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6 Simultaneity of Data Collection EOS is premised on the assumption that it is essential to collect global data or' various environmentalparameters simultaneously. Now important is data simultaneity to the ultimate utility of the data? Can the requirement of simultaneity be applied more narrowly than proposed? We conclude that arguments for collecting certain sets of data si- multaneously are strong. The arguments, which do not apply to all the measurements that NASA proposes to make with EOS, depend primarily on the importance of studying the interactions within natural processes occurring on short time scales and on the interdependence of certain pairs or small sets of measurements for precise quantitative interpretation. For measurements critical to two high priority areas of research in the USGCRP, the needs for several small sets of instruments to make simulta- neous measurements lead to a set of interwoven requirements for a suite of ten instruments that should be flown on the same satellite. (See the section below on Specific Analyses for Particular Scientific Objectives.) The instruments proposed for EOS and certain other space missions that are planned to be flown in the interim or contemporaneously with EOS are consistent with the scientific objectives for observing the Earth system developed in a series of reports from the NRC and elsewhere (Earth System Science: A Program for Global Change;, Earth Science Mom Space; Strategy for Earth Explorers in GlobalEarth Science; Mission to Planet Earth). The strategy is best summarized in Mission to Planet Earth, a volume in the 50
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51 Spatial Coincidence , - 1 Same Area Same View Path T C E a M P N Close to R ~Same Time A D (Few Min.) L E _ N Different E (> 10 Min.) Same Area Different View Path Same Time (Few Sec.) Congruent Simultaneous Close Sequential Sequential FIGURE 1 Nomenclature for "Simultaneity." Space Studies Board series, Space Science in the Ih~enty-First Century, which concluded that "the measurements must be global and synoptic, they must be carried out over a long teen, and different processes ...must be measured simultaneously (to a degree dependent on the rates involved)." DEFINITIONS OF SIMULTANEITY "Simultaneity" takes on several meanings when applied to EOS. In this report, we adopt the terminology of the EOS Investigators Working Group (see Figure 1~. In the current context, we take "simultaneous" to mean "within a few seconds." There is a more stringent requirement in several instances for "at the same time through the same atmospheric path," i.e., essentially "bore-sighted." There are also some special requirements for "at nearly the same time but through different view paths, i.e., different look angles." Given the above, observations taken within a few seconds of the same area on the surface through the same view path are termed `'congruent"; those acquired at the same time of the same area but through different view paths are termed C`simultaneous"; those acquired within a few minutes of the same area are termed 'Cclose sequential." Observations of a common area obtained at intervals greater than about ten minutes are termed``sequential.'' The most stringent requirement would be for C`congruent" obse~va- tions, the least stringent for `'sequential." But there are some special situations that require specifically 'Cclose sequential" observations made through different view paths. Several specific remote sensing studies illustrate the differences in
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52 nomenclature. For example, the Large Area Crop Inventory Experiment (LACIE) and AgriSTARS (former interagency programs to establish ap- plications of remote sensing to agriculture and other plant canopies) were based primarily on sequential observations using the same instrument (the Multi-Spectral Scanner on Landsat). Recent studies of global deforesta- tion and desertification have been based primarily upon sequential mea- surements obtained with the NOAA Advanced Very High Resolution Ra- diometer (AVHRR) and the Landsat Thematic Mapper. In contrast, global temperature soundings for weather forecasting are made by truly congruent measurements by the High Resolution Infrared Radiation Sounder (HIRS) and the Advanced Microwave Sounding Unit (AMSU) on the NOAA po- lar orbiters. NASA s Ocean Topography Experiment (TOPEX) requires congruent dual frequency measurements of altitude and a separate mea- surement of water vapor, the latter for correcting the effects of water vapor on the former. GENERAL CONSIDERATONS Four lines of argument enter into consideration of the need for simul- taneous or congruent measurement of parameters. First, understanding natural processes that change on time intervals of seconds to minutes may require measurements of several different simultaneously interacting parameters. Second, the physics of the techniques for interpreting some measurements depend on environmental conditions that can be sensed with companion instruments. For example, data from surface imagers must be corrected for atmospheric moisture. Environmental conditions have differ- ent degrees of persistence; some, such as atmospheric moisture, can vary on time scales of seconds to minutes, so that companion measurements should be congruent. Where congruent measurements are required but not made, no amount of subsequent data processing can recover the full information. Third, merging sequential data streams can place additional technical and financial burdens on ground-based data management and computational systems that might not be present if the data were taken simultaneously. Fourth, engineering considerations-such as the availabil- ity of launch vehicles, onboard electrical power, viewing angles, and data transmission capabilities-affect decisions on spacecraft size, complexity, reliability, and cost. In some cases, there can be advantages to larger spacecraft systems that make possible simultaneous measurement with a number of instruments. At the same time, these considerations must be balanced against possible scientific disadvantages, such as requiring all the instruments to fly on the same orbit. Such a requirement may not be op- timal for some measurement objectives and may not provide the flexibility
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53 needed to protect continuity in the case of a sensor or system failure. The latter are discussed below in our response to question 3. To Enhance Process Studies It is a rare natural process that involves only one parameter or takes place on only one spatial scale. Thus there are strong arguments that, to understand natural processes, measurements of several parameters on several spatial scales must be taken. In studying many global change processes, congruent imaging surveys are required at variable spatial and spectral resolutions. For example, congruent acquisition of MODIS and HIRIS imagery would provide large and small images in the same field of view. As the successor to AVHRR, MODIS (which has a pixel size of 1 km) will provide gross spatial variations in surface-cover materials on a global scale. Before they can understand processes, scientists must be able to identify the nature and condition of surface materials at higher spatial and spectral resolution. Ideally, global patterns could be inferred by simply averaging measurements of this type. The acquisition of global imaging surveys at HIRIS resolution (pixel size, 30m), however, is not practical from an engineering standpoint. Consequently, it is logical to develop a two-sensor system, one for global- scale measurements (MODIS), and another for detailed regional-scale studies (HIRIS). 1b use the instruments for process studies, congruent measurements are necessary. The requirement for congruency of MODIS and HIRIS observations results from two lines of reasoning. First, the same surface materials must be observed by both sensors. Many surface materials such as crops, scrub brush, flowering plants and trees, and suspended sediments in surface waters can change character significantly over time periods that are short relative to the repeat cycles of a single spacecraft. Further, the spectral signatures of the sensed objects often vary with the angle of the sun, or the angle of observation of the instrument. Second, before a detailed analysis within the global view can be made, HIRIS measurements must usually be taken within the field of view of the MODIS sensor. In principle, this could be accomplished by placing the sensors on two different satellites that would follow one another closely in orbit while maintaining their relative positions (i.e., flying in formation or position keeping). However, the NASA analysis suggests that the simplest means of assuring spatial coreg~stration of HIRIS and MODIS data is to place the two instruments on a common spacecraft so that they will use the same view path for their observations.
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54 To Interpret Physical Measurements As a general rule, no single measurement of radiance will allow the calculation of a geophysical parameter. For example, to measure accurately the distance traveled (by the signal) from a radar altimeter, from which altitude is inferred, signals at two frequencies are needed to correct for the influence of the ionosphere, and an independent measurement of water vapor in the atmosphere is needed to correct for influences of the atmosphere. Another example is the derivation of parameters on the surface of the Earth from an optical imaging instrument, which generally requires several optical frequencies and independent corrections for water vapor and particulates in the atmosphere. Several other examples are described in the NASA documentation of EOS instruments and research projects. Because these measurements must largely be taken of the same atmospheric column, and because the atmosphere changes on the time scale of minutes, the measurements must be congruent. To Facilitate Data Management Merging sequential data streams places technical and financial burdens on ground-based data management and computational systems in addition to those that would be present if the data were taken simultaneously. For instance, co-location of pixels from the NOAA Geostationary Operational Environmental Satellite (GOES) with soundings from the NOAA polar orbiters has proven too difficult to automate. However, in the current GOES system there is a crude sounding capability done with the same in- strument. The soundings can be registered with the images and corrections or correlations can be made that add to the usefulness of the images. To Incorporate Engineering Considerations As a practical matter, planning space missions must take into account a number of engineering and systems considerations. For example, the platform that supports the research instruments must have technological and logistical capabilities compatible with the payload. Technical capabil- ities affect the extent to which the scientific objectives of missions can be achieved. A major technical constraint appears to be in launch capacity. Ac- cording to the current capabilities and plans of the U.S. launch industry as supplied to us by NASA, only two vehicles Delta-II and Titan-IV are expected to be available to launch into polar orbit from the Western Test Range facility in the late 1990s. The satellite design is therefore constrained in size, shape, and mass by the failings of the vehicles. If scien- tific arguments were sufficiently strong, an intermediate launch capability,
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55 Atlas-IIAS, might be established at that facility, but it seems likely that the EOS program would have to incur the full costs in the absence of other users. Launch capabilities continue to evolve, however; and to the extent that the EOS program evolves, it should incorporate ways to consider the opportunities that other launch systems afford. (See the section in Chapter 7 on Launch Vehicles.) Flying a number of optical instruments on the same platform may have significant systems and scientific advantages in terms of calibration. Calibration of space instruments has, for years, been recognized as an important concern. Drift and lack of intercalibration of instruments have been a barrier to comparison of data from different satellites and can prevent quantitative analysis of potentially long-tune series obtained by combining data from different missions. Historical measurements of strato- spheric ozone, sea surface temperature, and atmospheric temperature are three examples. One way to calibrate optical instruments is to compare instruments flown at the same time looking at the same dark or bright scene on the surface. Means to obtain an absolute calibration of one of the instruments are essential to this technique. For example, instruments might be intercalibrated with MODIS-T, which can be tilted to view dark space and the moon. Flying a number of instruments on the same platform may have scien- tific disadvantages related to the selected orbit. In principle, every instru- ment has a preferred orbit and few of these preferred orbits are identical. Altitudes, inclinations, sun synchrony' equator crossing times, and repeat times all affect the quality of the information derived from instruments. Thus flying several in the same orbit entails compromise for each. For example, the projected orbit for EOS detracts from the ability of ALT to measure mean ocean circulation and from the ability of all instruments to monitor diurnal variations and cycles. Some of these disadvantages may be overcome by relying on data from complementary missions, as might be done in missions under consideration for the Earth Probes series or on operational satellites. Engineering considerations are discussed in greater detail in Chapter 7. SPECIFIC ANALYSES FOR PARTICULAR SCIENTIFIC OBJECTIVES The most thorough assessment of requirements for synergism among measurements that we have seen is contained in a draft letter report to NASA s Associate Administrator for Space Science and Applications from the Chair of the Payload Advisory Panel (PAP) of the EOS Investigator Working Group (B. Moore, private communication, April 2, 1990~. The assessment focuses on two of the high priority global change research issues: the role of clouds in climate and hydrological systems and the flux of trace
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56 gases among the biosphere, atmosphere, and oceans. In the analysis, which is summarized below, the PAP traces the needs for instruments from the geophysical measurements, through the combinations needed to make those measurements, to specific requirements for simultaneity or near simultaneity. Climate and Hydrological Systems: The Role of Clouds Understanding the role of clouds is crucial to improving general circu- lation models that simulate climate change. The PAP analysis notes that the liquid water content of clouds, their optical characteristics, and the extent of cloud cover all have important effects-sometimes conflicting-on the results of model simulations. Therefore, to improve the models, the PAP analysis concludes that it is important to obtain data on the optical, geomet- rical, and microphysical properties of clouds on a global basis. Parameters of interest include: optical thiclmess, emissivity, effective radius of cloud particles, atmospheric pressure at the tops of clouds (or their altitude), cloud top temperature, and the phase of cloud water (ice, snow, and/or liquid water). In most cases, the high temporal and spatial variabilities of many of these parameters require that their measurement be taken at about the same time because the average of the computed properties cannot be determined as a function of the averages of the measured parameters. The PAP notes, for example, that mid-level clouds move at 20 to 40 m/s so that if the CERES instrument, with 25 kilometer pixel, and the MODIS, with pixels less than 1 kilometer, must look at the same scene, they must take measurements within three minutes (i.e., close sequential) to assure 80 percent coherence. The HIRIS pixel is to be 30 meters on a side. Atmospheric temperature profiles are measured by AIRS and AMSU- A and -B; radiances at the tops of clouds are measured by CERES-IN; oceanic cloud water and water vapor are established by HIMSS; basic phys- ical properties of clouds, including heights, are obtained from MODIS-N; aerosol composition is determined from EOSP; and bulk radiative proper- ties of aerosols in the short-wave spectral region are provided by MISR. The PAP concludes that overall consistency among the cloud properties, humidity, temperature, and radiance measurements requires that the 8 instruments make essentially simultaneous measurements. Because of their larger pixel size, HIMSS and AIRS/AMSU-A and -B can be separated by as much as ten minutes. Thus to obtain these measurements, this set of instruments must view through the atmosphere either congruently, within a few minutes, or within ten minutes of each other. From the PAP analysis and our own discussions, we constructed a set of order-of-magnitude simultaneity requirements for
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57 an oh ~a: A: cat z ~ ' use ~ ~ to ~ c, O O I G ~UJ C: by # . B 1 o G E o C E D y N A 1 C S AIRS # 0 1 AMSU # 1 CERE~IN # HIMSS MEDIAN 1 1 1 ADDICT 1 1 HIRIS EOSP MISR ALT 10 1 10 1 1 1 # 1 1 # 1 1 O O 1 # O 1 - 1 1 10 # # # STIKSCAT 1 ~ 0 NOTE: Numbers are approximate allowable delays in minutes between measurements, where zero implies a congruent measurement. AIRS AMSU H IMSS C o CERES-IN U D S MODIS-N & MODIS-T HIRIS EOSP C M A T MISR E ALT STIKSCAT FIGURE 2 Simultaneity for observations calf the role of clouds in climate and of the biogeochemical dynamics of tmce gases based on the PAP analysis. the several instruments listed above for clouds and climate studies (see the upper right side of Figure 2). Biogeochemical Dynamics: Fluxes of Trace Species The major challenge in developing a measurement system for remote sensing of the biogeochemical dynamics of trace gases, such as carbon dioxide, is to observe biological activity both on the land and in the oceans. The ocean is a dark target compared with the land, so that ocean-observing instruments require higher sensitivity. The PAP points out that the differ- ence is particularly important in the near infrared, where precise MISR measurements must be made before the influence of aerosols on signals in the visible portion of the spectrum can be determined. Another com- plicating factor in making ocean color observations, indicative of biological activity in the surface layer, is the requirement that sunlight reflected from
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58 the water surface be avoided, which calls for a sensor that can tilt away from the vertical (i.e., the nadir). The spectral coverage of MODIS-N provides data on land vegetation on a regional scale; similar data on a highly localized scale are obtained from HIRIS; MODIS-T provides ocean "color"; aerosol properties are measured by MISR; sea state and winds are obtained from STIKSCAT; and ocean circulation is derived from ALT. Based on the need for spatial and temporal coverage, the PAP ar- gues that MODIS-N, MODIS-T, and MISR should fly together. Although MODIS-N and MODIS-T view along very different tracks, the PAP con- cludes that the measurements should be taken simultaneously so that the ozone absorbance of MODIS-T can be corrected by the MODIS-N mea- surement. Further, the detailed high-resolution data obtained by HIRIS, on a spatial scale where human influences can be observed, must be related to the low-resolution MODIS-N data if global inferences are to be drawn. According to the PAP, the two instruments should therefore view the same scene at the same time. The MODIS sensors form couples with AIRS and AMSU in surface emissivi~ studies and in deriving ozone concentrations to connect MODIS and HIRIS visible channels. The PAP also concludes that ALT and STIKSCAT should fly with MODIS-T, but that observations in close sequence may be sufficient in this case. Based on the PAP analysis (which is more extensive than the summary above) and our own discussions, we tabulated the requirements for simul- taneity to support the study of trace species fluxes (see the lower left side of Figure 2~. All instruments listed in the figure, except ALT, STIKSCAT, and EOSP, are currently part of the baseline payload for the EOS-A platform. Based on these two research objectives, the PAP concludes that the fol- lowing instruments should be flown on the same spacecraft: AIRS, AMSU, CERES-IN, EOSP, HIMSS, MISR, MODIS-N, MODIS-T, and HIRIS. This complement of research instruments, with the support instruments for com- munications and positioning (e.g., COMM and GGI), if selected for flight, would constitute a payload comparable to that envisaged for EOS-N We are not aware of any similar discussion of the currently proposed payload for the science objectives of the EOS-B satellite, although the posi- tioning instruments, and perhaps others, may require some coordination to correct for atmospheric water vapor. The scientific objectives of that series and the set of instruments planned for it are directed at the chemistry of the atmosphere, the dynamics of its upper reaches, atmosphere-ocean inter- actions, and solid earth processes. Documentation exists for simultaneous measurements for individual pairs or sets of a few instruments. But neither the scientific investigations nor the measurements appear to demand the same considerations of simultaneity as those for EOS-N In principle, the
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59 EOS-B investigations could be done by a suite of smaller satellites. Argu- ments presented by NASA for a large platform for the proposed EOS-B instruments are based on engineering and systems considerations rather than on scientific objectives. DOCUMENTATION While there are a considerable number of well documented studies that provide general scientific arguments for measuring a variety of environmen- tal parameters at the same time, there is little documentation linking the specific research objectives and the need for simultaneous measurements on EOS. No single peer-reviewed document or set of documents was avail- able to which we could turn to find arguments for or against simultaneity among individual space-based measurements. Specific scientific arguments for simultaneity ("congruent," "simultaneous," or "close sequential") are contained largely in working documents, often in the draft stage. These include a detailed analysis of facility-class instruments, a detailed analysis of PI instruments, a briefing on NASA:s `'Science Plan" for EOS, and two draft letter reports to NASA from the Payload Advisory Panel of the EOS Investigators Working Group. The analyses of the instruments contain a wealth of detailed arguments for simultaneous measurements related to individual scientific objectives. Individually, many are convincing, but they are not integrated in the analyses. The PAP's draft letter reports integrate some of the arguments. Our assessment of the quality of the available documentation is shown in Figure 3. Notwithstanding the lack of an overall analysis, we found adequate information in these working documents to construct a logical, scientific argument for congruent and simultaneous measurements for a certain set of parameters, and thus a certain set of instruments. Nonethe- less, because the simultaneity arguments are central to the scientific success of the mission, it is essential that they be carefully described and made available for peer review. We therefore recommend that NASA prepare formal documentation that collects the scientific, measurement physics, and other arguments for simultaneity into one or a small number of documents to provide the traceability from the scientific need to the system design and guidance for the development and operation of the program over its lifetime. The document should be updated periodically to maintain that traceability as the configurations and payloads evolve.
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60 Scientific (Processes) Available Strength of Argument Documentation Fragmentary in Good general arguments in working documents a series of consensus reports; specifics in the draft PAP letters. Fragmentary and Strong in key scientific areas in working documents and examples. Well developed Very strong, but relies on physics or scientific argument as a basis. None identified Potentially strong. Physics Systems Data Management FIGURE 3 Summary assessment of arguments for a large platform for research on the role of clouds in climate and on the fluxes of trace gases.
Representative terms from entire chapter: