satellite buses that are available at about 15 to 50 percent of the cost of their larger Delta or Atlas class counterparts, depending on the difficulty of the mission requirements.

Mission cost trends are more uncertain when using small satellites to perform larger missions. For example, a mission architecture that employs a constellation of small satellites to achieve a high sampling frequency may cost a great deal, even though the individual satellites may cost little. More controversial is a mission architecture that accommodates a specified complement of sensors with several small satellites rather than with a larger multisensor satellite. In this trade-off, there is no a priori right answer on relative mission architecture costs as they depend on many variables (see Chapter 6).

The chapters that follow address first NASA's and NOAA's core observational needs and then three specific aspects of flight missions: sensor payloads, satellite buses, and launch vehicles. Finally, the report examines a number of systemwide issues, first with respect to overall mission architecture and then regarding several key management concerns that go beyond hardware development considerations. Specifically, Chapter 2 provides an overview of the measurements planned by NASA and NOAA to support satellite-based research and operational Earth observation programs, and it introduces key issues common to the development of either large or small satellite programs to fulfill NASA and NOAA requirements for the EOS and NPOESS programs. Chapter 3 provides a tutorial on the principles guiding the design and accommodation of sensors on a satellite. It also presents a discussion of sensor costs and an overview of the trade-offs and physical limits that govern sensor design. Chapter 4 discusses the capabilities of small satellite buses and their suitability for performing Earth observing missions. It also addresses some of the issues and trade-offs related to acquisition and cost, including the use of commercial, standard, and catalog buses. Chapter 5 addresses the current dilemma regarding the fact that achieving the full promise of small satellites will require the availability of reliable U.S. launch vehicles with a complete range of performance capabilities.

Chapter 6 is a key chapter in this report. Small satellites on dedicated launch vehicles offer a very high degree of programmatic flexibility, which allows them to be included in system trade-off studies that analyze the cost and effectiveness of alternative mission architectures for current and future programs. These trade-offs are illustrated in an analysis of alternative mission architectures for the NPOESS mission. Chapter 7 examines issues related to the management of small satellite programs, including consideration of science-driven versus technology-driven approaches and of calibration and validation strategies.

Chapter 8 reviews the preceding chapters in the broad context of the overall study, providing an integrated summary of key findings and recommendations.

REFERENCES

Earth Observing System (EOS) Project Science Office. 1999. EOS homepage. Available online at <http://eospso.gsfc.nasa.gov/eospso_homepage.html>.


National Aeronautics and Space Administration, Small Spacecraft Technology Initiative (NASA SSTI). 1994. Fact sheet: Smallsat—A new class of satellite. Available online at <http://ranier.hq.nasa.gov/SSTI_Page/NewClassSat.html>.


Steitz, D. 1998. NASA terminates Clark Earth science mission. Press Release 98–35, February 25 . Washington, D.C.: National Aeronautics and Space Administration.



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