''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



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