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Committee on Earth Studies, Space Studies Board, National Research Council

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THE ROLE OF SMALL SATELLITES IN NASA and NOAA EARTH OBSERVATION PROGRAMS

NATIONAL ACADEMIES PRESS

Chapter One

At the request of the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA), the Committee on Earth Studies analyzed the capability of small satellites to satisfy core observational needs in Earth observing and environmental monitoring programs. The committee's study focused in particular on the use of small satellites to be inserted in the NASA Earth Observing System (EOS) program and the planned NOAA-Department of Defense (DOD) National Polar-orbiting Operational Environmental Satellite System (NPOESS) program.

The committee's study was begun in November 1995, during a period of much debate over the feasibility and merits of substituting smaller satellites for larger systems. Proponents of the small satellite approach believed that advances in miniaturization would allow development of much smaller sensors with performance sufficient for many Earth science and operational needs. These smaller sensors could be accommodated on capable, smaller spacecraft and launched with the new generation of smaller launch vehicles. Further, they argued, performing missions with smaller payloads, spacecraft, and launch vehicles would lead to lower costs, greater programmatic flexibility, more and faster missions, and accelerated infusion of new technologies. These features would help fill recognized gaps and provide new opportunities in the nation's Earth observation programs.

The committee approached the study by setting out to understand the observational needs for key NASA and NOAA Earth remote sensing programs, and to determine and assess the availability and capability of sensors, satellite buses, and launch vehicles suitable for small satellite missions. The committee examined opportunities presented by small satellite options with respect to mission architecture and assessed their implications for future NASA and NOAA missions.

SMALL SATELLITES VERSUS SMALL MISSIONS

The committee found that, in addressing the role of small satellite missions, it is important to distinguish between small satellites, small missions, and larger missions employing small satellites. In this study, the term "small satellites" refers to size-satellites in the 100 to 500 kg class capable of meeting NASA and NOAA Earth observation measurement requirements. The term "small mission" refers to cost-that is, a small mission is a comparatively low-cost mission. NASA's current Earth science strategy of performing a larger number of smaller missions (versus that planned in earlier conceptions of the EOS program) is predicated on the cost of each mission being relatively low. Although small satellites may help enable low-cost small missions, not all small satellite missions will be low cost. Low costs result as much from the relative simplicity of the mission (or the preexistence of mission elements) as from the size of the satellite.

The ability to achieve low costs when employing small satellites for larger missions is even more uncertain than when small satellites are employed for small missions. For example, performing a mission with a large constellation of small satellites to achieve a high sampling frequency may cost a great deal, even though the individual satellites may cost little. A more controversial example is to use small satellites as a substitute for larger satellites to accommodate a specified complement of sensors. In this trade-off, the cost of initially placing the sensors into orbit may be higher with multiple small satellites because it involves building and launching more satellites. The lowest cost architecture to maintain a functioning complement of sensors over a prescribed mission lifetime depends on the system availability requirements (i.e., the percentage of time the system must be able to deliver the specified data) and the design life and reliability of the mission elements (sensors, spacecraft bus, launch vehicles).

MEETING CORE OBSERVATIONAL NEEDS

NASA's and NOAA's core Earth observational needs span many disciplines, including oceanography, land processes, atmospheric sciences, meteorology, climate, and geodesy. While these aspects of Earth studies have shared remote-sensing spacecraft, the mission goals for the different disciplines often have different mission time horizons, different orbit requirements, and differing instrument sizes and require measurements of differing resolution, repeat cycle, and area coverage, for example. Although it is sometimes necessary (or at least very desirable) that some of these data be temporally and geographically coincident to some tolerance, accommodating these diverse mission goals with large, multisensor spacecraft generally involves compromises. The committee has sought to understand these requirements and compromises to help assess the capabilities and opportunities associated with small satellites.

A primary argument for a multisensor platform is a requirement for temporal or spatial simultaneity of data collection-for example, when studying the interaction between columnar water vapor and temperature, or when there is a desire to test for the presence of clouds in the field of view. However, the committee found the requirement for simultaneity difficult to prove. Generally, only a need to observe clouds or other rapidly changing conditions supported the argument for simultaneity. Rather, it is more important to ensure that a full suite of sensors is contemporaneously available to measure processes related to coupling of various components of the Earth system, such as air/sea fluxes, and that this suite is continued for a sufficient period of time. For operational systems, strict simultaneity is also not generally required. Because the sensors are not all co-boresighted and because some have inherently different sampling strategies, even operational satellite platforms that carry multiple sensors mostly provide contemporaneous rather than simultaneous observations. Even in those cases where simultaneity is required, there may be opportunities to use alternative architectures-for example, clusters of satellites flying in formation.

Although there are differences between the operational measurement requirements of missions such as NPOESS and the Earth science research requirements of missions developed by NASA's Earth Science Enterprise, there is clearly overlap as well. Moreover, many operational measurements are useful for research, especially for long-term climate studies. The separation of instrument variability from the often subtle long-term variations in climate-related processes requires careful calibration and validation of the sensor and its derived data products. As sensors are replaced over time, it is essential to maintain continuity of the data product despite changes in sensor performance ("dynamic continuity").

The requirements for research missions evolve rapidly with advances in science and technology. Long development times associated with large multisensor missions often run counter to this emphasis on flying the latest in sensor design. Research missions emphasize the quality of the individual observation and thus constantly push the technology envelope in an attempt to obtain better-quality data. By contrast, operational systems tend to evolve more slowly, in part in response to budgets that grow more slowly and in part in response to the well-defined operational nature of the missions. For example, the data processing infrastructure of the user community often involves numerical models that may be expressly designed to assimilate satellite measurements collected at specific times with specific observing characteristics.

CAPABILITY OF SMALL SATELLITES TO PERFORM EARTH OBSERVATION MISSIONS

A review of development trends points to continued efforts to increase capability, reduce size, and lower costs of small satellite buses. In particular, technology has advanced to the point where very capable buses are currently available for performing many Earth observation missions. However, some Earth observation payloads are too large, too heavy, too demanding of power, or generate too much vibration to be accommodated efficiently with small satellite missions. Future advances in payload technology should mitigate this situation, but there are fundamental laws of physics that in some cases restrict the degree of miniaturization that can be achieved while retaining sufficient performance to meet the observation requirements. Thus, the committee sees small satellites as a complement to larger satellites, not a replacement for them.

FLEXIBILITY AND NEW OPPORTUNITIES PROVIDED BY SMALL SATELLITES

Small satellites offer new opportunities to address the core observational requirements of both operational and research missions. Small satellites, in particular single-sensor platforms, provide great architectural and programmatic flexibility. They offer attractive features with respect to design (distribution of functions between sensor and bus); observing strategy (tailored orbits, clusters, constellations); faster "time to science" for new sensors; rapid technology infusion; replenishment of individual failed sensors; and robustness with regard to budget and schedule uncertainties. New approaches to observation and calibration may be possible using spacecraft agility in lieu of sensor mechanisms, for example. Small satellite clusters or constellations can provide new sampling strategies that may more accurately resolve temporal and spatial variability of Earth system processes. With advances in technology and scientific understanding, new missions can be developed and launched without waiting for accommodation on a multisensor platform that may require a longer development time.

Small satellite missions, as a new element of measurement strategy, may also help provide more balance between long-term operational or systematic observations and short-term experimental process measurements, as well as between focused missions and larger, more comprehensive missions. Programs can be more readily tailored to fiscal funding constraints when implemented as a series of smaller satellites (although this raises the risk of an incomplete data set unless the missions are planned and executed carefully).

AVAILABILITY OF RELIABLE LAUNCH VEHICLES

Achieving the full promise of small satellites will require the availability of reliable U.S. launch vehicles with a full range of performance capabilities. This is currently not the case. Present launch vehicle performance capabilities do not effectively span the range of potential payloads. For example, there is a significant gap in capability between the Pegasus-Athena-Taurus launch vehicles and the Delta II. Also, fairing volume (which determines the stowed payload size as well as the type and complexity of deployable systems such as antennas) is often limited and sometimes drives the size of the payload. More flexible launch systems are needed where volume constraints are less stringent. Further, early experience with the new small launch vehicles has included a number of failures, and the present paucity of reliable options is of great concern. This is likely due in part to the relative newness of these systems and a desire to minimize development costs for these commercial ventures. Continued development should overcome the difficulties and yield a suitable balance between cost and reliability.

It will take some time, and likely some additional failures, before any of these launch vehicles establish a reliability record approaching that of the Delta II. Plans by numerous suppliers to address these needs are encouraging.

COST OF SMALL SATELLITE MISSIONS

Small spacecraft do offer opportunities for low-cost missions, but very low costs are experienced only with simple spacecraft performing limited missions. Small spacecraft can be relatively expensive when they retain the complexity required to meet demanding science objectives (pointing accuracy, power, processor speed, redundancy, etc.). Commercial "production" satellite buses offer the potential for reducing costs. However, they generally have to be tailored-with attendant costs-to accommodate existing Earth observation payloads. Designing new payloads to match existing bus capabilities offers greater cost-effectiveness, but caution must be exercised not to compromise the scientific mission in so doing.

Several small missions-e.g., Clementine, QuikSCAT (Quick Scatterometer)-consisting of a single small satellite launched on one of the new class of small launch vehicles have been successfully performed at a relatively low cost. But the true cost of these missions is somewhat controversial in that they employed preexisting sensors and technology developed under separate funds. The true cost of a mission must also include the investment in technologies around which the activity is built. Leveraging advanced technology to lower mission costs is laudable, but understanding the true cost of the mission requires consideration of such prior investments, particularly when they are directly supportive of the mission (e.g., preexisting sensors).

SENSOR DEVELOPMENT

The factors driving mission development time and cost for Earth observation missions are typically associated with the development of sensors. "Standard" small satellite buses and launch vehicles are available to support faster missions, but development of new sensors will often control a program's schedule regardless of satellite size. Small satellites can provide a quicker path to operation and data collection if the required instruments-sized to the smaller spacecraft-are available, or if they are under development on a schedule that matches the development timetable of the spacecraft. Many of the early successes with smaller, faster missions depended on the availability of sensors developed elsewhere (e.g., Clementine, QuikSCAT). Whereas larger mission budgets and schedules have traditionally provided for their own sensor development needs, continued success with fast, cheap, small missions will require a reservoir of new sensor technology developed through alternative sources. If small missions are burdened with the development of their sensors, then the cost, the development time, and the time to science will increase accordingly.

MISSION ARCHITECTURE

The development of highly capable small satellites has given new flexibility to planners when designing mission architectures. Small satellites offer program managers flexibility that is useful for both operational and research missions. For example, operational missions might employ small satellites to ensure minimum gaps in critical data records, while research missions might use small satellites to ensure short time to science. Constellations or clusters of small satellites also afford new strategies for acquiring data or for accommodating fiscal funding constraints.

Larger, multisensor platforms have advantages as well. When needed, they provide a more stable platform and facilitate spatial and temporal simultaneity of measurements. Because fewer spacecraft and launches are involved, multisensor platforms offer a higher probability of placing a given complement of sensors into orbit without loss-and, often, at lower initial cost. Multisensor platforms frequently offer the simplest ground segment solutions, including mission operations, downlink and data system architectures, and calibration and validation of sensors.

The trade-off between small and large platforms is a complicated function of overall mission objectives, available budgets, tolerance for risk, and success criteria. These criteria are significantly different for research and operational missions. For example, operational systems are judged by performance, life cycle cost, and availability (the percentage of time the system can provide timely delivery of data). Loss of a single critical sensor can result in mission failure. Multiple launches of small satellites carry a higher risk of a launch or satellite failure, although the impact of such a failure with a larger multisensor satellite can be greater. Research missions are more tolerant to partial failure and place higher value on dynamic continuity and data quality as well as the flexibility to pursue new sensors and new science requirements aggressively.

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