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4 Current and Planned Earth Observing Satellite Missions: 1986 to 1995 INTRODUCTION Environmental satellites are the central observing element for a Mission to Planet Earth. They provide the primary means for collecting environmental data on a global, consistent, repetitive, and long-term basis. Prototypes of many of the elements of a Mission to Planet Earth are already proven and operational, or are being planned. This chapter reviews existing space and infor- mation systems, as well as those systems that are expected to be operational by 1995. The space-based elements are broken down for the sake of organizational clarity according to six broad areas of inquiry: land, oceans, atmosphere, radiation budget, atmospheric chemistry, and geodynamics. The discussion that follows is meant to be representative, not comprehensive. All missions and instruments, including those reviewed below and those not discussed, are listed in Table 4.1. A more comprehensive discussion of earth observing missions is providecl in the series of NASA reports on the Earth Observing System (EOS). 73

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74 TABLE 4.1 Observational Programs for Global Data Acqumi- tion: Representative Examples of Approved and Continuing Pros gram~i Program Agency/Status Objectives POES: Po~ar-orbiting NOAA/ Weather observations Operational Environmental Operating Satellites (e.g.. NOAA-7) GOES: Geostationary NOAAI Weather observations Environmental Satellite Operating System DMSP: Defense Meteor- U.S. Air Force/ Weather observations for ological Satellite Program Operating Department of Defense METEOSAT: Meteorology ESA/Operating Weather observations Satellite GMS Geostat~onary NASDA Weather observations Meteorology Satellite (Japan)/ Operating METEOR-2: Meteorological USSR/ Weather observations Satelilte-2 Operating LANDSAT Land Remote EOSAT/ Vegetation crop. and Sensing Satellite Operating land-use inventory LAGEOS-1: LaserGeo- NASA/ Geodynamics gravity dynamics Satellite-1 Operating field ERBE Earth Radiation NASA-NOAAI Earth s radiation Budget Experiment Operating losses and gains GEOSAT: Geodesy U.S. Navy/ Geodesy. shape of the Satellite Operating geoid ocean end atmospheric properties GPS Global Positioning U.S. Navy- Geodesy. crystal System NOAA-NASA- deformation NSF-USGS/ Completion 1989 SPOT" 1 Systeme Proba- France/ Land use. Earth tore d Observation de la Operating resources Terre- 1 IRS Indian Remote India/ Earth resources Sensing Satellite Operating Representative Space Shuttle instruments: ATMOS: Atmospheric NASA/Current Atmospheric chemical Trace Molecules Observed composition by Spectroscopy ACR. Active Cavity NASAlcurrent Solar energy output Radiometer SUSIM Solar NASA/Current Ultraviolet solar Ultraviolet Spectral observations Irradiance Monitor SIR Shuttle Imaging NASA/ Land-surtace obser- Radar Current/ln vations development MAPS Measurement of NASA/ Tropospheric carbon Air Pollution from Shuttle Current/ln monoxide development SISEX: Shuttle Imaging NASAlplanned Spectral observations Spectrometer Experiment of land surfaces LIDAR: Light Detection NASAlplanned Surtace topography and Ranging instrument atmospheric properties Program Agency/Status Objectives MOS-1: Marine Obser- NASDA State of sea surface vapor Sate~ite-1 (Japan)/ and atmosphere Launch 1987 LAGEOS-2: LaserGeo- NASA-PSN Geodynamics gravity dynamics Sate~tite-2 (Italy)/ field Launch 1988 SPOT-2: Systeme Proba- France/ Earth remote sensing toire d Observation de la Launch 1988 Terre-2 WARS: Upper Atmosphere Research Satellite NASA/ Stratospheric chemistry. Launch 1989 dynamics. energy balance ERS-1: Earth Remote ESA/Launch Imaging of oceans. Ice Sensing Satellite-1 1990 fields. land areas Ut Hb-1 Japan Earth NASDA Earth resources Remote Sensing Satell~te-1 (Japan)/ Launch 1991 Representative International Programs for Measurements In Situ Organlzatlon/ Program Status Objective GEMS: Global Environment UNEP/ Monitoring of Monitoring System Begun 1974 global environment World Ozone Program WMO-NASA- Atmospheric composition UNEP/ Operating Crustal Dynamics Project NASA-23 Tectonic plate movement nations/Begun and deformation 1979 Man and the Biosphere UNESCO/ Ecological studies Operating International Biosphere UN/Operating Long-term ecological Reserves studies ISCCP: International WMO-ICSU/ Measure interaction of Satellite Cloud Climatology Begun 1983 Project (World Climate Research Program) clouds and radiation ISLSCP: International WMO-ICSU/ Measure interactions of Satellite Land Surface Begun 1985 land-surface processes Climatology Project (World with climate Climate Research Program) TOGA: Tropical Ocean WMO-ICSU/ Variability of global Global Atmosphere Pro- Begun 1985 interannual climate gram (World Climate events Research Program) GRID: Global Resource Information Database UNEP/ Information on global Begun 1985 resources

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75 TABLE 4.1 (continued) Observational Programs for Global Data Acquisition: Representative Examples of Proposed Future Pros grams Agency/ Progr m Status TOPEX/POSEIDON: Ocean Topography Experiment NASA-CNES (France)/Start 1987, Launch 1991 NOAA/ Planned Ocean surface topography POES: Polar-orbiting Operational Environmental Satellite systemfollow-on missions (NOAA K,L,M) GOES: Geostationary Operational Environmental Satellite systemfollow-on missions (e.g., GOES-Next) RADARSATCanadian Radar Satellite Advanced capabilities for weather observations NOAA/ Planned Advanced capabilities for weather observations Canada/Start Studies of arctic ice, 1986, Launch ocean studies, Earth 1991 resources Passive and active microwave sensing MOS-2: Marine Obser- vation Satellite-2 GRM: Geopotential Research Mission NASDA (Japan)/ Launch about 1990 NASA/Start Measure global geoid 1989, Launch and magnetic field 1992 Individual instruments for long-term global observations: OCI: Ocean Color Imager NASA-NOAA/ Planned NASA/ Planned ERB: Earth Radiation Budget instrument Carbon-Monoxide Monitor NASA/ Planned NASA/ Planned GLRS: Geodynamics NASA/ Laser Ranging System Planned Total Ozone Monitor Laser Ranger Scanning radar altimeter NASA/ Planned NASA/ Planned NASA-NOAA/ Long-term global NASA Start Earth observations 1989, Launch 1994 NASA/ Planned Ocean biological productivity Earth radiation budget on synoptic and planetary scales Monitor tropospheric carbon monoxide Monitor global ozone Crustal deformations over specific tectonic areas Continental motions Continental topography Eos: Earth Observing System/Polar-Orbiting Platforms. NASA-NOAA program: NASA research payloads N OAA operational NOAA/ payloads Planned Surface imaging, sound- ing of lower atmosphere; measurements of surface character and structure; atmospheric measure- ments; Earth radiation budget, data collection and location of remote measurement devices Weather observations and atmospheric com- position; observations of ocean and ice sur- taces; land surface imaging; Earth radiation budget; data collection and location of remote measurement devices; detection and location of emergency beacons; monitoring of space environment Agency/ Obloctives Program Status Obloctivoe Long-term compre- hensive research, Operational, and commercial Earth observations NASA/Start Tropical precipitation 1991, Launch measurements 1994 Secular variability of Earth's magnetic field European Polar-Orbiting ESA/Planned Platform (Columbus) Raintall mission MFE: Magnetic Field Explorer NASA/Start 1993, Launch 1996 MTE: Mesosphere- NASA/Start Thermosphere Explorer 1995, Launch 1998 NASA/Start 1997, Launch 2000 GGM: Gravity Gradiometer Mission Chemistry and dynamics of upper atmosphere Gradient in Earth's gravitational field Representative International Programs for Measurements In Situ P ram rag O rg an lzatl on/ Status WMO-ICSU- Detailed understanding IOC-NSF- of ocean circulation NASA-NOAA/ 1987 enhance- ment ICSU/ Proposed Obloctlvo WOCE: World Ocean Circulation Experiment (World Climate Research Program) IGBP: International Geo- sphere-Biosphere Program (Global Change) GOFS: Global Ocean Flux NSF-NOAA- Study NASA/ Enhancement NSF-NASA- NOAA/ Enhancement NSF-USGS- NOAA/ Enhancement Sensing of the Solid Earth NSF-USGS- DoD-NASA/ Enhancement Ecosystem Dynamics NSF/ Enhancement Study of global change on timescale of decades to centuries Production and fate of biogenic materials in the global ocean Tropospheric chemistry and its links to blots GTCP: Global Tropospheric Chemistry Program Ocean Ridge Crest Processes Chemistry and biology of deep-sea thermal vents, plate motions, crustal generation Large-scale mantle convection. studies of continental lithosphere Studies of long-term ecosystems, biogeo- chemical cycles Greenland Sea Project ISCU/Planned Atmosphere - sea ice - ocean dynamics SOURCE: Earth System Science Committee, NASA Advisory Council, Earth System Science Overview A Program for Global Change, NASA, Washington, D.C., pages) 34~35, 1986.

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76 CURRENT PROGRAMS [and Obeying Systems Landsat . The Landsat system is a series of sequentially launched satel- lites commencing with Landsat-1 in 1972. The onboard instru- ments of the first spacecraft were a multispectral scanner (MSS) capable of Am ground resolution, three return beam vidicons (RBVs), and two wide-band video recorders. The MSS consisted of an electro-opto-mechanical scanner covering a swath 185 km wide in four spectral channels from 0.4 to 1.1 ~m. The data were transmitted to earth stations in digital format from a sun- synchronous orbit at an altitude of 918 km. The entire Earth was sequentially covered every 18 days. Landsat-2 and -3 were identical platforms launched at the same sun-synchronous altitudes, although their instrumentation varied. Landsat-2, launched in 1975, carried a five-channel MSS. The fifth channel was in the infrared range with a 270-m ground resolution; the four other channels, the RBVs, and the tape recorders were identical to those of Landsat-1. Landsat-3, launched in 1978, returned to the four-channel MSS, but used two RBVs oh crating in a panchromatic mode, increasing the ground resolution to about 40 m. Again, two wide-band recorders were used. Landsat-4 was a newly designed spacecraft launched in 1982 Into a sun-synchronous orbit at 705 km. The instruments included a thematic mapper (TM), which provided a ground resolution of 30 m in seven spectral bands, and a four-channel MSS similar to the one on Landsat-3. Data were transmitted in real time to the ground using a wide-band system in the Ku-band via the TDRSS satellite, or by X-band directly to ground. The TM failed soon after launch in 1982. Landsat-5, with an identical configuration to Landsat-4, was launched in 1984 and is still functioning. Operational control over the Landsat system was transferred from NOAA to the Earth Observation Satellite Co. (EOSAT) in September 1985. EOSAT is operating the Landsat system on a commercial basis. If they receive the necessary funding, Landsat-6 and -7 are scheduled for early 1990s launches. These spacecraft would have improved capabilities over existing instrumentation and possibly additional sensors, such as a wide-field-of-view imager for ocean phenomena.

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77 Systeme Probatoire d'Observation de la Terre (SPOT) The Systeme Probatoire d'Observation de la Terre (SPOT), another commercial remote sensing satellite system, is operated by the French space agency (ONES). The first of a series of at least four planned satellites was launched in early 1986 in a sun- synchronous orbit at 832 km. The instruments, two identical pointable multispectral linear-arrays, called high-resolution visible (HRV) sensors, operate in three spectral bands: 0.50 to 0.59 ~m, 0.61 to 0.68 ~m, and 0.79 to 0.89 ~m. The ground resolution is 20 m in a color mode operating across the entire band, and 10 m in a panchromatic mode operating from 0.51- to 0.73-pm bandwidth, ant] is capable of stereoscopic imaging. Viewing can be forward, backward, and sideways, and each HRV can operate in both modes simultaneously; however, only two sets of data can be acquired at the same time. SPOT images can cover the entire Earth in a 2.5-day repeat cycle. Data are transmitted in real time to 4 ground stations in France, Sweden, and eastern and western Canada, or stored onboard on two wideband tape recorders for non-real-time transmission to the ground. The 4 ground stations are being expanded to 10 in the near future to provide worldwide real time image coverage. Shuttle Imaging Radar (STR) A series of radar imaging experiments called SIR-B, carried out on a Space Shuttle flight in 1985, added another major di- mension to NASA's program in earth observations. An earlier experiment, SIR-A, was conducted in November 1981. The data it collected provided the first demonstration that radar sensors can penetrate deep into windblown sand deposits in hyperarid environments. STR-A imagery of portions of the eastern Sahara Desert revealed the presence of buried drainage channels that pro- vide important clues to the archaeological and geological history of southern Egypt. STR-B is the first space-borne radar capable of imaging Earth's surface at multiple angles of incidence measured from the local vertical. Initial results were dramatic and showed the SIR-B ability to obtain accurate relief maps of the Earth's surface.

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78 Ocean Observing Systems The major achievement In ocean observations in the past decade was the Month flight of Seasat In 197S, which demon- strated the feasibility of scatterometer measurements for mea- suring wmd and waves, the altimeter for measuring waves and currents, and the synthetic aperture radar for detailed high- resolution measurements of land, ocean, and ice surface. In ad- dition, Nimbus-7, which was also launched in 1978 and Is still flying, demonstrated the feasibility of measuring ocean color to be used for est~rnat~ng sediments and chlorophyll in the near-surface waters. The NOAA operational satellites also have shown the ca- pability for measuring sea surface temperature, which is now a regular operational product. The untimely demise of Seasat, due to a massive power failure, led to the design of follow-on programs for scatterometers and al- tuneters. The Geosat altimeter has obtained more than 5 times as much data as Seasat, with comparable accuracy. The NSCAT pro- gram, a NASA scatterometer, and the TOPEX/Poseidon mission, which includes a precision altimeter experiment, are discussed be- low. A fodow-on to the Coastal Zone Color Scanner on Nnnbu~7 is also being designed, and will fly as part of the EOS. Atmosphere Obeer~n~g Systems The primary observing systems for the atmosphere are the operational weather satellites in polar and geostationary orbit. These are operated by several nations, and the plans detailed here are predicated on their continued operation. As the task group looks to new requirements, it notes that NASA has both the expe- rience and the facilities to deal with the special problems involved in understanding the circulation of the atmosphere; namely, the processing of voluminous data, interpretation of results in mete- orological terms, and application of the results to meteorological issues. Investigation and assessment of data from the first Global Atmospheric Research Program experunent are proceeding. A substantial part of NASA's work is devoted to the development of new techniques. For example, an effort is under way to develop and fly an advanced temperature and moisture sounder whose ex- pected performance could approach that of radiosondes, but with far more complete spatial coverage.

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79 The emphasis of research on severe storms and local weather includes meteorological observations from space or high-fly~ng air- craft, and high-technology interactive computer techniques to as- sim~late and analyze data from multiple sources. One aspect of de- veloping new measurement techniques ~ the use of aircraft flights for field tests. Also being emphasized are the research applica- tions of the Visible-Infrared Spin-Scan Radiometer Atmospheric Sounder on NOAA's Geostationary Operational Environmental Satellites, and development of new algorithms for determining temperature, moisture, and winds at different heights in the atmo- sphere for use In numerical models. Flow scales in the atmosphere must be understood if progress is to be made in relating large-scale weather to local weather. Earth's Radiation Budget Observations from Nimbuses and -7 instruments and from NOAA's operational satellites are the foundation for a continuing series of data sets on Earth's radiation budget that will serve as a resource for climate research. NASA's Earth Radiation Budget Exper~rnents will continue to augment the data sets. Earth's radi- ation budget also is being addressed in other ways. Evidence from recent Nimbuses and Solar Maximum Mission observations con- firms that the total output of the Sun varies naturally by several tenths of a percent for periods of up to about 2 weeks. A number of instruments are being designed to monitor the long-term trend of solar variation and to determine its effect on climate systems, and these are discussed later in the chapter. Research programs have been initiated to develop an understanding of and models for the processes by which clouds are formed and interact with incident or reflected radiation, and to study the sources, compositions, and radiative effects of aerosols that volcanic explosions inject into the stratosphere. In addition, the International Satellite Cloud Clima- tology Project is expected to develop a global cloud climatology data set. Atmospheric Chemistry Investigators are developing techniques for measuring major trace species in the troposphere. Field measurements to test the most promising instruments will be followed by a Year program

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80 of measurements by aircraft to characterize the chemistry of the troposphere on a global scale. Research on the stratosphere and mesosphere also continues and has increasingly used more realistic two- and three-dimensional models. The chemical, radiative, and dynamic computer codes used in those models are being improved continually, with the goal of developing fully coupled chemical, radiative, and dynamic three-dimensional models that simulate the atmosphere very precisely. Also NASA, in cooperation with European, Canadian, and Japanese investigators, is using a variety of instrument techniques on balloon, rocket, and aircraft flights to obtain measurements of trace species in the stratosphere that will allow accurate comparisons with current experimental techniques. Data from Nimbus-4, -6, and -7, and the Stratospheric Aerosol and Gas Experiment have been validated and are becoming avail- able for detailed analysis. Solar Mesosphere Explorer data on ozone, nitric oxide, and water vapor will also soon be available for analysis. In addition, two instruments, the Imaging Spectrometer Observatory and the Atmospheric Trace Molecule Spectroscopy experiment, have been developed for use on the Shuttle to mea- sure those species in the mesosphere and stratosphere. The ON servatory already has flown on Spacelab 1, and the spectroscopy experiment may fly on a future Spacelab mission. Geodynamice Laser ranging, lunar ranging, and microwave interferometry (V[Bl) are being used to measure the motions of Earth's polar axis, variations in the length of day, and the motion and deforma- tion of Earth's crustal layer. A worldwide network of over 20 coop- erating space agencies participates in NASA's global geodynam~cs research. A second Laser Geodynamics Satellite (LAGEOS), be- ing built by Italy, is expected to be launched in 1993. Data from laser tracking of satellites, and altimeter data from Seasat and the third Geodynamic Experimental Ocean Satellite (GEOS-3) improved the accuracy of models for global gravity fields used in studies of earth and ocean processes. Similar data acquired by the Magnetic Field Satellite (Magsat) were used in studying secular and temporal variations of Earth's main field and inhomogeneities in Earth's crust.

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81 POTENTIAL INITIATIVES: 1986 TO 1995 The initiatives below are those that have been developed from the knowledge base gained from the programs described above. The first two of these, the Upper Atmosphere Research Satel- lite and the scatterometer, were approved as new starts in 1985. NASA had no new starts in 1986, but did obtain a new start for TOPEX/Poseidon in 1987. The other initiatives are further down the queue, but are expected to be strong candidates for new starts in the period 1988 to 1995. Finally, the task group anticipates that the Earth Observing System will be the major new start of this group. It is discussed in Chapter 5. Upper Atmosphere Research Satellite (WARS) This program's goal is to extend scientific understanding of the chemical and physical processes occurring in Eaxth's stratosphere, mesosphere, and lower thermosphere. Its primary objective is to observe the mechanisms that control the structure and variability of the upper atmosphere, the response of the upper atmosphere to natural and human-related perturbations, and the role of the upper atmosphere in climate and its variability. It wall use remote sensing instruments currently in development, including two in- struments being provided by British and French investigators, to measure trace molecule species, temperature, winds, and radiative energy input from and losses to the upper atmosphere. It also will make in situ measurements to determine magnetospheric energy inputs to the upper atmosphere. Plans include extensive inter- action among experimental and theoretical investigations, and an interactive central data facility with direct on-line access via re- mote terminals to facilitate that interaction among investigators. It is expected to fly in the early 1990s. Scatterometer Upper ocean currents, as well as surface waves, are gener- ated by the stress that winds exert on ocean surfaces. As earlier instruments aboard aircraft and Seasat have shown, a scatterom- eter can measure the small-scale roughness of a sea surface; the associated wind velocity, or stress, then can be calculated. Mod- ern oceanographic measurements show that ocean currents are

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82 much more variable than they previously were thought to be. An ability to obtain wind velocities will permit calculation of the velocities of the time-dependent, wind-driven, upper ocean cur- rents. Knowledge of those velocities will substantially improve understanding of the momentum coupling of the atmosphere and oceans. Knowledge of wind velocities also will improve forecasts of such factors as wave conditions and the intensity and loca- tion of storms. Scatterometer data would provide a unique global perspective of the oceans, significantly improving understanding of how the oceans work. A scatterometer is tentatively planned to be flown on the U.S. Navy's Remote Ocean Sensing Satellite (NROSS). Other plans include flight of a scatterometer aboard the European Space Agency's (ESA) ERS-1 satellite. Both the NROSS and the ERS-1 are expected to be launched in the early l990s. Ocean Topography Experiment for Ocean Circulation (TOPEX)/Poseidon The Ocean Topography Experiment, a joint U.S./French ini- tiative, is expected to provide significant capabilities for observing the circulation of the oceans on a global basis. Its objectives will be to measure ocean surface topography over entire ocean basins for several years, integrate those measurements with subsurface mea- surements, and use the results in models of the oceans' density fields to determine the oceans' general circulation and variability. The information from all those activities will be used to develop an understanding of the nature of ocean dynarn~cs, calculate the heat transported by the oceans, understand the interaction of cur- rents with waves, and test the capabilities available for predicting ocean circulation. TOPEX/Poseidon is planned to be launched on Ariane in 1991. Ocean Color tanager The success of the Coastal Zone Color Scanner, which was launched on Nimbu+7 in 1978 and now is in its eighth year of operation, clearly indicates that a follow-on instrument could de- termine global primary productivity, which forms the base for the various marine food chains. The synoptic, global measurements of chlorophyll concentration that a satellite color scanner can provide

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83 will serve as the primary data base to which complementary ship, airplane, and buoy data can be added to yield pr~rnary productiv- ity estimates of high accuracy for key oceanic regions. An improved version of the Coastal Zone Color Scanner, the Ocean Color Trnager, has been designed. Plans are being formu- lated to make it possible, for the first time, to relate wind forcing data acquired by a NASA scatterometer to data on ocean current response from the planned TOPEX/Poseidon mission, the redid tribution of oceanic nutrients by the currents, and the resulting changes in primary productivity from the Ocean Color Imager. With appropriate in situ observation, it will be possible to quanti- tatively relate biological variability to the physical characteristics of the global oceans. Shut tI - Spacelab Payload Basic processes in which electromagnetic energy and particle beams interact with plasmas occur in many systems within the universe, but can be studied most easily in the most accessible space plasma that near Earth. Spacelab's capabilities are well suited for making those studies. A beginning was made with the flight of the OSS-1 pallet, which used a small electron gun to study vehicle charging and wave generation. Spacelab 1 had a Japanese electron accelerator with pallet-mounted diagnostics, and a future Spacelab may include an electron gun and a plasma diagnostic package on a subsatellite. Under current planning is a more ambitious mission, called the Space Plasma Laboratory, on which those instruments will be joined by including a V[F- HF wave injection facility being developed in cooperation with Canada. Because of Spacelab's versatility, the mix of instruments can be changed between flights and the entire payload can be upgraded in an evolutionary fashion. Also planned is the assembly, into a single payload, of several solar radiance instruments (the French-developed Solar Ultraviolet Spectral Irradiance Monitor, the Active Cavity Radiometer, and the BeIgian-developed Solar Constant Variation instrument) and two atmospheric instruments (the Atmospheric Trace Molecule Spectroscopy experiment and Imaging Spectrometer Observatory).

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84 Tethered Satellite System The Tethered Satellite project is a cooperative undertaking be- tween the United States and Italy to provide a new facility for con- ducting earth science and applications experiments. The Tethered Satellite will make measurements as far as 100 km from the Space Shuttle. It will make possible long-term scientific exper~rnentation not heretofore feasible. This will include the generation and study of large-amplitude hydromagnetic waves, magnetic-field-aligned currents, and high-power, very low frequency and extremely low frequency waves in the ionosphere-magnetosphere system. It also will permit studies of magnetospheric-ionospheric-thermospheric coupling and atmospheric processes below 180 km; high-resolution crustal geomagnetic phenomena; and the generation of power us- ing a conducting tether. Italy has agreed to provide the satellite for the planned atmospheric (tethered downward) and space plasma (tethered upward) missions. Magnetic Field Satellite The first Magnetic Field Satellite, Magsat-l, acquired the ini- tial detailed, global data on the scalar aunt vector magnitudes of Earth's magnetic field. However, that field undergoes major changes over the period of a few years due to variations in the motions of the outer core. The position of the magnetic pole drifts westward, but the rate of drift is not constant. Resulting uncertainties in magnetic maps limit their usefulness to from 3 to 5 years. However, those changes provide information on im- portant and enigmatic properties of Earth, such as the origin of the main magnetic field and its variations with time; the struc- ture and electrical properties of the mantle; and the relationship among variations in the magnetic field, the mass distribution of the atmosphere, and the rotation rate. The Magnetic Field Ex- plorer will obtain scalar and vector field data that, in conjunction with data from Magsat-1 and the Geopotential Research Mission, will be used to examine magnetic field changes for periods ranging from months to decades. It also will provide an updated data set required for a future magnetic field survey.

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85 Geopotential Research Mmelon (GRM) Accurate knowledge of Earth's gravity and magnetic fields is essential to scientific studies of the planet, particularly those involving the solid Earth, the oceans, and energy and mineral resources. Earth's gravity field is known to an accuracy of 5 to 8 meal for resolutions of 500 to 800 km, and the geoid (mean ocean sea level) to an accuracy of about 50 cm. Those accura- cies are inadequate to resolve key scientific questions relating to the motion of Earth's crust (mantle convection) and the structure and composition of Earth's interior. Magsat-1 provided a map of crustal magnetic anomalies that showed a high degree of corre- lation with large-scale geological and tectonic features. However, its orbital altitude was too high to yield a map with the accuracy and resolution required for both solid earth science and geological prospecting. Greater accuracy and resolution are needed, and they can be achieved only by a mission at a significantly lower altitude. The Geopotential Research Mission will provide the most ac- curate models yet available of the global gravity field, geoid, and crustal magnetic anomalies. It wiD employ two spacecraft approx- imately 300 km apart in the same 160-km circular polar orbit. To determine the gravity field, a drag-free sphere will be positioned at the center of mass of each spacecraft in a cavity that will shield it from all surface forces and therefore permit it to be affected only by gravitational forces. The relative motion of the spheres as they are accelerated and decelerated while passing over a gravity anomaly will be a measure of the size and intensity of the anomaly. The accuracy to which the position of each sphere in the along-track direction can be measured by Doppler tracking wall be 1 Amps every 4 s. That accuracy in the Doppler data will permit analy- sis to determine the global gravity field to approximately ~ meal and the geoid to approximately 5 cm, both to a resolution of 100 km. Earth's magnetic field will be surveyed by scalar and vector magnetometers, similar to those flown on Magsat, mounted at the end of a rigid boom extending from the leading spacecraft. The magnetic field data will have an accuracy of 2 nT and a resolution of 100 km.

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86 Earth Obeervmg System (EOS) The Earth Observing System ~ an integrated set of exper- iments that builds on ad of the above to form the basis of the Mission to Planet Earth. It ~ described in the following chapter. COMPUTERS, COMMUNICATIONS, AND DATA MANAGEMENT The Space Science Board's Committee on Data Management and Computation (CODMAC) recently completed two extensive studies of space data issues. These reports, Data Management and ComputationVolume 1: Issues and Recommendations (1982) and Issues and Recommendations Associated with Distributed Computation and Data Management Systems for the Space Sci- ences (1986), address data issues over a broad range of space science disciplines. They conclude that data management prom lems account for many of the shortcomings in the science returns of space observation programs. The Task Group on Earth Sciences concurs with the CODMAC findings, and notes that the high data rates from earth observing satellites wiD strain the system more than any other discipline.