4. ERROR ANALYSIS OF TECHNIQUE 5. MAKE SIMULATOR STUDY USING TECHNIQUE FIG. 2. Procedure for selection of simpli- fied piloting technique. next step is to compute the maneuver based on use of the refer- ence and to compare the results with the optimum maneuver. If the maneuver looks reasonable, an error analysis is made to de- termine the sensitivity of the procedure to reasonable operational errors. The most promising techniques at this stage are tried on a simulator. LUNAR DESCENT As an example of the technique, consider Fig. 3, which is con- cerned with lunar landing. It was assumed that after applying thrust, a lunar excursion module separated from a spacecraft which was in an 80-nautical-mile-altitude orbit around the moon. The module descended on a Hohmann transfer ellipse to a peri- cynthion altitude of 50,000 (ft). It then made a gravity-turn pow- ered descent toward the lunar surface. On examining the orienta- tion of the thrust vector relative to various references, it was PARKING ORBIT HOHMANN TRANSFER- FIG. 3. Lunar descent. 42

noted that the angle between the excursion-module thrust vector and the line of sight to the orbiting spacecraft remained very nearly constant during the powered descent phase (see Fig. 4.). Subsequent error analyses indicated that the orbiting spacecraft would be a suitable reference for manual control during the de- scent (Barker & Queijo, in press; Barker, in press). Some en- hancement of the orbiting spacecraft, possibly by the use of a high-intensity flashing beacon or a filtering technique (Penning - ton, 1964) would be required to assure visual acquisition for the range of the maneuver. It is worth noting that the large variation in the angle K shown at the lower end of the altitude scale would, of course, require the astronaut to obtain some other visual reference. At this point, it so happens that the excursion module is now operating within the altitude-speed range of many high- performance airplanes, and previous airplane experience would indicate that an out-of-the-window view of the surface would suffice from this point to touchdown. K, DEC 20 10 0 10 20 30 40 50 x 10 ALTITUDE, FT FIG. 4. Lunar landing. Studies along these lines have been made and others are in progress for many tasks associated with lunar landing missions. Manual procedures utilizing visual references for guidance are being developed for most of these tasks. Some of these pro- cedures already have been tried on available sumulators, and some must await the activation of more sophisticated simula- tion devices. 43

ORBIT ESTABLISHMENT Some additional lunar mission tasks, the procedures developed for each task, the visual cues required, and the generation of the cues in simulation devices are now reviewed. Consider the task of establishing an orbit around the moon. The problem is illus- trated in Fig. 5. The vehicle approaches the moon on a hyper- bolic trajectory. The task is to establish an 80-nautical-mile- altitude circular orbit, which means reducing the vehicle radial and tangential velocity components and altitude to the desired orbital values. BRAKING PERIOD HYPERBOLIC APPROACH TRAJECTORY CIRCULAR ORBIT FIG. 5. Orbit establishment. An analytical study showed that the lunar horizon would be a convenient reference for thrust-vector orientation in the pitch plane, and that stars would, of course, be good yaw or azimuth references. In fact, Mercury experience indicated that the astro- naut could align the capsule in yaw within a couple of degrees just by observing the convergence of the surface features through the window. In other words, it appears that the astronaut could navigate by aiming his vehicle, using a scribed windshield or some other simple sighting device, possibly as shown in Fig. 6. Here the lunar surface and several stars are shown. Elevation or pitch angles could be set using the lunar horizon. Conver- gence of the landscape on the grid could be used for azimuth alignment, while the stars and the grid could be used to obtain a desired angular displacement. In order to provide these cues 44

YAW ANGLE, DEC -15 -10 -505 10 15 10 PITCH ANGLE, DEC --I-5 FIG. 6. Window for vehicle alignment. in a simulator, it is necessary to show the horizon, surface features, and stars with proper relative motion to correspond to spacecraft movement. At present, there is no device for project- ing or displaying properly all of this information, so it has been necessary to revert to the use of a cathode-ray tube (CRT) to generate representative stars and a horizon. The scribed lines on the CRT correspond to the spacecraft window lines. This simulator will be used for a preliminary evaluation of the pro- cedures for establishing orbits. By the summer of 1965, the Lunar Orbit and Landing Approach (LOLA) simulator should be operational, and there will be a good means of generating the horizons and surface features. At that time, star projectors will be used for star displays. The LOLA simulator is described in detail subsequently in this paper. ORBIT EPHEMERIS DETERMINATION The next task to be examined is that of orbit ephemeris deter- mination. The problem is simply that of finding the characteris- tics of an orbit as determined from on-board sightings. Two basic procedures for doing this have been proposed as shown in Fig. 7. One depends on the use of lunar landmarks and measure- FIG. 7. Orbit ephemeris determination. 45

meats of such parameters as rotation of a line-of-sight, altitude, altitude rate. etc. The other depends on taking sightings on orbit- ing spacecraft and determining range and range rate. Thus, there are two different techniques to evaluate, and the visual cues to be generated are completely different. The one requiring lunar sur- face features will be evaluated on LOLA. The other requires the generation of a spacecraft image. In this case, the spacecraft will probably be represented by a light spot. LUNAR LANDING The most critical phase of the lunar mission will be the final part of the powered descent and touchdown. Experience with air- planes and helicopters has demonstrated that man can perform landings much better and more reliably than any automatic sys- tem. The lunar landing, therefore, is one task area which will be investigated with as much realism as practical. Here, of course, the visual cues required are the lunar surface features, and since appreciable accelerations are involved motion cues become im- portant. The final portion of the lunar landing will be studied on the Lunar Landing Research Facility, which is described subsequently. LUNAR ORBIT AND LANDING APPROACH SIMULATOR Currently under construction at the Langley Research Center are two rather sophisticated simulators. One of these is the LOLA simulator, which is illustrated in Fig. 8. This simulator consists of four models of the lunar surface, viewing systems to transmit views of the models to the display area, and a four-porthole dis- play system. The four models were selected on the basis of a desired simulated altitude range of about 200 miles to 200 ft for a wide range of trajectories. Design considerations included a minimum distance from viewing optics to the models of 3/4 in., and a practical size for construction and housing in an existing structure. The region around Crater Alphonsus was selected as the landing site for simulation studies because of scientific in- terest and because regions of the most rugged mountains on the moon lie on the approach. Therefore, it presents the pilot with an exacting navigational task. Orbital inclinations up to 15° can be simulated with the spherical model. The surface of the spheri- cal model will be smooth with the lunarscape painted on plastic gores which are then mounted on the surface. All other models 46

FIG. 8. Lunar Let-Down Simulator. are in relief with painted shadow patterns to give the proper ap- pearance. All models are internally or back-lighted. Direct solar illumination was chosen as the lighting condition so that the lunar surface features would appear under low contrast. This should reduce the facility to see the features and provide an ad- verse viewing condition, as compared with other lighting condi- tions in which shadows are more prominent. The models are viewed by clusters of television cameras mounted on transport mechanisms. The transport mechanisms have three translational degrees of freedom. In addition, the camera clusters are gimbaled to provide three angular degrees of freedom so that motion with a full six-degrees-of-freedom can be simulated. One group of four TV cameras furnishes the display information to the pilot at any given instant. A simulated vehicle will have four portholes, a TV camera providing each with a 65° simulated field of view. The display will present the pilot with realistic terrain features, such as the irregular lunar horizon and close-up features when in the final landing approach. Pilot control signals are transmitted to the computer which, in turn, drives the transport and gimbal mechanisms so that the pilot, in effect, flies the cameras over the lunar surface. During a descent, the system viewing the spherical model of the moon will furnish display information to the pilot until the lower limit of travel is reached. Before this lower limit is reached, the second camera cluster is automatically switched on for Map 1. Similar switching will be made through the remainder of the descent. 47

The size, scale factors, and altitude range of each model are shown in Table 1. TABLE 1. Model Size Altitude Range Model scale Sphere 20-ft D 200 mi. to 7 mi. 1 in. = 9 mi. 1 15 ft x 40 ft 7 mi. to 1.5 mi. 1 in. = 2 mi. 2 35 ft x 25 ft 1.5 mi. to 3/8 mi. 1 in. = 1/2 mi. Ellipse 34.9-ft major axis 3/8 mi. to 200 ft. 1 in. = 200 ft. 22-ft minor axis In order to use the sphere and Map 1 before the TV system is operational, a 180° motion-picture camera-projector has been developed. Preprogrammed trajectories will be filmed. The mo- tion pictures will then be projected within the sphere giving a 180° field of view. The pilot will be an observer and will not have control over the display. This presentation will be used to test man's ability to perform observational tasks which would pre- cede any control action and to determine his orbital ephemeris. LOLA should help define those control tasks best performed by man or machine, and thus will determine an effective man- machine integration for the lunar mission. Studies with the pre- programmed trajectories should begin in the latter part of 1964, and the complete system should be in operation about a year later. LUNAR LANDING RESEARCH FACILITY Because gravity on the moon is one-sixth that of the earth, thrust levels for lunar operations are very low compared with those re- quired for vertical taken off or landing (VTOL) night on earth. To produce reasonable horizontal accelerations for braking and maneuvering during lunar landing, large attitude angles, up to 30° or more, will be required. A facility designed to study the piloting problems close to the lunar surface is presently under construction at Langley. This simulator with its associated equipment is known as the Lunar Landing Research Facility. Simulation with this facility begins at about the altitude where LOLA stops. An over-all layout of the facility is shown in Fig. 9. The gantry supports a traveling crane from which the vehicle is suspended. The crane system supports five-sixths of the weight 48

FIG. 9. Lunar Landing Research Facility. of the vehicle through serv- controlled vertical cables, while the remaining one-sixth of the weight pulls downward and simulates the lunar gravitational force. The overhead crane is slaved to move with the vehicle linear motions to keep the cables vertical. A gimbal system on the vehicle permits angular freedom in pitch, roll, and yaw. Vehicles weighing up to 20,000 (Ibs), and as large as the full- scale lunar excursion module used in the Apollo Project, can be tested on this facility. The pilot can maneuver in complete 6° of freedom in a volume 400 ft long, 165 ft high, and 50 ft wide. Through the use of a catapult, initial velocities up to 50 feet per second (fps) horizontally and 40 fps vertically can be provided. A photograph of the general research test vehicle is shown in Fig. 10. The vehicle gross weight is 10,000 Ibs including a two- man crew and 3,300 Ibs of fuel. Fuel is 90 per cent hydrogen peroxide. The main motors provide 6,600 Ibs of thrust with a ten-to-one throttling range. Attitude motor thrust is ground- adjustable to produce angular accelerations from 0.1 to 0.5 radians per second per second (rad/sec^) about all axes. The fuel load will permit about 3 minutes of operation. The pilot's bubble can be masked to determine the effect of the viewing area on his ability to land safely. It is anticipated that requirements for instrument displays will be developed as the simulation program proceeds. The establishment of require- ments for performing a lunar landing will be accomplished by 49

FIG. 10. General research vehicle for Landing Research Facility. measuring pilot performance. Piloting techniques, visibility, and abort modes will be major items of study using this simulator. Construction of this facility was started in September 1962. Re- search studies were started in 1964. CONCLUDING REMARKS In conclusion, the Langley Research Center has been examining, through analytical and simulation studies, simple guidance tech- niques for pilot control of various tasks associated with the lunar mission. These simplified techniques and pilot utilization should increase the reliability of Project Apollo and other manned space missions. Results of simulator studies conducted thus far have shown that, given proper information, pilots can perform rendez- vous and docking, although these missions have not actually been performed in space. The usefulness of simulator devices, how- ever, has been demonstrated in Project Mercury. Simulation devices such as LOLA and the Lunar Landing Research Facility will provide information necessary for the lunar and other space programs. 50

REFERENCES Barker, L. K. A technique for thrust-vector orientation during manual control of lunar landings from a hohmann transfer. Washington, B.C.: NASA tech. Note, in press. Barker, L. K., & Queijo, M. J. A technique for thrust-vector orientation during manual control of lunar landings from a synchronous orbit. Washington, D.C.: NASA tech. Note, in press. Pennington, J. E. Langley research center simulators and studies related to space rendezvous and docking. Paper presented at meeting of Armed Forces-NRC Committee on Vision, (April 23-24) 1964, Wash- ington, D.C. (Included elsewhere in these Proceedings) 51

VISUAL MASKING USING DIFFERENT TEST STIMULUS PATTERNS Robert C. Boyle1 NASA Ames Research Center Moffett Field Temporal delay in human visual processes assumes a greater importance as the velocities of travel increase. In fact, at very high velocities an observed event can occur and be past before the observer is even aware that he saw the event. This visual latency increases under conditions of low illumination. The data from this present study give some idea of how visual latency in- creases as object luminance decreases. One approach to the study of visual perceptual latencies is by means of a visual phenomenon called visual masking, which has been studied under various other names. It was called masking by Pieron (1925), rapid light adaptation by Boynton and Kandel (1957), perceptual blanking by Lindsley (1961), and perceptual interference by Kietzman (1962), and Boyle (1963). In this study the term visual masking refers to the gradual reduction of cor- rect responses as to the orientation of a patterned test stimulus as the temporal interval between the test stimulus and a succeed- ing brighter masking stimulus is decreased. A latency model of visual masking was used to explain the obtained results. It has been stated by Cheatham (1952) and Keitzman (1962) that a latency model of visual masking cannot explain the divergent results that are obtained when using different test-stimulus patterns under otherwise similar conditions. It is the thesis of this experiment that even though the constants of an equation based on a latency model of visual masking may vary some with different test-stimu- lus patterns, the variation, despite its significance, will be rela- tively minor and the equation will retain its general latency form. 1. Resident Research Associate, National Academy of Sciences. 52

METHOD Figure 1 shows a block diagram of the experimental system. Fourteen numbered program steps and the four test slide posi- tions are prepunched on paper tape and operate through the relay logic block to present the various conditions. The neutral-density filter selector servo-system is controlled by external panel con- trols. The test-stimulus positioning servo-system is directly FIG. 1. Block diagram of experimental system. controlled by the prepunched paper tape. Activation of the pre- sentation switch by the subject starts a timing counter, begins the pulse generator, and prepares the tape reader to advance the paper tape to the next position. To indicate the test-stimulus direction, the subject activates a four-position response switch which stops the timing counter and initiates test-stimulus re- positioning. The preselected stimulus position, the subject's selection of the stimulus position, and his response latency (mainly to insure that the subject is not drowsy) are printed out on the digital printer. The buzzer informs the subject when the presentation switch is rearmed. The optical and monitor system is shown in Fig. 2. The optical system consists mainly of a monocular Maxwellian-view optical system, with Sylvania R1131C glow modulators as light sources, and a red fixation-light source. The pulse generator initiates the pulses which "fire" the glow modulators, GMTp and GMgp. These in turn are activated by an ultraviolet source (UV) to eliminate erratic operation of the glow modulators in the dark. The filters Fj and F2 eliminate visible light from the ultraviolet light source. 53

GMTF —r-^XII ""1 I w rt BS 84 L| ^ YL3 =ti*> EYE 1 1 1 1 iu M3 M4]V |_pH S3 i r -'uv 1 , , 1-2 -m -n r~k ' lU D\ ' — > .CRO UL— J£> GMBF My F4 S2 _., f,,- ,-•-., |CAL . FOUIPMFNT FIG. 2. Schematic diagram of optical system. Glass slides MJ and M2 reflect a portion of each light beam to a photomultiplier (PM) which permits equating the luminances of the two light sources by means of an oscilloscope (CRO). Be- yond Mj and M2 the light beams are collimated by lenses Lj and L2 (508-millimeter (mm) focal lengths). F4 is a filter holder for neutral density filters, and Fg represents a pair of servo-con- trolled filter wheels in the light path of the test flash. Similarly, 82 is the fixed reticle holder for the blanking stimulus, and 8^ is the servo-controlled reticle holder for the test stimulus. The blanking-flash beam is reflected at 90° of arc from itself by means of a pentaprism (P), combined with the red fixation pattern (83) at glass slide M^, and is superimposed upon the test-flash beam at a beam splitter (BS), This combined beam is then focused upon the cornea of the subject's eye by lens Lg (508-mm focal length) through a 3.65-mm artificial pupil (8,). To insure that the illumination is always set at the same level, a calibration circuit (CAL) is incorporated in the system. A beam of light is passed through a chopper and is reflected from mirror Mg onto the photomultiplier (PM). Mounted on Mg is a cadmium sulfide photocell that is one leg of a bridge circuit. The light source is varied until a zero reading is obtained from a meter in the bridge circuit. Then the amplitude of the calibration light source can be determined from the oscilloscope, and the test and blanking-flash amplitudes are set at this same level. Figure 3 shows the various reticles used in the experiment. A, B, and C were the three equal-area test stimuli used, E the blanking-flash stimulus, and D the fixation pattern. Each test slide has four positions: up (|), down (-), left (\), and right (/). As seen by the subject, the blanking flash was superimposed upon the test flash in the open central area of the fixation pattern. 54

FIG. 3. Reticle patterns used in experiment. A, B, and C are equal area test flash patterns, D is fixation pat- tern, and E is blanking flash pattern. White areas repre- sent lighted portion of pattern as seen by subject. The visual angle subtended by the test and blanking flashes was 1°42'. Both stimuli were effectively square waves of 10 milli- seconds (ms) in duration with luminances of 4358 millilamberts (ml) for the blanking flash and 2566 ml for the test flash (no fil- ters) as measured at the plane of the subject's eye by means of a Pritchard Photometer. The red fixation pattern was easily visible and was present continuously throughout the session. SUBJECTS The three subjects studied were Moffett Field Naval personnel. They were all trained on all three test stimuli over a 2-week period. All three had emmetropic vision with 20/20 visual acuity or better for the right eye as measured by a Bausch and Lomb Ortho-Rater. Throughout the experiment each subject used only his right eye. PROCEDURE Given subject aligned himself in the apparatus and dark-adapted for 10 minutes. At the end of this time, a ready signal was given and, when he was prepared, the subject could initiate the signal presentation by pressing a hand-held switch. The subject's task 55

was to identify the position (up, down, left, or right) of the test stimulus. He had to indicate his response each time by means of the four-position response switch or the programmed tape would not advance to the next step. Two conditions were run. Generally, the conditions were randomized except that one test-stimulus pattern was completed before a new test-stimulus pattern was begun. Each condition consisted of a preliminary warm-up run of 12 presentations, followed by 6 more runs of 23 presentations each with a minimum of 7-1/2 seconds between presentations. The first three presentations of each run were eliminated from the data as they were for light-adaptation purposes only. The dependent variable was the percentage of correct re- sponses (0 to 100 per cent) corrected for a 25 per cent chance level. The independent variables were test-flash luminance (1.61 to 5.61 log microlamberts (jil) in six 0.1-log neutral-density filter steps for each condition), test-stimuli forms (three), and the in- terval between test- and blanking-flash onsets (10, 13.5, 18, 26, 39, and 70 ms). The blanking-flash luminance was 6.64 log pi. Each condition was repeated three times, and then averaged to obtain a better estimate. RESULTS Preliminary data are shown in Figs. 4, 5, and 6. From Fig. 4, it can be seen that visual masking decreases as test-flash luminance is increased or as the interval between the test flashes and blank- 100 - (MILLISECONDS) 39 26 18 13.5 10 PARAMETERS TEST PATTERN ®, IBF=6.64 log p-L ANO TIME BETWEEN TF ANO BF ONSETS, msec 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 TEST-FLASH LUMINANCE, log micro lamberts FIG. 4. Percentage correct (adjusted for 25 per cent chance level) as function of test-flash luminance and interval between test- and blanking-stimulus onsets for coarse-grating stimulus. Each curve represents average of three repetitions of each condition for all three subjects. 56

100- 70 (MILLISECONOS) 26 18 PARAMETERS TEST PATTERNS IBF=6.64logflL ANO TIME BETWEEN TF AND BF ONSETS, msec 1.9 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 TEST-FLASH LUMINANCE, log microlamberts 6.0 FIG. 5. Percentage correct (adjusted for 25 per cent chance level) as function of test-flash luminance, in- terval between test- and blanking-stimulus onsets, and test-stimulus patterns. Each curve represents average of three repetitions of each condition for all three subjects. PARAMETERS IBF 6.64 UuiuL PERCENT CORRECT 1.0 2.0 3.0 4.0 5.0 6.0 7.0 S.O TEST-FLASH LUMINANCE, loii nncrolambcrts 9.0 FIG. 6. Time between test- and blanking-flash on- sets as function of test-flash luminance and test- stimulus patterns at constant 50 per cent correct level. Plotted points obtained from curves of Fig. 4, and smooth curves calculated from indicated equa- tions. Break from calculated curve (for which data were not obtained in this experiment), occurs beyond 10 ms as indicated by dashed line at that interval. ing flashes is increased. Figure 5 gives the same data as Fig. 4, but for all three test stimuli. It can be seen that the coarse grat- ing is more easily seen while the rectangular form is generally the hardest to see. Test-flash luminance values were taken from all of the curves 57

of Fig. 5 at the 50 per cent correct level. These are the points plotted in Fig. 6. A preliminary equation was fitted to each set of these data. They are included in Fig. 6. The smooth curves were plotted from values calculated from the equations. When the data were plotted in the form of Fig. 6, it could easily be seen that, as the test-flash luminance was increased, the interval between stimulus onsets had to be decreased in order to operate at the same level of performance (50 per cent correct). It should also be noticed that all three curves are quite similar to each other. DISCUSSION The equations derived from the data were based on a latency model. In other words, it was hypothesized that the time interval between stimulus onsets could be determined from the difference between some inverse function of the test-flash luminance and the blanking- flash luminance, as shown in Equation (2). A sche- matic of this model is shown in Fig. 7. If one imagines an elec- trode implanted in the brain to measure evoked potential latencies at the point where visual masking occurs, TF refers to the time the test flash was initiated, and TTF to the latent period after which an evoked potential (TFe) to the test flash appeared. D refers to the time after the test flash (TF) was initiated that the blanking flash (BF) was initiated, and Tgp is the latent period after which an evoked potential (BFe) to the blanking flash IT B • TTT F T , . • 1 . — T1UF IM UM 1 iQrrnMDQ-- — .-- ( FIG. 7. Hypothesized relationship between test- flash evoked potential latency (TTp), blanking-flash evoked potential latency (TBj.), interval between test-flash and blanking-flash onsets (D), and the dif- ference in latency [C&TJj] between TTF and TBF under visual masking conditions. TF and BF refer to physical light stimuli, while TFe and BFe refer to physiological potentials evoked by TF and BF stimuli. 58

appeared. (AT)i refers to the time differential between test-flash and blanking-flash evoked potentials under visual masking con- ditions. (AT)^ is assumed to vary directly with the percentage of correct responses. The entire relation can easily be stated in mathematical form. = TTF-TBF+(AT)i f(Z). (2) D is the interval between test- and blanking-flash onsets, Lpp is the test-flash luminance, Igp is the blanking-flash luminance, and Z is percentage correct (corrected for chance) converted to standard score form. The equations shown in Fig. 6 follow the latency form of Equation (2). The constant on the right would be some inverse function of blanking-flash luminance (Boyle, 1963) if that parameter had been varied. In Fig. 6 there is probably a significant difference between the coarse-grating curve and the other two curves. This would indicate that the neural interaction between test- and blanking- flash evoked potentials is dependent on the form of the test stimu- lus. This could be expected because slightly different retinal elements are activated by the different test patterns, even though the over-all test pattern areas are equated. Nevertheless, as hypothesized, it should be noticed that all three curves are very similar to each other. This is also verified by the equations in Fig. 6, in which the differences between the various constants of the equations are relatively minor, and all of the equations have the same general form. Another point of interest is that when the data are plotted in the form of Fig. 6, an approximate idea can be obtained as to how long it takes the human eye to perceive a given object over a wide range of object luminances. Transposing the term of Equation (1) to obtain TTF = D + TBF - (AT)i' (3) and assuming some low value of percentage correct, one or below, (AT)^ can be assumed to be zero or close to zero. Therefore, if a value is known for the evoked potential latency of the blanking flash (TBF), then the evoked potential latency of the test flash (TTF) could, in effect, be determined. From work with humans, Ciganek (1961) has determined that 59

a value of 28.6 ms would be the minimum latency of a visual evoked potential for a very bright stimulus which filled the entire eye. For the small foveal blanking flash used in this experiment, although very bright, this is too small a figure and, therefore, quite conservative. Accordingly, it would take longer to perceive a visual stimulus than the figures would indicate. There is also an unproved assumption about the relationship between evoked potentials and the phenomenon of visual masking when any re- corded value of evoked potential latency is used. Referring to the 2 per cent correct level in Fig. 4, and keeping the above limitations in mind, adding 28.6 to 70 would give 98.6 ms as the time required to see a foveal stimulus just 1 per cent of the time, very near to the absolute threshold of the test stimulus. Even with the same test stimulus 2.6 log units above this threshold, it would still take 38.6 ms to perceive the stimulus. These la- tencies could have important ramifications for the operation of very high velocity spacecraft. As an example, at a velocity of 30 ft per 1 ms, which is in the order of the velocity required to escape the earth's gravitational field, a dim object which would require 98.6 ms to be perceived would appear to be 3,000 ft away when, in actuality, it would be in the same position as the ob- server. Even with the much brighter object which would require 38.6 ms in order to be perceived, the object would appear to be 1,200 ft away. Such visual latencies suggest that, in cases of the extreme velocities associated with space flight, the traditional concepts of pilot observation of the external environment will have to be modified. REFERENCES Boyle, R. C. An investigation of the latency hypothesis of perceptual interference resulting from successive visual presentations. Un- published doctoral dissertation, Univer. of Calif., Los Angeles, 1963. Boynton, R. M., & Kandel, G. On responses in the human visual system as a function of adaptation level. J. opt. Sec. Amer., 1957, 47, 275-286. Cheatham, P. G. Visual perceptual latency as a function of stimulus brightness and contour shape. J. exp. Psychol., 1952, 43, 369-380. Ciganek, L. Die elektroencephalographische lichtreizantwort der mensch- lichen hirnrinde. Bratislava: Verlag der Slowakischen Akad. der Wissenschaften, 1961. Kietzman, M. L. The perceptual interference of successively presented visual stimuli. Unpublished doctoral dissertation, Univer. of Calif., Los Angeles, 1962. 60

Lindsley, D. B. Electrophysiology of the visual system and its relation to perceptual phenomena. In M. A. B. Bazier (Ed.), Brain and be- havior, Vol. 1, Washington, D.C.: Amer. Inst. biol. Sci., 1961, pp. 359-392. Pie"ron, H. Recherches experimentales sur la marge de variation du demps de latence de la sensation lumineuse (par une methode de masquage). Annee psychol., 1925, 32, 5-24. 61

SEXTANT SIGHTING PERFORMANCE IN THE AMES MIDCOURSE NAVIGATION AND GUIDANCE SIMULATOR Robert J. Handle and Bedford A. Lampkin NASA Ames Research Center Moffett Field The Ames Research Center is studying the role of the crew in the navigation, guidance, and control for the midcourse phase of manned space missions. In pursuit of these investigations, a lunar midcourse navigation and guidance simulator has been constructed at Ames. This device presents, for one thing, a controlled visual task to the human operator in a relatively realistic manner. So that the reader may form some idea of its capabilities as a re- search device for visual problems, the major features of the simulator and the conduct of an exploratory study of sextant- sighting performance in the simulated task environment are discussed. The visual scene is a 25° portion of the sky, including a simu- lated moon which is translatable in accordance with long-period vehicle-moon relative motions for a typical trajectory. Fig. 1 shows the moving cab mounted on an air bearing. The cab is driven by an on-board cold gas system. The air-bearing support is a portion of a 105-inch sphere that allows rotational motion up to ±10° of arc in pitch and roll, and ±90° of arc in yaw. The on-board cold gas control system has been successfully used to stabilize the cab both manually and automatically during opti- cal sighting tasks. It has also provided limit-cycle operation. Fig. 2 depicts the celestial visual scene that has been stimu- lated in the midcourse simulator. The direct optical planetarium approach to visual-scene generation has been utilized. The visual scene, located about 40 ft from the viewing point, is composed of 64 stars that are contained in the 25° segment of the sky surround- ing the moon. This segment results from the choice of a specific 62

FIG. 1. Moving cab on air bearing. KIFFA BOREALIS FIG. 2. Simulated celestial scene. trajectory which is taken as being typical of several possible lunar trajectories. The simulated stars are simply 0.005-in. diameter holes in the end of a tube lighted by a grain-of-wheat lamp. They have a subtended angle of about 2 arc (sec) and maintain their relative positions within ±5 sec of arc for as long as 8 (hr). Unfortunately, the direct optical planetarium is subject to optical parallax due to the finite distance between the light source and the viewer. To minimize the errors in angular measurements due to parallax, four of the star images were collimated by means of 6-in. 63

parabolic mirrors with small light sources at the focus to represent the stars. Fig. 3 is a picture of the collimating device. The images produced in the parabolic mirrors, when viewed from the simu- lator cab, appear to be at infinity and have the same direction when viewed anywhere within the viewing limits of the parabolic mirrors. To extend the viewing area these collimated stars are being fitted with 12-inch parabolic mirrors. Use of the larger mirror will of course extend the magnitude of rotational oscilla- tions which may be employed in research on the effects of motion on sighting accuracy. (This very brief description of the simula- tion facility is taken from an unpublished paper by Donald Smith of Ames Research Center.) FIG. 3. Collimating device. The research using this facility concerns mainly the identifi- cation and verification of performance capabilities of the human operator as an active participant in the positive fixing and guid- ance of the vehicle during translunar or midcourse flight. This use of the simulator rests upon the conviction that man can be a useful adjunct to, or replacement for, fully automatic primary systems. Thus, his capabilities in this task environment must be fully explored and delineated (Christensen, 1963). One of the important tasks that will concern the astronaut is that of obtain- ing navigational information. For the moment manual sextant sighting performance is being investigated as a possible minimum manual system for gathering navigational data. Later, this task will be integrated with the larger task of inputting the data to the on-board computer and, ultimately, with the broader task 64

context of vehicle alignment, sighting, computation, and velocity correction. The immediate larger goal is to simulate up to an 8 hr segment of the midcourse phase of the translunar trajectory. Sextant sighting, which has been the initial concern, is now discussed briefly as the first task element. Many manual naviga- tional schemes have been proposed in the literature for finding one's way about in the solar system (Haviland & House, 1963; Havill, 1963; Lillestrand & Carroll, 1963; Moskowitz & Wein- schel, 1963; U.S. Navy Department, 1962; White & Wingrove, 1962). These are generally of two kinds: (a) extensions of con- ventional celestial navigation techniques for the explicit, point- by-point determination of present and future position; and (b) implicit guidance through the determination of the elements of the orbit and their departure from or agreement with a reference orbit which has been predetermined to intersect the coordinates of the desired target point in space. Both of these methods involve considerable mathematical com- putation, but each, in most cases, depends first on the seemingly simple task of measuring an angle with an optical sighting device. In marine and air navigation, the accuracy of the celestial fix depends largely on the measurement of angular altitude of the body of interest above the natural sea horizon or the bubble hori- zon. In navigation in space, several kinds of angles may be of interest: 1. the angle between the line of sight of a lunar, terrestrial, or planetary landmark and a star; 2. the angular extension of any of the planets, moon, or sun; 3. the angle between a star and the limb of a planet, moon or sun; and, 4. the angle between the earth or moon and the vehicle- centered horizon. Since the marine sextant may be rotated through any angle to measure the angle between any two points of interest, it is the instrument that has been used. The bubble sextant is disqualified, of course, because of its dependence on a gravitational field. Specifically, the Navy Mark n Mod 0 hand-held sextant has been used, mainly because of its availability. It has a three- power scope and a 10° of arc field of view. A modern sextant with interchangeable telescopes will be used in future studies. It probably appears an audacious bit of romance to bring for- ward this time-honored device for evaluation against the preci- sion requirements of space navigation. However, as an entrance point to the study of the fundamental sighting task its employment 65

is inescapable and appears thus far to be quite fruitful. This is particularly true where it is desired to estimate relative accura- cies under various sighting conditions. The inherent sextant errors bias only the determination of the true angle. The vari- ance of the operator's sighted angles about their mean is the criterion measure for a given sighting session, and the change in this score with the conditions of the study is the experimental variable of interest. Fig. 4 is a photograph of the marine sextant now being used. Its design, as for all sextants, is based on the optical principle that the angle between the first and last directions of a ray of light that has undergone two reflections in the same plane is twice the angle that the two reflecting surfaces make with each other. Since the index arm is mechanically linked to the index mirror, the position of the arm on the limb indicates the angle between the two mirrors. The limb thus must be calibrated so that 0.5° of arc reads 1° of arc. Originally, the limb was a sixth of a circle, hence the name sextant; however, on modern sextants it is usually more than a sixth. There are several sources of error associated with this sextant. Some are due to mirror and telescope misalignment, others are due to eccentricity in the index arm, errors of graduation, and lack of parallelism be- tween the mirror and shade glasses (Hill, Utegaard, & Riordan, 1961). Obviously, these are not highly accurate instruments in terms of error tolerances for space navigation. Position fixing on the open sea does not have stringent accuracy requirements nor does FIG. 4. Marine sextant. 66

air navigation necessarily, where critical corridors and termi- nal points are approached using other, more precise techniques. In fact, the German Hydrographic Office considers any sextant used for marine navigation purposes to be "free from errors for all practical use if the error goes up to twenty seconds" of arc. However, the theoreticians in space navigation and guidance em- ploy an accuracy model of, at most, 10 sec of arc in sighting per- formance in their analyses of navigation requirements (McLean, Schmidt, & McGee, 1962; Smith, 1964; Smith & Harper, 1964). The literature is not at all replete with controlled studies of sextant sighting accuracy. One study, in which a modified modern sextant was used, measured performance in sighting on actual celestial objects in the night sky (Yachter & Goetz, 1962). Two stars, a star and a planet, and a star and a moon crater were the targets. The sextant was fitted with a 6 x 30 telescope and mounted on a modified telescope mount. The micrometer vernier permitted angular interpolation to 3 sec of arc. For the measurements of angle between two stars, and a planet and a star, the standard deviation was of the order of 10 sec of arc, and for the star-crater pair it was of the order of 26 sec of arc. The Ames project used a hand-held sextant and a sextant gimbal mounted to the cab. The three-power telescope had a 10° field of view. The vernier was readable to 6 sec of arc with a dubious interpolation to 3 sec of arc. The interest was not, how- ever, in the ascertainment of absolute sighting performance; but rather the determination of whether there was a difference in performance between the hand-he Id and the gimbaled sextant, and also whether oscillatory motion affected performance to any great extent. Accordingly, an arbitrary limit-cycle function was programmed on an analog computer to drive the cab using the cold gas jet sys- tem. The limit cycle was restricted to the yaw axis, spurious motions in the other axes being damped to relatively small ampli- tudes. Three levels of rate were used: 1/2° of arc/sec, 1° of arc/sec, and 1-1/2° of arc/sec. A static condition was also in- cluded. The oscillations were contained within a ±2° band about the line bisecting the stars of interest. Two of the collimated stars, oriented in a near vertical position, were used as targets. The task was to measure the angle between these stars by super- positioning one star over the other in the sextant field of view. The subjects were three Air Force navigator instructors at nearby Mather Air Force Base, an advanced Air Force navigation 67

school. Also included were four professionals engaged in related studies at Ames Research Center. Since these investigations are still in process, it would be pre- mature to present data at this time: however, some informal statements regarding apparent trends are in order. Training is an important variable, at least for the task en- vironment being provided. It was found that, using two sighting sessions per day, a week or more was necessary to bring the subjects down to asymptote. Twenty-four sightings were taken in each session. This was true for the navigators as well as for the professionals. For the navigators, transfer of training cannot be evaluated because of their lack of recent intensive use of their bubble sextants and the fundamental differences between previous tasks and the present one. One of the major differences is due to the use with modern bubble sextants of manual or automatic in- tegrating devices that allow for the averaging of a continuous sighting over a minute or two of time. The marine sextant is a single-shot device and is so used in sea navigation. It was notice- able that the subjects regressed considerably in their learning after the intervention of a weekend. However, the losses were quickly recovered after some additional sessions. It is manda- tory that the extent of retraining required after varying periods of nonpractice be determined, if a strictly manual sighting scheme is to be seriously considered as an alternate mode for space navigation. The cab motion does not appear to have a systematic effect on performance at the relatively large rates used. However, the effects of less discernible rates in both the limit cycle and the long-period relative motions of the vehicle and bodies in the solar system have yet to be estimated. Indications are that the gimbaled sextant has a slight edge, in terms of accuracy, over the hand-held. The subjects preferred the gimbaled sextant, particularly after having had some experi- ence with both types. However, this is not sufficient to qualify the gimbaled sextant and disqualify the hand-held sextant, the difference amounting to some 3 sec of arc in standard deviation. This is not very much when it is known that the standard devia- tions for all subjects ranged from 5 sec of arc to 46 sec of arc with a mean of 22 sec of arc. The three-power telescope may not be the best for most accu- rate performance. The modern sextant with a 6 x 30 telescope has been tried, and, although there was considerable random motion in the two star targets due to hand tremor, performance 68

was better. Since this determination was based on only a few subjects, the contribution to performance of various telescope magnifications using the newer sextant will be further investi- gated using a larger number of subjects. In the process of defining the details of the task, it becomes apparent that the visual processes involved require separate treatment for their investigation. Accordingly, plans are being made to assess matters such as the relation of visual acuity to performance; the effects of fatigue; the effects of reticle geome- try such as cross hairs versus concentric circles versus gun- sight type displays; the variance of performance with varying geometries of celestial targets; the effects of varying contrasts in the field of view; and the effects of training, lack of practice, and retraining on all of these. These are general statements because they result from an initial judgment of what is important in sighting performance as identified in the simulated task en- vironment. The opportunity to make these judgments is provided by the midcourse navigation and guidance simulator. REFERENCES Christensen, J. M. Psychological aspects of extended manned space flight. Wright-Patterson AFB: Behavioral Sci. Lab., tech. doc. Rep., 1963, No. AMRL-TDR 63-81. Haviland, R. P., & Hause, C. M. Celestial navigation in space, advances in the astronautical sciences. Amer. Astronautical Soc. Proc. of 9th Ann. Mtg. (Los Angeles), 1963, Vol. 13. Havill, D. C. An emergency midcourse navigation procedure for a space vehicle returning from the moon. Moffett Field: NASA tech. Note, 1963, No. D-1765. Hill, J. C., Utegaard, T. F., & Riordan, G. Duttons navigation and piloting. (4th Prtng). Annapolis: US Naval Inst., 1961. Lillestrand, R. L., & Carroll, J. E. Horizon based satellite navigation systems. In IEEE transactions on aerospace and navigational elec- tronics, Vol. ANE-10, No. 3, New York, 1963. McLean, J. D., Schmidt, S. F., & McGee, L. A. Optimal filtering and linear prediction applied to a midcourse navigation system for the circumlunar mission. Moffett Field: NASA tech. Note, 1962, No. D-1208. Moskowitz, S., & Weinschel, P. Instrumentation for space navigation. In IEEE transactions on aerospace and navigational electronics, Vol. ANE-10, No. 3, New York, 1963. Smith, G. L. Secondary errors and off-design conditions in optimal estimation of space vehicle trajectories. Moffett Field: NASA tech. Note, 1964, No. D-2129. 69

Smith, G. L., & Harper, E. V. Midcourse guidance using radar tracking and on-board observation data. Moffett Field: NASA tech. Note, 1964, No. D-2238. United States Navy Department, Space navigation handbook. Washington, D.C.: U.S. Government Printing Office, 1962, Navpers No. 92988. White, J. S., & Wingrove, R. A survey of guidance and navigation prob- lems for the manned lunar mission. J. Inst. Navigation, 1962, 9,2. Yachter, M., & Goetz, R. A. Astronomical angular measurements via an astro sextant. Amer. Bosch Arma Corp.: Arma Div. Rep., 1962, No. DR-62-E652-8. 70

COMMENTS ON MAJOR GORDON COOPER'S OBSERVATIONS FROM ORBIT John H. Taylor Scripps Institution of Oceanography University of California The Mercury series of manned earth-orbital space flights ended with the mission referred to as MA-9, piloted by Major L. Gordon Cooper. During certain of the 22 orbits, Major Cooper reported having seen objects on the surface of the earth that must neces- sarily have subtended very small visual angles from the capsule altitude. There was an immediate and vociferous reaction on the part of the scientific and lay communities, and during the months following MA-9 there was a considerable bulk of commentary pub- lished in the press, notably in Aviation Week. Opinions ran the gamut from flat denial of the possibility of Major Cooper's sight- ings being genuine to acceptance of his reports based upon one or another "explanatory" principle. These included such things as hallucination, the magnification due to the atmosphere, and a postulated improvement in visual acuity due to weightlessness. Most of these hypothetical effects can be dismissed or shown to be insignificant, e.g., the magnification due to the whole atmo- sphere would have the effect of raising the object about 8 feet— not much help in 100 miles. In September of 1963, the Visibility Laboratory was asked by Dr. Robert B. Voas, then of the Manned Spaceflight Center at Houston, to investigate the situation in terms of visual and atmo- spheric optical considerations in the hope of settling the contro- versy. Analysis by the Laboratory of Major Cooper's reports was contained in a letter from Dr. S. Q. Duntley to Dr. Voas, and eventually formed the basis for a National Aeronautics and Space Administration press release. Further, it led to plans for a con- trolled experiment which will assess the ability of the Gemini astronauts to discriminate ground features. 71

This ex post facto visibility problem was approached in much the same manner that one usually goes about making visibility calculations. Ideally, the needed inputs regarding the character- istics of the target and its illumination, the transmission proper- ties of the path of sight, and the visual performance capabilities of the observer should be quantitatively known. In the present case, however, it was necessary to reconstruct the situation on the basis of partial information gathered after the event and upon some assumptions which are thought to be reasonable. The first step was to get as much information as possible about the objects which Major Cooper reported, the manner in which they were illuminated, and the backgrounds against which they were seen. For this purpose, Mrs. Jacqueline I. Gordon and the writer made a trip to Houston where transcripts of the taped in-flight verbal reports and the detailed orbital information re- garding the areas of the sightings were secured. The post-flight Pilot's Report was read, and Major Cooper was questioned at some length about his experiences. It should be emphasized at the outset that Major Cooper is a remarkably careful observer; he is meticulous in differentiating fact from inference. Not only does he have excellent visual acuity, as measured clinically, but he has had a tremendous amount of experience in the reconnais- sance of angularly small, distant objects. From his Wisconsin boyhood hunting days through his Air Force test pilot work in high-altitude jet aircraft, he emerges as a genuine specialist in the types of observations that he later reported from orbit. To give a single example, Major Cooper asserted that he often ob- served, while flying over the Salton Sea in clear weather at 40,000 ft, the wakes of motorboats, the boats themselves, occa- sionally a smaller wake near the boat, presumably caused by a water skier, and, once in a while, a small light dot at the front of the wake which could have been the skier and his lifejacket. It is entirely possible to detect a target this small (about 10 sec of arc) given sufficient contrast and good visual acuity, of course, and herein lies, in the opinion of the author, the crux of the matter. Before, however, one can evaluate Major Cooper's orbital sight- ings, it is necessary to arrive at reasonable estimates of the apparent size and contrast of the reported objects, and to relate these to existing visual-performance data. The appropriate data for use in this case are those reported for the visibility of fine lines, e.g., Hecht and Mintz, angularly small targets, e.g., Black- well, and vernier acuity, e.g., Berry. It is clearly inappropriate 72

to appeal to ordinary clinical wall-chart acuity, with its tradi- tional 20/20 rubric, as it has become entrenched in the popular mind. Since the exact optical properties of the atmosphere during the observations cannot be known, it was decided to use data from the Visibility Laboratory's instrumented aircraft obtained under the clearest weather conditions. It was assumed that the atmo- spheric clarity at the time of Major Cooper's sightings was as good as or better than that measured during the instrumented- aircraft flights. This is believed to be a reasonable assumption for both the areas of interest. The first area near El Centre, California, is characterized by extremely clear dry air much of the time, and information secured from the local weather station confirmed that this condition obtained during the period of interest. The second area, the high Tibetan plateau, is probably overlaid by a very clear air mass, owing largely to the fact that its eleva- tion is about 13,000 to 16,000 ft. Photographs taken from the capsule tend to corroborate this, although data from ground weather stations there were unavailable. Four specific reported sightings were examined. They com- bined measured atmospheric transmission data, known visual- performance capabilities, and both measured and assumed prop- erties of the objects purportedly seen. The estimated inherent characteristics of the reported objects were arrived at by some rather interesting sleuthing. For each of the four cases investi- gated, pertinent excerpts are given from Major Cooper's state- ments (derived from the on-board tapes, the Pilot's report, and the interview, followed by a brief description of the findings and the conclusions to which they lead. Case I Between El Centro and El Paso Major Cooper could "easily see lots of roads, both paved and unpaved." He saw two unpaved roads running east-west, one on either side of the U.S.-Mexican border. On the northern road he observed a cloud of light dust, "lighter than the terrain" under conditions of "no wind" so that the dust cloud hung over the road. He stated that the cloud seemed to be caused "by a vehicle traveling from west to east," and that he could discern "a lighter dot" at the eastern end of the dust cloud. The U.S. Border Patrol confirmed the presence of the roads paralleling the border. The Patrol uses a specially designed ve- hicle called the International Scout, somewhat similar to the Jeep, 73

but covered by a flat white top to reflect the desert sun. Dimen- sions of this vehicle, which were furnished by the Patrol, have been used in the calculations. The terrain background reflectance, from information supplied by the San Diego Museum of Natural History, was estimated to be similar to that measured in other desert locations during field expeditions where the soil and vege- tation closely resembled those in the border area. Assumed values of reflectance of terrain, road, vehicle, and dust cloud, together with physical dimensions of the vehicle and an 8 ft wide road, have been combined with an assumed contrast transmittance of the path of sight of 0.77 in order to derive the probabilities of seeing, as shown in the last column of Table 1. Parenthetically, it might be noted that, had the vehicle been stationary, it would very likely not have been seen; the dust cloud at the time of ob- servation not only added to the positive signal contrast but may have obscured the vehicle's shadow which, without dust, would have tended partially to cancel out the lighter vehicle at the angu- lar size noted. One may conclude that this sighting was entirely credible. Case II During a pass over the high Tibetan plateau (ground elevation 16,000 ft) Major Cooper reported seeing, on an east-west road, a dust cloud blown by a "wind out of the south" which he inferred to be a "stiff breeze" from the angle it appeared to assume rela- tive to the ground. At the confluence of the dust cloud and the road, he reported seeing "a light spot" which he interpreted to be a vehicle. An attempt was made to discover the most likely characteris- tics of both the road and the terrain; and what is believed to be reasonable data from the Laboratory files was taken for use in the calculations. The probability of seeing the road, as indicated in Table 1, is in excess of 0.99. If one guesses that the vehicle might have been a 2.5-ton truck with a light top, the probability of its detection is 0.50. Case in Near some of the Tibetan roads, Major Cooper reported seeing small villages and, occasionally, "squarish houses." "I noted... the wind direction on the ground due to smoke coming out of smokestacks and out of the fireplaces (sic) of houses." Tibetan dwellings in the area of interest are found (in National Geographic Magazine photographs) often to be rather large, 74

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multifamily houses with white (whitewashed ?) sides and dark roofs. The lighting which prevailed at the time of the sightings should have caused the sides of these dwellings to be brightly lit, and to form, in consequence, a high positive contrast with the terrain background. Using terrain reflectance values which seem reasonable for the region, it is calculated that a brightly-lit build- ing side having a projected area of 138 sq ft in the direction of Major Cooper's line of regard would be seen with a probability of 0.50. This probability increased markedly with size, so that, for example, a vertical wall having twice the area, i.e., 276 sq ft, would generate an optical signal of detection probability greater than 0.90. Some detective work led to the discovery that the smoke issuing from these structures may have produced a posi- tive contrast sufficient to be seen with a probability of 0.50, for the usual local fuel in these timberless regions is yak dung, which yields a dense, light smoke. (It has been suggested that this fact may have led to the traditional expression used by Tibetan housewives: "Oh my baking yak!") In any case, it is believed the sightings are reasonable. Case IV In what is believed to be part of western China, Major Cooper reported seeing a railroad track running in a northeast-southwest direction. The track was seen as darker than the terrain, and at one point on it he reported "an extended target, lighter than the track" with a plume of white (smoke or steam) at its northeastern end. This he interpreted to be a train proceeding in a northeast- erly direction. He stated that he believed the wind direction to have been southerly, owing to the angle formed between the white plume and the track. Under the assumed conditions it is believed that the roadbed should have been visible with a probability of 0.90. The white plume of steam or smoke under the same conditions would al- most certainly have been detected (probability greater than 0.99). There were other fascinating sightings reported by Major Cooper, such as a boat and its wake on a river in Tibet, nightside observations of cities and villages, lightning, and his remarks concerning the apparent color of terrestrial features from orbital altitude. Only in the few cases outlined above, however, did mak- ing the assumptions required for visibility calculations seem at all justified. At this point, a few comments can be ventured which may help in understanding why many people greeted Major Cooper's reported 76

sightings with skepticism, or felt obliged to ascribe his observa- tions to one or another extrinsic causes. The term "visual acuity" refers to a variety of discriminations of which an observer is capable. In all cases, it relates to the detection of a spatial dif- ference or discontinuity, and the subject is tested to find the smallest such difference he can detect. This value, generally expressed in terms of the subtended angle of the spatial element or its reciprocal, is taken as a measure of the visual acuity. A wide variety of test objects has been used in the investigation of this function, and the numerical results are widely disparate and depend on the nature of the visual task involved. Simplest of such tests, which are referred to as tests of the "minimum visible," involve the detection of presence of an object, such as a point or a line. Somewhat more complicated are those tests in which the objects contain some spatial discontinuity within themselves, such as a pair of small targets or a broken ring, in which the "twoness" of the points or the location of the gap must be discriminated. These tests are referred to as measures of the "minimum sepa- rable." Still other tests involve higher-order discriminations, such as form recognition, of which the ordinary clinical wall chart of Snellen requiring the recognition of letters, is typical. They are called measures of the "minimum cognizable." It is evident that the last-named measures of acuity are most often used in medical practice, and that the numerical values resulting from such tests are most familiar to the majority of the population. Since the Snellen charts are based upon the no- tion that 1 min of arc is required for the perception of form (based upon a statement of Hook, quoted by Robert Smith in 1738), it is firmly implanted in the popular mind that 1 min of arc angle represents the value of best acuity. After all, is it not often said that 20/20 scored on the Snellen test (from the line on which the letter stroke width subtends 1 min of arc) means per- fect vision"? Major Cooper's Snellen acuity happens to be 20712, or 0.60 min of arc, although, as is indicated below, this value is merely suggestive of his superior vision and does not represent a limiting value of visual resolution. Measures of acuity other than the conventional clinical wall charts yield quite different values, and, generally speaking, the simpler the test the more "acute" vision becomes. Only two studies are cited, although there are dozens in the experimental literature. These two have been chosen because the test objects are more closely analogous to the real objects sighted during the MA-9 Hight. 77

The first step is to summarize the data of Hecht and Mintz, who determined the minimum angular diameter required for a long wire to be seen against a uniformly luminous background. The subtended angle of the wire, which was seen as a dark sil- houette (contrast - -1.0), was found to decrease with increasing field luminance, reaching its limiting asymptote at 0.007 arc min. These data were taken from a single observer (Hecht), aged 45 years, and it is probable that Major Cooper, similarly tested, would better this result by a palpable factor. While the terrain backgrounds against which roads, rivers, and railroad tracks were seen were probably not as uniformly bright as those used in the experiments, still these data are most closely appli- cable to the visibility of such earth features. One variety of visual acuity comes from tests in which the observer is required to detect the presence of a discontinuity in an extended line. This measure, called "vernier acuity" from its resemblence to the visual task required in the reading of vernier instrument scales, is analogous to the situation in which an extended line is suddenly displaced by some small angular amount. An hypothetical example might be the case where a truck and its shadow combine to produce a pair of such apparent dis- placements. Experiments have shown vernier acuity values in the range of 1 sec of arc, or about 0.017 arc min. Both of the studies referred to concerned targets of essen- tially -1.0 contrast, the lower limit for targets darker than their backgrounds. Targets which are darker than their terrain back- grounds may approach this value, but, owing to contrast losses suffered because of the presence of the atmosphere, will always be of lesser contrast and concomitantly reduced discriminability. The quantitative features of this situation may be calculated in order to arrive at visibility estimates. When targets are brighter than their effective backgrounds, however, no upper limit on con- trast is imposed, and it is common to see angularly tiny objects (such as stars, distant lights, sun glints, and the like) provided only that sufficient light from these objects reaches the eye. The light-colored vehicles reported by Major Cooper may be a case in point. A final point should be made in regard to the use of laboratory data in predicting the performance of an observer in a real-life situation. By and large, the numerical results of these experi- ments are estimates based upon large numbers of observations, and almost always refer to that value of angle that is necessary for discrimination to be successful one-half of the time. There 78

are statistical considerations that make this a desired value which need not be gone into here. It must be emphasized, how- ever, that the numbers so derived represent only a single point on a continuum—that there are larger visual angles which will result in greater certainty of seeing, and smaller ones which yield lower probabilities of seeing. That is to say, smaller tar- gets than those indicated will occasionally be seen, albeit less frequently. This fact, together with the likelihood that Major Cooper is a superior observer, and with the unquestionable fact that he is highly experienced in high-altitude observation, make it very probable that estimates based upon laboratory data may be conservative, indeed. In sum, it is concluded that the terrestrial objects reported by Major Cooper from the Faith 7 capsule could, in fact, have been seen under the conditions that have been assumed to have prevailed during the MA-9 mission. It is not necessary to invoke any exotic environmental or psychological factors in order to account for these sightings. Finally, reconstructing the event merely indicates the possibility of the sightings, and in no wise proves them to have been made. An opportunity to perform con- trolled experiments during future space flights is, therefore, anticipated with great enthusiasm. The first of these is described by Dr. Duntley elsewhere in these Proceedings. 79

GEMINI IN-FLIGHT VISUAL-ACUITY EXPERIMENT S. Q. Duntley Scripps Institute of Oceanography University of California As a result of astronaut Gordon Cooper's reports of sighting small objects on the ground from Mercury Flight MA-9, there is considerable operational and scientific interest in an experi- ment which will test the existing methods of predicting the visual capabilities of observers in space. It is hoped to determine under carefully documented conditions the effects of prolonged weight- lessness, 5 PSI oxygen breathing, and other environmental con- ditions peculiar to space flight on the astronaut's visual-per- formance capabilities as a function of time. It is intended to obtain information by measurements prior to flight on the visual capabilities of the astronauts who will be involved in the seven-day or longer missions. Also, they will be trained in the tasks which they will have to perform in flight. The astronauts' performance in two visual tasks in flight will then be measured in flight as the mission progresses. The astronauts involved in missions GT-5 and GT-6, and/or GT-7 will be required to measure their own visual acuity during the mission with the aid of an in-flight vision tester, which will be provided by the Visibility Laboratory of the University of California, San Diego. This task will involve the use of the tester by each man once a day throughout the flight. He will report the result of his test to the ground each day. In addition, during or- bits which pass within range of a prepared ground target-area, the astronaut in the right-hand seat will be ask to determine the orientation of each of (approximately) 12 rectangular targets which will be arranged in a line from west to east approximately 10 nautical miles long. The astronaut in the left-hand seat will orient the spacecraft so that the right-hand astronaut will have .the optimum view of the target area. The astronauts will be 80

familiar with the location of the target site and its general con- figuration, and a suitable method will be provided for locating the target area. All targets will be above detection threshold but will bracket the astronauts' ability to determine their orientation. The observing astronaut will call out the orientation of the targets, and his answers will be conveyed to the ground by radio. Depend- ing on the results of the experiment, the size of the ground tar- gets may be changed between days of the mission to insure that the proper range of target size and contrast is presented. The optical condition of the window being used by the observer will be monitored continuously throughout the observing period (approximately 2 minutes) to determine the amount of earthlight being scattered by the window. This is necessary in order to ob- tain Quantitative information on the astronauts' performance, as the apparent contrast of targets will depend on the manner in which the contrast is degraded by passage through the window. The necessary information will be obtained by an in-flight photom- eter which will be mounted on the 16-mm camera bracket in the right-hand corner of the right-hand window. This photometer will be aligned with a small, circular light trap which will be mounted outside the window on the hatch immediately in front of the pho- tometer, about 9 inches in front of the window. The output from this photometer will be telemetered through the high-level dumped telemetry system. In order to determine if the scattering from the window is uniform and, if not, what the degree of nonuniform- ity is, the spacecraft will be rolled over so that the right-hand observer is looking into black sky but sunlight is obliquely illumi- nating the window. The astronaut will then remove the photometer from the 16-mm camera bracket and scan the window manually, using the black sky as his light trap. The output from the tele- photometer will again be telemetered during this operation, and the telemetered information will be time-correlated with the voice record which he makes during this task. A meter on the rear of the telephotometer will permit the astronaut to make his own determinations of the scattering from the window, or mea- sure the luminance of any other target of opportunity that may interest him. The experiment will be performed only on a seven-day or longer mission as the purpose of the experiment is to determine the "longitudinal" effects of spacecraft environment. It will be necessary as a very minimum to perform the ground observa- tion portion of the experiment near the start of the mission and near the end. It is expected that this observation will actually be made on each pass within range of the target area. A Visibility 81

Laboratory instrumented trailer-van will be at the target site suring the mission to document the light and atmospheric condi- tions at the targets. An Air Force C130, instrumented by the Visibility Laboratory, will fly over the target area at the time of the orbits used for sighting to document the pertinent optical properties of the atmosphere as a function of altitude. All of this information will be used to determine the nature of the optical signal available to the astronaut, and the Laboratory will then correlate this with his visual performance. A National Aeronautics and Space Administration van will be outfitted by the Visibility Laboratory and set up in Houston to measure the visual capabilities of the astronauts and to train them in the use of the in-flight vision tester. This will require 8 to 12 two-hour sessions for each of the astronauts who may be assigned to the mission. As this training may occur six months prior to flight, a brief refresher training will be given to the astronauts within two to four weeks prior to flight. In addition to the training at Houston, the astronauts will be flown in a C130 over the target area to familiarize them with its appearance and with the location of permanent landmarks. A scale model of the target will be laid out in this area and will be viewed by the astronauts through open hatches or ports from an altitude of 20,000 feet or less. The in-flight vision tester will be completely self-contained and require no interfaces with the spacecraft other than stowage. It will be used by both astronauts once each day. The astronaut in the right seat will use it on the orbit prior to his first orbit over the target for that day and report his results to the ground on passage over a suitable communication site in the United States. The device will be binocular, with an adjustable interpupillary distance which will be pre-determined for the astronaut and will be held by means of a biteboard inserted in the astronaut's mouth. Each astronaut will have a biteboard prepared for him, which will then properly position the vision tester. The astronaut will rotate a knob on the tester to a series of detented stops which will align targets in the field of view of the instrument. The astronaut will make a binary-type decision, i.e., yes-no, vertical- horizontal, or a-b, and will note his answer on a small card by punching the knob used to rotate the drum, thereby causing a pin to puncture a card if he determines "a." If he determines "b," he will not puncture the card but rotate the drum to the next position. The card, which is removable, will contain the results of the vision test, these results to be read by the astronaut either into his tape recorder or directly by radio link to the ground. 82

FLASH BLINDNESS John L. Brown, Chairman

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Vision Research: Flying and Space Travel; Proceedings of Spring Meeting, 1964. Edited by Milton a. Whitcomb and William Benson Get This Book
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Vision Research: Flying and Space Travel is a record of the proceedings of the Committee on Vision meeting in 1964. The papers presented at the meeting concerned visual problems related to low altitude, high-speed flight, space travel, and incapacitating effects on pilots resulting from inadvertent viewing of a nuclear detonation.

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