CIESIN Reproduced, with permission, from: National Aeronautics and Space Administration (NASA). 1993. Earth Observing System (EOS) Reference Handbook, ed. G. Asrar and D. J. Dokken. Washington, D. C.: National Aeronautics and Space Administration, Earth Science Support Office, Document Resource Facility.


The primary goal of the rescoped EOS Program remains that of the restructured EOS--that is, to determine the extent, causes, and regional consequences of global climate change. The extent (e.g., the change in average temperature and the time scale over which it will occur) is presently unknown. Causes can be either natural or human-induced. Both must be understood to determine how to alter human behavior and avoid climate changes that prove most detrimental to the environment. The regional consequences (e.g., changes in precipitation patterns, length of growing seasons, severity of storms, sea level) must be understood to determine which aspects of climate changes are most harmful, and how to adapt to those changes that the human species cannot avoid.

EOS IWG defined the following science and polity priorities for EOS observations, based on IPCC, EPA, and CEES recommendations:

1) Water and Energy Cycles

2) Oceans

3) Chemistry of Troposphere and Lower Stratosphere

4) Land Surface Hydrology and Ecosystem Processes

5) Glaciers and Polar Ice Sheets

6) chemistry of the Middle and Upper Stratosphere

7) Solid Earth


The instruments flying as part of the restructured EOS Program were chosen to address these key scientific issues associated with global climate change. The original EOS Program covered a broader range of global change issues, including studies of stratospheric chemistry and its controlling influence on ozone depletion, and aspects of solid Earth physics and the exosphere. The baseline EOS Program included a total of 30 selected instruments. By focusing on climate change, the required instruments were reduced to 17 that needed to fly before 2002. Six were deferred, and seven were deselected during the restructuring process.

With input from the EER Committee and detailed recommendations from the EOS Payload and Science Advisory Panels, NASA reconfigured EOS to fly the 17 instruments required for global climate change studies, as follows: 1 ) Three intermediate spacecraft series to be launched on IELVs, 2) one smaller spacecraft series to be launched on MELVs, and 3) two small spacecraft series to be launched on SELVs. The names of the spacecraft series, initial launch date, launch vehicle date, and disciplinary focus follow:

The launch of the first EOS-AM spacecraft was rescheduled to June 1998, 6 months earlier than the originally planned launch of the first large EOS observatory (i.e., EOS-A). By reducing the size of the spacecraft and its payload, it became possible to launch earlier. The launch dates of the remaining EOS spacecraft were scheduled to occur over the ensuing 4 years, through the year 2002. Refer back to Figure 4 for a timeline and listing of instruments making up the restructured EOS Program.

The restructured program had the EOS-AM and -PM satellite series both employing sun-synchronous polar orbits, but with different crossing times. The EOS-AM spacecraft primarily would observe terrestrial surface features; thus, a morning crossing time when cloud cover is at a minimum over land) proved preferable. In contrast, EOS-PM included a next-generation atmospheric sounder--a candidate for deployment on future NOAA operational satellites. The instruments on this platform were suitable for an afternoon crossing time. Both EOS-AM and -PM would observe characteristics of terrestrial and oceanic surfaces, and the atmosphere. By having measurements at two different times of day, it would be possible to study diurnal variations in these features. EOS-COLOR, -ALT, and -CHEM were also slated for sun-synchronous polar orbits, and EOS-AERO was to have a 57deg. inclination.

Certain instruments were to be flown on more than one spacecraft. The Moderate-Resolution Imaging Spectroadiometer (MODIS), which is capable of observing both the Earth's surface and atmosphere, was included on both EOS-AM and -PM because of its synergy with other instruments on these platforms. That is, MODIS observations obtained simultaneously through the same atmospheric column are important in interpreting data from the other instruments. By flying on two separate spacecraft, MODIS--now the central instrument of EOS--would provide important redundancy to the program. MODIS would yield cloud information to complement the radiation budget observations taken by the Clouds and Earth's Radiant Energy System (CERES) instrument, which also was scheduled to fly on both the EOS-AM and -PM satellites as well as TRMM. Two MODIS instruments would provide complete global ocean color measurements by avoiding sun glint over the northern hemisphere oceans and the lack of illumination over the southern oceans, to be accomplished through their complementary ascending and descending orbits. The continuity of ocean color data beyond 2000 would be assured by including MODIS on both platforms. By flying on TRMM and both EOS-AM and -PM, CERES would provide Earth radiation budget from three different orbits at different times of the day, thus capturing diurnal changes.

The Stratospheric Aerosol and Gas Experiment III (SAGE III) instrument, which measures atmospheric aerosols, would be flown on EOS-AERO and -CHEM to provide measurements from two different orbits (57deg. inclination and polar, respectively). This strategy would also guarantee complete global coverage.

As stated earlier, the science objectives resulting from the restructuring exercise narrowed the overall study of global change down to an examination of global climate change. The extent, causes, and regional consequences of global climate change were to be determined by 1 ) providing a continuous calibrated data set of key Earth science variables in order to monitor variability and detect trends; 2) observations that will lead to an enhanced understanding of processes in order to improve predictive models; and 3) an information system for the receipt, processing, archiving, and dissemination of data to the scientific community and policymakers. The latter two remained virtually intact from the baseline EOS Program approach; rather, the observations to be collected and the instruments that were to take the measurements came under scrutiny. The selected EOS instruments and spacecraft ensured continuity of important time series of climate measurements, addressed the high-priority science and policy issues identified by IPCC, and were consistent with technical, budgetary, and schedule constraints.

The program plans that came out of the restructuring exercise were tempered by the caveat that follow-on EOS spacecraft payload configurations could change, depending on the evolution of scientific understanding and/or technical developments. for example, the Advanced Spaceborne Thermal Emission and reflection Radiometer (ASTER) instrument on the first EOS-AM platform possibly was going to be replaced by the High-Resolution Imaging Spectrometer (HIRIS) on EOS-AM2. The Payload Advisory Panel stipulated that actual decisions on instruments to fly on follow-on spacecraft did not need to be made immediately; rather, their rationale was to continue technology development efforts to ensure that subsequent generation instruments were available when needed. This proved a wise approach, given that the restructuring recommendations were approved by congress in March 1992, and the Red and Blue Team reviews were initiated by the NASA Administrator a mere 2 months later.


The rescoped EOS Program retains the focus on global climate changes instituted in the restructuring exercise. The EOS Program still emphasizes data collected over a 15-year period; however, many important measurements were canceled or deferred due to the high-risk technologies involved and associated cost. The descope of EOS to an $8 billion threshold required difficult tradeoffs to maximize science. As stated in the EOS Chronology section, the amount of contingency funds held to handle unexpected problems had to be reduced substantially. This contingency had to be balanced against the savings that resulted from complete elimination of instruments and their associated scientific information. This section identifies the principal factors considered by the Payload Advisory Panel and the Red and Blue Teams in the rescoping effort. Resolution is expected in 1993. A brief synopsis of key rescoping developments follows:

Table 6 lists areas of scientific uncertainty identified by IPCC and the rescoped EOS Program instruments that will address each issue. EOS remains a long-term program, providing continuous observations of the causes of global climate change; therefore, each EOS spacecraft will be repeated twice on 5-year centers to provide at least 15-year coverage. The only exceptions involve EOS-AERO (four follow-on launches on 3-year centers) and the one-time EOS-COLOR mission because of the lifetime limitations associated with small ELVs. The development of EOSDIS, its support of precursor data sets, and provision of a reduced set of essential data products at the launch of each EOS element has been maintained.

The principal reductions in cost result from the initiation of a common bus development, a decreased number of at-launch data products, increased international and interagency cooperation, increased risk, and rescoped payloads. The rescoped payloads primarily affect those platforms planned beyond 2000--namely EOS-ALT and -CHEM. The EOS-AM1, -PM1, -AERO, and -COLOR science objectives have been preserved by maintaining their instrument complements consistent with the recommendations of the EOS Payload Advisory Panel and the restructured program. Figure 6 provides a graphic representation of the rescoped EOS satellites and the science objectives sought.

The rescoped program places a greater degree of risk on meeting the science objectives beyond 2000, because increased reliance on other agency and international collaborations has been assumed where firm commitments are still being negotiated. In particular, the deletion of HIRIS was predicated on the joint DoD/NASA partnership in Landsat. This action has been resolved since initiation of the Red and Blue Team reviews, with management responsibility to be completely transferred from NOAA to the integrated DoD/NASA team with the launch of Landsat-7.

Developing a common spacecraft bus preserves the science objectives by increasing payload flexibility, by simplifying instrument design through a known interface, and by allowing for a launch opportunity every 18 to 30 months. This approach will only be realized if minimal redesign is required for each spacecraft and instrument grouping. The chief concern of the Red and Blue Teams and EOS Payload Advisory Panel was to determine optimum characteristics that best support the groupings identified in Table 5. This resulted in EOS-AM1 having a unique design, with -PM1 the first common bus to be used on all subsequent spacecraft (except EOS-AERO, -COLOR, and -ALT). IELVs remain the launch vehicles of choice, because this class best accommodates the payload needs developed during the restructuring and rescoping deliberations. The EOS-ALT and -AERO series can be accommodated on smaller ELVs.

Continuing the current EOS-AM1 development effort provides the greatest assurance of meeting the June 1998 launch readiness date. The EOS Program no longer requires EOS-AM2 to have the same performance characteristics as -AM1. This development effort should not be considered for naught, because this spacecraft could serve as the basis for future Landsat-class missions. This spacecraft will be able to handle multiple high-resolution instruments, which require more power, pointing, and data-handling capabilities than other EOS missions.

The payloads after 2000 were shifted primarily to take advantage of ongoing discussions between NASA and the international community. This may allow scatterometer measurements to be advanced by approximately 2 years, if sufficient resources are made available to accommodate the NASA Scatterometer follow-on (NSCAT II) on Japan's ADEOS II. This provides greater assurance of the continuity of ocean wind stress and topography measurements needed to study ocean circulation and air-sea exchange of energy and chemicals. The NSCAT accommodation on the original EOS-CHEM1 payload was considered a flight-of opportunity in order to continue the observations begun with ADEOS I, which is scheduled for launch in 1996. Such a scenario would have required that NSCAT operate for at least 6 years prior to the launch of the EOS-CHEM1 mission. The rescoped scenario assumes flight of NSCAT II in 1999, significantly reducing risk of a gap in scatterometry data.

Two French instruments presently onboard the TOPEX/Poseidon mission [i.e., Solid-State Altimeter (SSALT) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS)] will provide needed altimetric observations by replacing the U.S. Altimeter (ALT) and GPS Geoscience Instrument (GGI), respectively. However, the science return will be reduced. SSALT currently does not operate in two frequencies (required for the correction of ionospheric effects), and the substitution of DORIS provides precise tracking but eliminates the sparse, accurate, atmospheric temperature profiles that GGI would generate. Both the Red and Blue Teams and Payload Advisory Panel took these factors into consideration when weighing science return against the costs involved.

The continuation of the Landsat Program allowed the placement of the Tropospheric Emission Spectrometer (TES)--instead of HIRIS--on EOS-AM2, together with MODIS and possibly the Measurements of Pollution in the Troposphere (MOPITT) instrument. This reconfiguration also allowed the inclusion of several flight-of-opportunity instruments [i.e., Active Cavity Radiometer Irradiance Monitor (ACRIM), Solar Stellar Irradiance Comparison Experiment II (SOLSTICE II), and Microwave Limb Sounder (MLS)] on the EOS-CHEM series. These instruments will generate measurements of atmospheric chemical composition, radiation, and dynamics complementary to observations currently being collected by the MLS and SOLSTICE II instruments aboard UARS.

As in the restructured EOS scenario, SAGE III measurements will be provided by both EOS-AERO and -CHEM satellites, which will fly in 57deg. inclined and polar orbits, respectively. By placing this instrument in different orbits, full global coverage can be guaranteed. The spacecraft for the EOS-AERO mission has yet to be determined; however, NASA negotiations for an international partnership in the aerosols series should be completed in 1993.

Through the rescoping exercise, the EOS Program has decreased instrument contingency funds to be more representative of a multiple copy procurement, and has phased instrument developments to control initial costs and bring the overall budget within the Congressionally mandated ceiling. As a result, the total number of instruments to fly on the EOS platforms (including international contributions) has been reduced to 22, of which 15 will fly before 2003. ASTER is slated for only one flight, and negotiations are still underway to accommodate MOPITT on the second EOS-AM platform; a slot for an as yet to be determined Japanese instrument for flight on the EOS-CHEM series has been held as reciprocation for the flight of NSCAT II on the ADEOS series. Of course, instrument complements could change with the evolution of scientific understanding and/or technological enhancements. Refer to the EOS Instruments section for descriptions of those instruments that remain part of the EOS Program.

The reconfiguration of the EOS payloads has both benefits and pitfalls; yet, in the prevailing budget environment, decisionmakers have to balance the science return against costs incurred. Any rearrangements require review by EOS IWG to determine if they represent on acceptable solution towards satisfying the identified IPCC science and polity priorities. Furthermore, the potential of achieving the international commitments assumed in the rescoped program must be quantified to determine the consequences of data gaps should these collaborations not be realized.