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8 Sensors for Small Spacecraft BACKGROUND AND STATUS In the past, the NASA budget for science missions was allocated primarily for medium and large spacecraft (Delta class or larger). Since volume or weight was not usually a constraint, the instruments that were developed to support the scientific missions were also quite large. While the capability of NASA's instruments have advanced over the years, little investment was made in instrument miniaturization. Prior to BMDO's Brilliant Eyes and Brilliant Pebbles programs and ARPA's (formerly DARPA) Advanced Spacecraft Technology Program in the mict 1980s, the same philosophy of using large spacecraft also existed in DoD, and resources were focused on improving instrument capability rather than on miniaturization. The BMDO ant! ARPA program requirements for small spacecraft resulted in a large investment (hundreds of millions of dollars) on the development of lightweight, low-power instruments ant! components. Although impressive advancements were made in miniaturization, many of the instruments were tailored toward specific military requirements and are not directly applicable to NASA scientific missions. While NASA can build on BMDO and ARPA technology advances, NASA must invest in smaller scientific instruments that are compatible with small scientific spacecraft. NASA PROGRAMS NASA has a long history of successful space science missions carried out with large and small spacecraft. It continues to have ambitious plans for the future, with emphasis on doing more with small, less expensive spacecraft. For this approach to yield the maximum return of scientific data, the mission sensors must be capable of providing more return for less weight and less electric power consumption. NASA's current major emphasis is on Mission to Planet Earth, a multidecade examination of the Earth as a coupled, interacting system. As currently planned, Mission to Planet Earth will be carried out with both intermediate and small spacecraft. The long-term nature of the program provides the opportunity to employ small spacecraft to a greater degree later in the program if a vigorous technology program supporting small spacecraft and sensors 66

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Sensors for Small Spacecraft is conducted. Because of the high-weight and large-power requirements of many of the instruments, and the need to acquire data simultaneously with several different sensors, the current state of instrument technology necessitates the use of intermediate-sized spacecraft for the majority of the mission requirements. In support of the Mission to Planet Earth program, a wicle variety of sensors, both active and passive, has been, and is being, developed for use on numerous instruments. The sensors range in weight from as low as 19 kilograms to as high as i,300 kilograms (for a planned synthetic aperture radar). In electric power requirements, they range from iS watts to 2.2 kilowatts (for a planned Laser Atmospheric Wind Sounder). A listing of the Mission to Planet Earth instruments with their weight and power requirements is given in Appendix F. Although Appendix F covers only Mission to Planet Earth, the technology discussed is representative of currently available NASA instrument technology. Several of the instruments were scheduled for flight on the joint ARPA/NASA Collaboration on Advanced Multispectral Earth Observation (CAMEO) spacecraft, which did not receive funding for fiscal year 1994. This spacecraft was intended to provide a demonstration of multispectral remote sensing to meet DoD's tactical wide-area surveillance needs while also supporting civilian climate research and environmental monitoring. It had the technical objective of demonstrating a multispectral imaging system compatible with small spacecraft (Nicastri, 19931. Although much of NASA's sensor development has been for larger instruments, a significant amount of work in sensor miniaturization is being concluctec! at the Center for Space Microelectronics Technology at JPL. The Center for Space Microelectronics Technology was founded by NASA and several DoD agencies in 1987. It concentrates on innovative high-risk, high-payoff concepts and devices for future space missions (IPL, 19931. For example, the Mars Pathfinder rover (Rocky) utilizes a microseismometer developed by the Center for Space Microelectronics Technology (Space Microelectronics, 19931. DoD PROGRAMS . 67 DoD and associated agencies have been long-standing users of remote sensing Information and, as a result, have made significant investments in sensor technology. Although in some instances the technology developed is directly applicable to civilian applications (e.g., the preparation of accurate maps anti the imagery of snow and ice cover), in other instances the uses are tied to the particular tactical and strategic needs of defense forces. In the past several years, BMDO and other defense agencies have invested sizable funds in the development of advanced sensors anti instruments for acquiring and tracking ballistic missiles and for remote sensing. Several technologies under development by ARPA include a multispectral sensor, which detects a broad range of frequencies simultaneously, and superconducting materials. In support of the Brilliant Eyes program and the now-cancelled Brilliant Pebbles program, a sensors program was initiated by BMDO to develop advanced sensor technologies under a large number of separate . .

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68 Technology for Small Spacecraft projects. When these technologies are combined, they will produce a highly capable tracking and measurement system. The projects include work on passive and active sensors, on laser radar and interactive discrimination, on signal processing, and on radar and optical discrimination. A long-term development program in indium antimonide for midwave infrared sensors and arsenic-doped silicon for long-wave infrared sensors for use in passive infrared cameras has been funded by BMDO and may offer alternatives to the sensor developers in both the defense and civil sectors in the future. Many of the technologies developed under the BMDO sensor programs have been combined to develop lightweight and low-power star trackers; infrared cameras; ultraviolet and visible wavelength cameras; and laser radar and interactive discrimination capabilities. These instruments have been tested under a variety of programs, such as the Midcourse Space Program Space Experiment and the Infrared Background Signature Survey (Katz, 1993~. Advances in miniature devices such as the tunneling transducer, electron tunneling accelerometer, magnetometers, and infrared sensors have been made by BMDO-funded research at the Center for Space Microelectronics Technology (17Ie Update, 1993~. Many of these devices have the potential to significantly reduce the weight of small spacecraft. In the near future, many of the instruments clevelopec! by BMDO will be flown on several spacecraft. The Deep Space Program Science Experiment, usually known as the Clementine program, launcher! in January 1994, is testing instruments during its orbit of the moon and subsequent rendezvous with an asteroid. Another activity is the MST} program, where the objective is to test BMDO-developed miniature sensor technology. As noted above, JPL is developing a number of the Mission to Planet Earth sensors in- house and has a long record in both the planetary and Earth sciences. The MST} program, however, was carried out independently of JPE's Earth sciences activities, and the focus was on the fastest possible, lowest-feasible-cost launch of available experimental hardware, with emphasis on the management techniques necessary to make this possible. Several sensors currently are flying aboard DOE's Array of Low-Energy X-Ray Imaging Sensors (ALEXIS), including six x-ray telescopes and a device to measure the effect of the Earth's ionosphere on radio signals. FINDINGS AND PRIORITIZED RECOMMENDATIONS For essentially all of the NASA instruments described above, technology currently exists to conduct research ant! operational Earth observation measurements to a precision that satisfies the concerned science and applications communities. Although there will always be a desire for better capabilities, the current environment of limited budgets requires that a drive toward less expensive missions is necessary. Thus, in the near term, the most desirable advances are those that would permit high-quality measurements to be made with smaller, lighter, less-power-demanding systems. These would enable more frequent missions, perhaps with higher risk, and the financial ability to provide for a backup launch in the event of a failure of the first. The NASA technology program for sensors and instruments should be directed toward that objective.

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Sensors for Small Spacecraft Technology advances in guidance and control and in orbit location accuracy through use of GPS and low-thrust, high-response thrusters could enable the use of multiple small spacecraft flown in clusters to take sensor measurements where simultaneity is required. This technique could allow more scientific missions to be conducted with small spacecraft. In order to enhance sensor technology for small spacecraft, the pane! makes the following recommendations: I. The feasibility of achieving the required simultaneity of measurements of different instruments using a cluster of small spacecraft should be evaluated, and, if feasible, technology should be developed. The employment of GPS ant! very low-thrust and high-response attitude-control thrusters might enable this technique. 2. A research and development program should be directed toward the development of miniaturized, power-efficient, high-performance instruments in the following areas: multifrequency radar altimeter and scatterometer systems; advanced coherent licIar systems; multispectral Earth observation systems operating in the ultraviolet, visible, and infrared wavelengths, employing lightweight optics and advanced detector-array technology; advanced, passive, larger-aperture, high-sensitivity, low-weight, microwave radiometry employing lightweight deployable antennas, room-temperature superconducting sensors, and advanced on-board processors; and lightweight, deployable-mirror optical systems with deformable mirrors correctable to the diffraction limit, for ultraviolet, infrared, and visible long baseline interferometry using several small spacecraft, ultimately resulting in an extremely large- aperture phased array for astronomical observations. 3. A continuous research and development program should be conducted to improve the performance and reduce the weight and power required for infrared detector arrays; cryogenic detector coolers; and deployable antennas for radiometry and radar. 69