GPS will undoubtedly play an important role in Earth gravity field determination in the coming years. Due to the higher altitude of TOPEX and the fact that current gravity models have such small errors, it would be desirable to have a GPS receiver flown on a satellite in a lower orbit (< 500 km), where due to less attenuation, substantial improvements in the gravity field model should be realized. Geodetic GPS receivers are planned for several upcoming missions including GPS/Met (U.S.), SAC-C (Argentina), Oersted (Denmark), and Sunsat (South Africa) although the primary role of these missions is not to determine the gravity field, significant gravity field improvements will be gained nonetheless. GPS will also play a pivotal role in the development of airborne gravity measurements. In addition, if a dedicated satellite mission to measure the gravity field is ever approved, GPS will certainly play an important role. Other global tracking techniques such as DORIS and PRARE, depending on the type of orbits they support, will play a role as well.
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 NASA/GSFC have recently embarked on the development of a joint 360 x 360 gravity model [ Rapp and Nerem, 1994; Kunz et al., 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 the most accurate high resolution global gravity model available to date.
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. The development of realistic eddy resolving multi-layer ocean models forced by real wind fields will be an important resource over the next few years for determining the role played by the atmosphere and oceans in the excitation of the Earth's gravity and rotational variations [Rosen, 1993]. Future improvements in the long-wave-length models of the time-invariant gravity field will depend on accurately modeling or estimating both the tidal and non-tidal gravity variations, as the current uncertainties for the low degree coefficients are at the same level as the known temporal variations of the coefficients. While tidally-induced variations have been modeled for many years, the importance of modeling non-tidal variations in gravity is only now becoming apparent.
The development of the commercially available FG5 absolute gravity
meter will clearly make precise gravity measurements more
readily available. Absolute gravity will also benefit from
increased participation by the international community. The
International Gravity Commission (of the IAG) currently is directing
the establishment of an International Absolute Gravity Base Station
Network (IAGBN) with a goal of achieving better than 10-7 m/s
accuracy at each of 36 sites around the world.
Future prospects for developing improved planetary gravity models will depend on the availability of satellites from which tracking data 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. If they are actually approved, proposed missions to Pluto and Mercury would also provide data for gravity investigations, but these are unlikely to occur before the end of the decade. There have also been discussions about a dedicated lunar gravity/topography mission employing sat-ellite-to-satellite tracking, but these plans have not progressed beyond the preliminary planning stages.
Probably the most frustrating aspect of gravity science in the past decade has been the failure to launch a satellite mission dedicated to studying the Earth's gravity field. Several missions have been studied over the years, but none have ever formally been approved and flown. A dedicated satellite mission to study the gravity field is needed if any dramatic improvements in the gravity field are to be gained, especially for improving the model of the marine geoid for ocean circulation investigations using altimetry.
Acknowledgments. The Library at NASA/Goddard Space Flight Center provided a comprehensive list of references which supported the preparation of this article. Numerous colleagues provided references and suggestions which helped improve this report.