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Introduction NASA's Earth Observing System (EOS) has been designed to build on 30 years of scientific and technical experience in remote sensing from space. As the technology has matured from the early weather satellites, the Earth science community has had the opportunity to integrate remote sensing techniques and data products into its research. The new data have answered many questions but raised more. The existence of global measurement techniques and modern computational capabilities has led to significant advances in the conceptual approach to the study of a natural environment from a disciplinary to an interdisciplinary focus and from a local or regional to a global perspective. Over the past decade, these developments have contributed to the emergence of a new field of scien- tific inquiry, Earth system science, and to international research activities such as the World Climate Research Program (WCRP) and the Interna- tional Geosphere-Biosphere Program (IGBP), and to the initiation of the U.S. Global Change Research Program (USGCRP). Satellite measurement programs are a key element of Earth system science. In the EOS program, NASA proposes a major step in the evolution of the science and technology of global remote sensing and microwave imagers that provide detailed images of the Earth's surface; tropospheric and stratospheric sounders that provide vertical profiles of parameters such as temperature, trace gas concentration, and humidity; and accurate positioning instruments. EOS will carry two classes of instruments: facility and principal investigator (PI) instruments. The facility class instruments are those that NASA will supply in response to the general mission; the 31

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32 PI class instruments were selected through a competition and are aimed at the specific focused research interests of the selected investigators. The objective of EOS is to integrate a number of related, but previ- ously unavailable or disparate, space-based measurements into one contin- uing system and thereby to enhance research capabilities significantly. It will also provide a test bed for the development of the next generation of operational, Earth-observing instruments and measurement techniques. EOS is designed to yield a long-term, continuous set of high-priority mea- surements on a global basis that are consistent with the needs of the U.S. Global Change Research Program (USGCRP). The measurements will be combined with complementary data obtained from space, suborbital, and surface-based sources into the EOS Data and Information System (EOS- DIS). The comprehensive contents of EOSDIS are intended to enable the scientific community to document, monitor, and model environmental change, to broaden its understanding of the entire Earth system, and to improve predictive capabilities. Both the EOS space-based measurements and EOSDIS are planned as interdisciplinary, interagency, international endeavors, all of which are essential features of the USGCRP. While EOS as proposed would provide extensive capabilities in remote sensing and data management, it is best understood in the larger context of NASAs Mission to Planet Earth and the even broader context of the programs and plans for remote sensing of other agencies and nations. The framework of the Mission to Planet Earth includes a number of near- term smaller satellite missions (Earth Probes) to provide essential data and added flexibility to the overall program. Both EOS and Earth Probes are included in the FY 1991 USGCRP. Geostationary platforms for continuous synoptic observations are being considered for later flight. This report is based on a review of the most recent NASA documen- tation on the current status of the proposed EOS, including the 1990 EOS Reference Handbook (published by NASA), several briefing reports, and oral briefings. In particular, we, the panel, had access to the draft report of the Space Studies Board's Committee on Earth Studies (SSB/CES), which examines the EOS initiative in a broader context. That report will be issued after this one. We also received a copy of a review of EOS by the American Geophysical Union, draft letters on instrument selection and data simultaneity from the Chairman of the EOS Payload Advisory Panel to NASAs Associate Administrator for Space Science and Applications, and the latest information on plans for the USGCRP. Currently EOS is at the point in the NASA process where instruments have been selected for development, but not yet selected for flight. Thus the information we had for this review was a "snapshot" in time of an ongoing process of development. All details of the mission have not yet been established, and even some of the major decisions have not yet been

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33 made. Mindful of this situation, we dealt with the information as part of a changing process, so that what we have provided here is an assessment that should itself be viewed as a snapshot of a developing program. Because of the limited time available, we were not able to review many aspects of this complex program in detail. Moreover, as noted below, much of the documentation for the program is fragmentary and informal. In spite of this limitation, we believe that our general conclusions are sound. Because of the importance of this program to Earth scientists and to the U.S. Global Change Research Program in general, we recommend that a continuing ovemew by independent nongovernmental scientists and engineers be maintained. We were charged to address four questions. They appear verbatim at the openings of Chapters 5 through 8 below and are followed in each case by our responses and recommendations.