missions for measuring the gravity field (Antreasian et al., 1991; Bettadpur et al., 1992;
Gleason, 1991; Jekeli, 1993; Visser et al., 1994). The use of GPS positioning has been
planned for many of these missions, such as Aristoteles (ESA, 1991), which proposed to
carry a gravity gradiometer, and the Gravity and Magnetics Earth Surveyor (GAMES)
mission (Frey et al., 1993), which proposed to fly a satellite-to-satellite laser tracking
system. While GPS is not the primary gravity instrument for these missions, it both
provides strength for resolving the longest wavelengths of the gravity field, while also
providing an inertial position for the primary instrument. Unfortunately, neither the
Aristoteles or GAMES missions were approved, thus adding to a long list of unsupported
gravity missions that have been proposed over the last few decades. There are several
proposed satellite missions (Gravity Probe B (GP-B), Satellite Test of the Equivalence
Principle (STEP» designed to test different aspects of relativistic theory while also
improving the gravity field using the GPS receivers that they would carry, but due to the
satellite altitude, these improvements would only benefit the long wavelengths of the
model and would not satisfy the geoid accuracy requirements of oceanographers, which
has been one of the driving factors in the justification of a dedicated 'gravity mission.
Global gravity modeling using high-degree spherical harmonic expansions will improve
as more satellite tracking data, altimeter data, and surface gravity data becomes available.
A significant amount of surface gravity data has been unavailable because it has not been
released by the different governments who have control of these data. However, this is
beginning to change as previously unavailable data from many regions, especially Asia,
are becoming available (Kogan and McNutt, 1993; Makedonskii et al., 1994). In
addition, the Defense Mapping Agency (DMA) and NASNGSFC have recently
embarked on the development of a joint 360 x 360 gravity model (Nerem et al., this issue,
Rapp and Nerem, 1994) which will be based on nearly all of DMA's terrestrial gravity
holdings including newly available data from the former Soviet Union and GSFC's
comprehensive collection of satellite tracking data. This collaboration will likely result in
substantial improvements in our knowledge of the high resolution global gravitational
field.
SLR will continue to be essential for measuring temporal variations of the gravity field
given the passive simplicity of these orbit targets and the decadal span of precision data.
With the launch of Lageos 2 in 1992 and Stella in 1993, in combination with the older
geodetic satellites Lageos, Starlette, and Ajisai, estimates of the temporal variations of the
gravity field will undoubtedly be improved in accuracy, and spatial and temporal
resolution. Geophysical modeling of temporal gravity variations will also continue to be
important, since their combination with the satellite estimates will improve our
knowledge of the solid Earth, ocean, and atmosphere.
Future prospects for developing improved planetary gravity models will depend on the
availability of satellites from which tracking may be obtained. The Venus gravity model
will be improved as the Magellan post-aerobraking data set is more completely analyzed.
The gravity fields of Jupiter and its moons will be more accurately determined from
Galileo tracking data (Anderson et al., 1992; Schubert et al., 1994). Towards the end of
the decade, tracking data from the MGS mission will provide substantial improvements to
the gravity model of Mars. The Lunar Prospector mission should provide improved
lunar gravity field models towards the end of the decade.
REFERENCES
Anderson, I. D., 1. W. Armstrong, I. K. Campbell, F. B. Estabrook (1992), T. P. Krisher,
and E. L. Lau, Gravitation and celestial mechanics investigations with Galileo, Space
Sci. Rev., 60, 591-610.
Antreasian, P. G., I. B. Lundberg, and B. E. Schutz (1991), Simulation of the GRM drag
compensation system, I. Astron. Sei., 39(4), 487-518.
7
Gleason, 1991; Jekeli, 1993; Visser et al., 1994). The use of GPS positioning has been
planned for many of these missions, such as Aristoteles (ESA, 1991), which proposed to
carry a gravity gradiometer, and the Gravity and Magnetics Earth Surveyor (GAMES)
mission (Frey et al., 1993), which proposed to fly a satellite-to-satellite laser tracking
system. While GPS is not the primary gravity instrument for these missions, it both
provides strength for resolving the longest wavelengths of the gravity field, while also
providing an inertial position for the primary instrument. Unfortunately, neither the
Aristoteles or GAMES missions were approved, thus adding to a long list of unsupported
gravity missions that have been proposed over the last few decades. There are several
proposed satellite missions (Gravity Probe B (GP-B), Satellite Test of the Equivalence
Principle (STEP» designed to test different aspects of relativistic theory while also
improving the gravity field using the GPS receivers that they would carry, but due to the
satellite altitude, these improvements would only benefit the long wavelengths of the
model and would not satisfy the geoid accuracy requirements of oceanographers, which
has been one of the driving factors in the justification of a dedicated 'gravity mission.
Global gravity modeling using high-degree spherical harmonic expansions will improve
as more satellite tracking data, altimeter data, and surface gravity data becomes available.
A significant amount of surface gravity data has been unavailable because it has not been
released by the different governments who have control of these data. However, this is
beginning to change as previously unavailable data from many regions, especially Asia,
are becoming available (Kogan and McNutt, 1993; Makedonskii et al., 1994). In
addition, the Defense Mapping Agency (DMA) and NASNGSFC have recently
embarked on the development of a joint 360 x 360 gravity model (Nerem et al., this issue,
Rapp and Nerem, 1994) which will be based on nearly all of DMA's terrestrial gravity
holdings including newly available data from the former Soviet Union and GSFC's
comprehensive collection of satellite tracking data. This collaboration will likely result in
substantial improvements in our knowledge of the high resolution global gravitational
field.
SLR will continue to be essential for measuring temporal variations of the gravity field
given the passive simplicity of these orbit targets and the decadal span of precision data.
With the launch of Lageos 2 in 1992 and Stella in 1993, in combination with the older
geodetic satellites Lageos, Starlette, and Ajisai, estimates of the temporal variations of the
gravity field will undoubtedly be improved in accuracy, and spatial and temporal
resolution. Geophysical modeling of temporal gravity variations will also continue to be
important, since their combination with the satellite estimates will improve our
knowledge of the solid Earth, ocean, and atmosphere.
Future prospects for developing improved planetary gravity models will depend on the
availability of satellites from which tracking may be obtained. The Venus gravity model
will be improved as the Magellan post-aerobraking data set is more completely analyzed.
The gravity fields of Jupiter and its moons will be more accurately determined from
Galileo tracking data (Anderson et al., 1992; Schubert et al., 1994). Towards the end of
the decade, tracking data from the MGS mission will provide substantial improvements to
the gravity model of Mars. The Lunar Prospector mission should provide improved
lunar gravity field models towards the end of the decade.
REFERENCES
Anderson, I. D., 1. W. Armstrong, I. K. Campbell, F. B. Estabrook (1992), T. P. Krisher,
and E. L. Lau, Gravitation and celestial mechanics investigations with Galileo, Space
Sci. Rev., 60, 591-610.
Antreasian, P. G., I. B. Lundberg, and B. E. Schutz (1991), Simulation of the GRM drag
compensation system, I. Astron. Sei., 39(4), 487-518.
7