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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 system—follow-on
missions (NOAA K,L,M)
GOES: Geostationary
Operational Environmental
Satellite system—follow-on
missions (e.g., GOES-Next)
RADARSAT—Canadian
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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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
Computation—Volume 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.
Representative terms from entire chapter:
radiation budget
