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Introduction
In November 1995, the Committee on Earth Studies of the National Research Council's Space Studies Board began a study to analyze the capability of small satellites1 to satisfy core observational needs in Earth observation and weather monitoring programs of the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA). NASA's interest in the possible use of smaller satellites for its Earth Observing System (EOS)2—the space-based component of the Office of Earth Science's Earth Science Enterprise—arose from budgetary pressures, the desire within the scientific community for more missions and shorter mission time lines, the perception that there are both gaps and unrealized opportunities in the enterprise's space segment, and the determination of NASA officials to accelerate the infusion of new technologies into their space programs.
The Department of Commerce (specifically NOAA) and the Department of Defense (DOD, specifically the Air Force) have also been examining potential roles for small spacecraft3 as they proceed with plans to develop a converged polar-orbiting weather satellite system, NPOESS (National Polar-orbiting Operational Environmental Satellite System), scheduled for launch in approximately 2009 (availability date 2008). Prior to the start of the NPOESS program, both agencies had been planning block changes and upgrades to their existing polar-orbiting weather satellites, Polar-orbiting Operational Environmental Satellites (POES) and the Defense Meteorological Satellite Program.4 The NPOESS program is being executed by the Integrated Program Office with representatives from DOD, NOAA, and NASA. NASA's particular interest in NPOESS involves its potential to fulfill long-
term systematic measurement requirements formerly planned for follow-on missions to the first series of EOS satellites, especially the PM5 series because of its focus on monitoring weather- and climate-related variables. Each NPOESS satellite is currently planned as a multisensor spacecraft. However, alternative system architectures are possible that would distribute the sensor complement among a larger number of smaller satellites.
The committee's study originated during a period when satellite builders and policymakers were engaged in a spirited debate over the feasibility and merits of substituting smaller satellites for larger systems. Advances in miniaturization were said to allow much smaller sensors that retained sufficient performance for many Earth science and operational needs. These smaller sensors could be accommodated on smaller spacecraft, which would be smaller still because of miniaturization of various spacecraft subsystems. Reducing the size, volume, and weight of both payload and spacecraft would then allow the use of either the new generation of smaller launch vehicles or clustering of spacecraft on a single launch of a larger launch vehicle. It was argued that performing missions with smaller payloads, spacecraft, and launch vehicles would lead to dramatically lower costs.
The debate over the use of small satellites had sometimes been portrayed as a dispute between innovative satellite designers and government bureaucrats or industry officials who either lacked vision or had financial incentives to maintain the status quo. The committee found that these characterizations were either inaccurate or a simplification of more complex circumstances. It is noteworthy, for example, that the historic providers of large Earth remote sensing satellites have also provided small satellite systems for space physics research, planetary exploration, and other space missions. In addition, it was evident to the committee that any credible discussion of small versus large had to include a detailed analysis of the many interrelated technical and programmatic issues associated with the design and development of satellite systems.
In responding to its charge (Appendix A), the committee set 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. Further, the committee examined opportunities presented by small satellite options with respect to mission architecture and assessed their implications for future NASA and NOAA missions.
During the study, both NASA and NOAA made programmatic decisions that affected the committee's course. NASA restructured its Earth science program such that missions that would follow the initial EOS AM, PM, and Chemistry satellites would be smaller, more flexible, and responsive to advances in technology and science. NASA also planned to integrate EOS missions with operational weather satellite programs (e.g., NPOESS) for long-term systematic measurements. Further, the NOAA-DOD-NASA Integrated Program Office opted to develop new sensors, as opposed to continuing with heritage EOS sensors, for critical NPOESS measurements through competitive procurements. Thus, both NASA and NOAA plans now recognize and embrace current capabilities and ongoing advances in sensor and spacecraft technology for future Earth observation missions. Consequently, the committee altered its planned response to its charge and de-emphasized the study of specific new technologies in favor of an increased emphasis on the implications and impact of capable small sensors and satellites on mission architecture and management trade-offs. Among the questions emphasized in this modified approach were these:
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Are there sustained opportunities for low-cost, quick-response, focused missions, leading to a reduced "time to science" (analogous to the commercial sector's "time to market")?
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Would affordable constellations of small satellites open the door to enhanced science via more frequent or continuous sampling strategies?
The committee explored the scientific merits and technical capabilities of small satellites; the development status (e.g., availability and reliability) of the necessary system elements; and the programmatic aspects of implementing small satellite missions. The criteria used to assess small satellite utility and to examine mission architecture trade-offs included performance capability, risk, mission flexibility and robustness, and the potential for streamlined management processes—all with a focus on the potential for lower mission-life-cycle costs. Several case studies (Appendix D) were examined to assess the reality versus the promise of small satellites to date and to help identify paths to greater success in the future.
Both operational and research programs were considered, and the distinction between them underlies much of the discussion in this report. Characteristics of operational programs include an established community of data users who depend on a steady or continuous flow of data products, long-term stability in funding and management, a conservative philosophy toward the introduction of new technology, and stable data-reduction algorithms. Research programs often require greater measurement accuracy, more attention to calibration, programmatic flexibility, and faster time to science; depending on the cost of the mission, they can be more tolerant of risk.
The study emphasized the launch and space segments of Earth observation missions. Although treated more superficially in this study, ground segment operations (communications, command and control, and data routing and processing) and space system infrastructure (ground and space assets) may weigh heavily on mission architectures that involve many satellites—and may merit a study of their own.
During the course of its work, the committee heard presentations from companies long involved in producing small satellites for both commercial and research use. The committee also heard from industry representatives involved with NASA's Small Spacecraft Technology Initiative (SSTI),6 a very aggressive program initiated by NASA in 1994 that attempted to demonstrate faster, better, and cheaper approaches to the development of small satellites. Key questions addressed to all study participants were whether the use of smaller satellites could reduce overall mission cost and what the controlling factors were.
In considering the potential for small satellites to reduce the cost of Earth observation missions, it is important to distinguish between small satellites, small missions, and larger missions employing small satellites. As noted in footnote 1, the present study's analysis of small satellites refers to those in the 100 to 500 kg class carrying one or a few sensors that are capable of acquiring data of the kind and quality required by NASA and NOAA for their EOS and POES/NPOESS programs. In this report, the term "small mission" refers to a comparatively low-cost mission. NASA's current Earth science strategy of performing a larger number of smaller missions (versus that planned in the early 1990s) is predicated on the cost of each mission being relatively low.7 Low-cost satellite buses help enable low-cost missions, but total mission costs include those for the satellite (i.e., payload plus bus), launch vehicle, and mission operations. A number of small missions consisting of a single small satellite launched on one of the new class of small launch vehicles have been successfully performed at relatively low total cost,8 albeit at a high specific cost—i.e., cost per pound (see Chapters 4 and 5). These missions take advantage of small
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SSTI was developed by NASA's Office of Space Access and Technology to advance the state of technology and reduce the costs associated with the design, integration, launch, and operation of small satellites (NASA SSTI, 1994). TRW and CTA Space Systems were each awarded a contract by NASA to design and launch small Earth observing satellites, which were subsequently named Lewis and Clark, respectively. The Lewis spacecraft was successfully launched from Vandenburg Air Force Base into its initial orbit on August 22, 1997. However, on August 26, 1997, an in-flight anomaly led to loss of attitude control and a discharged battery, which resulted in the eventual loss of the mission. On February 25, 1998, NASA issued a press release announcing the termination of the Clark Earth science mission. The mission was terminated after an investment of some $55 million "due to mission costs, launch schedule delays, and concerns over the on-orbit capabilities the mission might provide." NASA retained Clark's launch vehicle services. See Steitz (1998). Appendix D of this report provides further discussion of the Lewis and Clark missions. |
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Small mission costs are typically constrained by a limit prescribed in the Announcement of Opportunity (e.g., the Earth System Science Pathfinder mission cost was capped at $90 million in its 1996 Announcement of Opportunity). |
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The true cost of a mission must also include the investment in technologies around which the activity is built. When ample advanced technology development has been done with prior investment that can be leveraged by a mission, the development costs of the mission itself may appear small. Discussions of the true cost of the mission should acknowledge such prior investments, particularly when they are directly supportive of the mission (e.g., preexisting sensors). |
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.
