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7 EOS Platforms Depending on the outcome of the question of simultaneity, are the EOS platoons, as currently configured, the optimal means for collecting data, or are Here better allernanves ~ are more cost effective or timely? These alcoves could include, for example, smatter mul- fiple platforms flying in formation or additional near~erm precursor missions that are capable of flying subsets or preliminary versions of EOS instruments. As discussed in Chapter 6, scientific arguments for simultaneous mea- surements have been developed by NASA for two specific research areas: the role of clouds in climate and the fluxes of the trace gases. With regard to these two cases, we conclude that the number of instruments that must fly together requires at least one large satellite. Dividing the proposed instruments for these measurements among several smaller satellites and flying them in close formation is technically feasible, but the smallest co- herent set of instruments for one of the smaller satellites is still sufficiently large to require a launch vehicle larger than the Delta rocket. The scientific requirements for continuity in data sets has led the community of researchers and NASA to plan for a long time-series of measurements. EOS plans call for a 15-year record of observations using series of identical satellites, each with a 5-year lifetime, for each set of measurements. Measurements to carry out the USGCRP emphasis on the role of clouds and the fluxes of trace gases, for example, are planned for a series of large spacecraft called the EOS-A series. It seems likely that 61
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62 scientific understanding and technical capabilities will change during the course of the EOS program. Accordingly, although continuity of specific data sets will be an important consideration, it may also be desirable to alter the instruments or the platforms, or both, at some time in the future. NASA has incorporated the design feature of common interfaces into the EOS program. This feature should make changing instruments easier than on many past missions. There is, however, no current plan for incorporating new understanding, concepts, or technology into EOS as they evolve during the life of the EOS program. Scientific arguments for simultaneity in terms of the research objectives of the second proposed, large EOS-B satellite have not been developed, and it appears that these objectives could be achieved with a number of smaller, independent satellites. NASA s current assessment of comparative costs as presented to us, suggests that flying the projected EOS-B instruments on a large platform is the least expensive option, although the differences in cost among some alternative configurations appear to be relatively small. In principle, the science investigations proposed for EOS-B could be done by a suite of smaller satellites. Since a number of the instruments do not require extensive development, these could perhaps be launched sooner. Significant opportunities exist for gathering key global change data though a number of U.S. and foreign research and operational satellite missions, including the proposed Earth Probes series, prior to the scheduled first launch of EOS in 1998. Some of the EOS instruments are intended to continue monitoring certain environmental parameters so that the precursor missions that fly similar instruments will be prerequisites, not substitutes. Interim missions, including WARS, TOPEX, and the currently proposed missions in the Earth Probes series are intended to gather data that are essential for the USGCRP. It is our view that if budget constraints arise, it would be preferable to delay the the launch of EOS rather than to forego or diminish the effectiveness of these near-term projects. (See the section below on Precursors, Small Missions, Earth Probes, and Operational Systems.) ENGINEERING CONSIDERATIONS Four alternative configurations of satellites to carry EOS instruments have been analyzed to date by NASA: the baseline mission comprising two large satellites, EOS-A and EOS-B, on identical platforms; a mix of one large satellite and three satellites of intermediate size; six intermediate satellites; and 12 small satellites. Each satellite would be designed to last five years and would be replicated twice, for a net mission of 15 years. The large satellites would be flown on the Titan-IV launch vehicle, the intermediate ones would be sized to fly on the Atlas-IIAS, and the small
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63 payloads could be launched on Delta-II rockets. The NASA analyses do not account for the potential that some sets of satellites would have to fly in close formation to achieve the desired degree of simultaneity. Launch Vehicles Space missions are constrained by the availabilities and capabilities of the launch vehicles available for the purpose. NASA, in its analysis, considered a large number of factors, including cost, mass, power, data rate, launch vehicle, the ability of the launch option to satisfy the mission requirements, the number of spacecraft required, launch schedule, produc- tion schedule, operational complexity, data processing requirements, ability to fit the instruments on existing spacecraft, and direct data downlink and broadcast requirements. The NASA analysis is constrained in two ways. First, there is currently no planned Atlas launch capability for the Western Test Range. As a consequence, the costs of a launch pad and the ground support crews would have to be provided, adding to the overall cost of any program that planned to use such a capability. It is not clear whether the launch rate would justify maintaining the crews at the Western Just Range or transferring them from the Eastern Test Range on demand. Second, the Delta option could not accommodate some of the larger instruments, such as HIRIS, ITIR, MLS, GLRS, and HIMSS, without changes in the instruments or the spacecraft. Based on the NASA analysis, the all-Delta scenario seems to us to be technically unrealistic. Thus from consideration of the capabilities of the respective launch vehicles, there are several discrete "levels" of capability rather that a continuum. If, as discussed above, the scientific investigations or the measurement capabilities to support them require the simultaneous flight of sets of instruments, the designers are led at this time toward the large spacecraft as the "optimum" configuration. Nonetheless, we believe it would be prudent at this time to continue to consider a mixed launch vehicle scenario so that the scientific return of instruments currently designated for EOS-B can be increased or achieved sooner. The option should not be eliminated solely on the basis of consideration of the launch vehicles. The Platform Systems There is no inherent risk in a structure of the size of the EOS-A satellite. The major development risk lies in the subsystems and complex interactions inherent in integrating and flying many instruments together. Systems interactions in cooling, viewing angles, data management, me- chanical and electronic noise, and other factors have complicated multiple
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64 instrument space missions in the past. These are engineering challenges that can usually be identified and solved. Nonetheless, there is always a risk that some interaction will not be anticipated and mitigated in advance. This is not an issue that we could deal with in our deliberations, and it is in any case never conclusively solved at this stage in the development of a space mission. The data system will require tape recorders that are a significant extrapolation from current technology. While we accept the assertion that the extrapolation is straightforward, past experience suggests that extra tape drives should be supplied where they will be critical to the operation of the mission. The experience in the NOAA TIROS series has been that half of the tape drives fail on orbit before the end of the mission and usually early in the mission. Providing extra tape decks, which is NASA:s current plan, will require extra space on the spacecraft, but it can be supplied with the larger platform. Similar arguments can be made for most of the subsystems. In general, the larger platform has greater capability to provide backup or redundancy. Extra weight or volume capacity on the part of the launch system further allows for the use of cheaper or less exotic materials and simpler designs, and makes possible arrangements for passive cooling of several instruments and unobstructed views of Earth by several instruments in relatively simple ways. The large platform also allows NASA to consider a direct downlink for data as another service for users and as a backup for telecommunication via the TDRSS, which is the baseline configuration. The user community has derived much benefit from the use of low-rate, real-time direct broadcast from the current meteorological satellites. The capability would be advan- tageous for EOS instruments, and it is currently planned for the EOS-A and EOS-B satellites. CONIINUI1Y AND RELIABILITY The USGCRP gives high priority to the establishment of an integrated, comprehensive, long-term program of documenting the Earth system on a global scale. The EOS will be a key component of that overall program, which will include surface-based measurements as well. It is essential that EOS be both comprehensive in its coverage of parameters in space and time and that it provide for continuity of calibrated data for the most critical of them. Data continuity is particularly important when the required information is used both to establish a baseline about global change and to monitor change as it occurs. If the critical instrument is not in place when major events occur, such as an E1 Nino or a major volcanic eruption, then
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65 opportunities to document and learn about the Earth system will have been lost. Furthermore, identifying trends in highly varying data is always compromised when a portion of the time series is missing. If data on clouds or the radiative balance were missing for either extreme of the solar cycle, for example, understanding of the related phenomena would be severely degraded. Loss of continuity also severely compromises the warning capability of the observing system. Consequently, the reliability of EOS data is important to the USGCRP. Considering the potential for long-term drift in instruments, the issue of calibration warrants particular attention. In our view, NASA has established reasonable criteria for the needed reliability for the scientific mission. For example, the large spacecraft plat- form is being designed to have a 75 percent probability of full capabilities for its expected operating life of 5 years, as well as a 75 percent probability of having 80 percent of its design power capability at the end of 7.5 years. NASA's current requirement for the instruments is a probability of at least 85 percent that they will be working satisfactorily at the end of 5 years, while for certain measurements of the facility instruments the requirement is at least 90 percent. NASA's preliminary analyses indicate that these probabilities are within reach and can be met. Continuity failures are more difficult to quantify. Three types of failures can occur, threatening the continuity of measurement: first, the launch vehicle can fail. NASA:s plan for this contingency is to provide a spare platform and set of instruments that would be available for launch as soon as possible after the failure. In the interim, the lifetime of a platform already operating in orbit could be extended until such time as the replacement is launched. The longer the lifetime of an operational platform is extended, the greater will be the likelihood that the continuity of some data streams will be lost. If a failure occurs during the launch of the first platform in either of its proposed series, NASA projects that the delay for the beginning of that series would be 2.5 years. An interruption in the time series of this magnitude would seriously disrupt USGCRP research objectives. The second type of failure is of the platform itself, once it is in orbit. Platform systems, such as the attitude control system, data management system, power supply, and telecommunications system, are essential to the operation of all instruments. The least reliable element of the platform infrastructure is widely regarded to be the tape recorders, six of which are currently planned to assure that at least two are always operational. The spare platform and set of instruments is intended to provide backup in the event of a platform failure, but it is clear that continuity in some of the data records would be lost. An interval would be necessary to determine the cause of the failure, take corrective actions as required, and launch a
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66 replacement platform, even if the platform, instruments, and launch vehicle were available at the time of failure. The third type of failure is of an instrument. Most of the instru- ments have a technical heritage in devices that have already been flown. Nonetheless, several proposed instruments require significant advances in technology. NASA currently estimates that development of the most com- plex instruments on EOS-A will take between 42 and 60 months, with some technological risk entailed. One way to help assure the continuity of critical measurements in the event of an instrument failure is to place redundant sensors on the platform. The approach would, however, inevitably result in a reduction of the size and scientific scope of the complement of sensors on the platform. Another approach, applicable to either partial or catastrophic failures, would be to provide so-called "hot spares" for critical instruments, spare instruments, and launch vehicles ready to go with minimum delay. This approach also has disadvantages: it is costly, and program managers are likely to want to determine the cause of failure and take corrective actions before launching a spare, increasing the interval of discontinuity of the data. NASA estimates the cost of providing for a typical hot spare to be in the $250M to $350M range. Further, depending on the instrument and the acceptable degree of simultaneity with other measurements, close formation flying may be required. If the lost instrument were to be in a set requiring congruent measurements, the hot spare could not restore the relationship. A third approach, which might also apply in the event that an in- strument is not ready at the scheduled time for launching, would be to use existing instruments on the platform to provide backup. In a working document entitled EOS Instnument Standard Data Products, NASA is as- sembling backup strategies for each data product in the event of loss of an instrument or channel in an instrument. The products being examined include such parameters as biomass characteristics, broken ice distribu- tion, carbon monoxide profiles, temperature profiles, ice sheet height, and many others. Backup products would be degraded from the originals but would be constructed from channels of other instruments to give crude but workable data for gap-filling purposes. This approach and other aspects of contingency planning are still under development by NASA Finally, given the extensive remote sensing capabilities and plans of other nations, full coordination of EOS with the foreign programs could contribute to maintaining critical redundancies. NASA should place high priority on such coordination. Any approach to providing for the contingency of instrument failure will entail balancing costs, technical capabilities, and other considerations with the scientific goals of the program. The question of the reliability
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67 and continuity of data is central to the mission, so that careful contingency planning Is unportant. We conclude that an important first action is to identify the mission-critical sensors for which contingency plans have highest priority. NASA considers the MODIS-N and -T as the "mission critical" instruments for the first platform series, so much so that the agency is prepared to delay the mission until these instruments are ready. We agree with this assignment of priority, particularly since the MODIS instruments are part of a suite that will provide significant benefits from simultaneous measurement and which serve the highest priority scientific objectives. A careful analysis needs to be done to determine whether there are others. A related question is the need for companion instruments with any such mission-critical instruments. After the mission-critical sensors have been identified, the next step is to identify ways in which an EOS instrument failure could be covered, and with what degradation to the science, with other instruments that are already flying. For example, the following types of questions need answers for each standard data product. ~ what extent can a data gap in MODIS be met with data from AVHRR, Landsat, and SPOT? How well can a gap in observations by AIRS be met with data from HIRS and AMSU? How well will ALT coverage be continued with ERS-2, U.S. Navy altimeters, and others? What will be the alternative SAR coverage with the Canadian, European, and Japanese missions? In summary, NASA's current science strategy does not fully address the issue of continuity of key data sets throughout the 15-year lifetime of the mission. We conclude that NASA needs a contingency plan for instrument failures. Changing scientific priorities may lead to different designations of "mission-critical" instruments. The plan could distinguish among instruments, but the scientific requirement for simultaneous, con- gruent measurements, which provide the rationale for a large platform for the highest priority science objectives, should be the guiding principle of the plan. The panel's suggestion that the instruments for EOS-B could be flown on a number of smaller satellites could also be used to help address continuity and backup issues. The data continuity issue is so important that it deserves continuing careful financial analysis and consideration. NASA s analysis indicates that the large platform approach gives the better overall reliability. Because this analysis is based necessarily at this time on design criteria rather than actual designs, we find the prelimi- nary analysis to be less than conclusive. Nevertheless, the NASA analysis adds another argument to those already mustered in support of the large spacecraft for the two highest research priorities.
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68 COSTS At this time, NASAs cost estimates are not sufficiently precise to support a conclusion about whether a configuration of one large platform with several smaller complementary spacecraft is more or less costly than the proposed two large platforms. The NASA cost estimate for an observing system with a comparable payload that consists only of small spacecraft, however, indicates that such a configuration would be roughly 50 percent more expensive than either of the other two options. The relative costs for four mission configurations as analyzed by NASA are given in Figure 4. The analyses are normalized to the baseline concept for two platform series. For each case, both an optimistic and a pessimistic estimate are given. The analyses are based on NASA cost models and the assumption that the same total set of instruments would be flown in each case. The assumption is probably not valid. The costs associated with contingency plans, which may differ among the configurations, are not included either. Aside from the totals, several features of these cost analyses are striking. First, the vehicle costs in each case are a small fraction of the total system costs. At some number of satellites in orbit, which happens to just about coincide with the number in a mixed fleet, an additional TDRSS is needed. This is driven by the need for committed access to a high data rate channel for several of the satellites. With the Atlas and mixed ELV scenarios, additional costs will have to be met to sustain a dedicated launch capability at the Western Test Range. Obviously, the costs for integration and testing increase with the num- ber of spacecraft. While the relationship is not linear, since the smaller spacecraft are simpler, there is real increase just from the numbers. This is especially true in the case of the Delta launch scenario where each space- craft may have to be individually designed for each instrument to fit within the shroud. The costs of operations will also grow as the number of satellites and the number of orbits are increased. Further, in the area of science, atmospheric corrections or co-location of images will require increasingly complex algorithms as simultaneity is lost and resulting costs increase. Because the precision of the NASA estimates is not high, there is little to choose in terms of cost between the baseline case, the mixed case, and the lesser assessment of the all-Altas configuration. Questions about the costs of establishing a polar orbit launch capability for Atlas-IIAS rockets and providing the requisite ground support personnel suggest that the differences in costs among the respective configurations may be larger than the analysis indicates. Nonetheless, since the differences between the
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69 1.8 1.6 1.4 8 1.2 ~1' 0.8 ~: 0.6 0.4 0.2 o . . ~ i] Launch vehicles and pad modification ---e; Communication Data, information system & operations ~ Management - - Be- Integration ~ testing ~ ~ Spacecraft with instruments . f : _ ~ :-:-:-:-:-:-:-:--: ·:-:-:-:- .; .:. .'eS,,'269~2 ~DJ // // Y" . //f At// ..................... .................... .................... :::::::::::::: .................... ::::::::::::: .................... .................... ...;............... :::-:::::::: :-: ::::::::::::: :-: .................... .................... ::::::::::::: .................... :::::::::::::: ................. ' .......... .......... . ........... T Optimistic Conservative Optimistic Conservative Optimism Conservative BASELINE MIXED ATLAS DELTA Note: Direct downlink communication aysteme not induded in the mixed, Atlas, or Delta options FIGURE 4 NASA estimates of relative costs of alternative mission configurations with identical instrument payloads. SOURCE: C. J. Scolese, NASA Goddard Space Flight Center, April, 1990. mixed scenario and the baseline are not large, we recommend that NASA continue studies now under way to optimize the scientific return from the instruments carried in NASA's strawman payload for EOS-B. PRECURSORS, SMALL MISSIONS, EARTH PROBES, AND OPERATIONAL SYSTEMS The first of the EOS platforms is scheduled to be launched in 1998. In the interim, significant opportunities exist for gathering key global change data in precursor missions in the Earth Probes series. Also, the agency plans to develop the EOSDIS as early as possible in order to make available to
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70 the research community other historical data sets and data streams from on- going operational satellites that can be used to experiment with prototypes of the data and information system in evaluating information management concepts. The scientific justification for several precursor missions, including Earth Probes, and the use of data from existing operational systems (e.g. Landsat and the meteorological satellites) was discussed earlier in Chapter 5. Because Earth Probes are planned as smaller satellites with shorter development times, they have considerable potential for providing high- priority precursor measurements to EOS. They also can advance the time in which some of the measurements critical to understanding global change could be made (see the SSB/CES report, Strategy for Earth Explorers u' Global Earth Sciences, 1988~. An important concern in the near term is the discontinuity of key measurements such as global stratospheric ozone levels, the Earth's radiation budget, ocean topography and winds; and the biological productivity of the oceans, made by satellite missions launched in the 1980s. Certain missions proposed for the Earth Probes line could provide opportunities for extending those measurements until acquisition of schemata sets is assumed by the EOS spacecraft. However, there is no Earth Probe mission proposed to fill the gap in Earth radiation budget measurements, as mentioned above. Certain other Earth Probes missions could also provide information on parameters not measured before on a global scale, e.g., the Earth's gravitational field and tropical rainfall. There are other opportunities for early flight and for providing conti- nuity of currently important measurements that do not seem to be as well exploited as they might be. These are flights of opportunity on already planned missions. Satellite series like the NOAA polar orbiters and the satellites of other nations can provide opportunities for modest instruments. The flight of the SBUV instrument on the NOAA polar orbiting series and the planned flight of the TOMS instrument on a Soviet meteorological spacecraft are instructive examples. Several important existing measurement capabilities are candidates for such "piggy-back" flight. They include flying another ERBE on the NOAA series, an active cavity radiometer on almost any satellite series, and an ocean color instrument. The contributions of these measurements to the objectives of the USGCRP are described in Chapter 5. In addition to possibilities for precursor-or interim flight of selected instruments, some measurements could be better made, or must be made from other orbits. For instance, biological processes and radiation studies related to cloud motion require sampling at various times of day, which cannot be made solely from sun synchronous polar-orbiting spacecraft such as EOS. ~ the extent that these measurements are critical to achieving
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71 the objectives of global change research, the Earth Probe line can pro- vide a flexible mechanism for such observations from more appropriate orbits. During the EOS era, Earth Probe missions will also be essential in complementing the EOS measurements by flying instruments that may be incompatible with the design and orbit of EOS platforms. It is expected that the Earth Probes will be augmented by foreign spacecraft, some of which could provide flight opportunities for U.S. instruments. For instance, discussions are currently under way to position the Japanese spacecraft contribution to EOS in a lower inclination orbit. A plan is needed to determine how to coordinate such possibilities with the USGCRP. Of the several instruments that can be better flown in a different orbit, we believe that the Synthetic Aperture Radar is of prime importance. If its technological development is successful it will supply two quantitive measurements unavailable on a global synoptic scale in any other manner. These are soil moisture and an estimate of biomass in a standing plant canopy. Both of these measurements are important for the two top priority scientific areas, i.e., the hydrologic cycle and fluxes of atmospheric gases. The SAR should be considered for inclusion in the EOS program as a free flier as soon as possible. Existing U.S. operational systems supply both interim data and a con- tinuing contribution to global change research. They include the NOAA polar orbiting and geostationary meteorological satellites, the Defense Me- teorological Satellite Program (DMSP), and the Landsat system. Many scientific studies proposed for EOS assume the continuation of these mea- surements. For instance, the top priority study area the role of clouds in the hydrologic cycle-assumes the continued measurement and mapping of clouds by the meteorological spacecraft. The second priority area, fluxes of atmospheric gases, assumes the continued global mapping capability of a Landsat system intermediate in capability between MODIS and HIRIS. NOAA and NASA are jointly planning for flight of atmospheric sounders and for a common interface for a number of the instruments, an approach that we endorse. Further, the European partners in EOS are planning to fly a polar orbiting platform with a morning crossing of the equator to cover the requirement for operational weather data, again with a common interface. This continuing use of operational data and integration of the hardware approaches will benefit scientific investigations of the future. These research and operational missions are complementary to, not replacements for, the main EOS missions. They will improve the scientific return of the space-based Earth observing program, and they can help to ensure continuity of key observations for the next decade or longer. Some, such as SeaWiFS, are important for the success of planned international field programs that will improve understanding of global change.
Representative terms from entire chapter: