GRAVITY-FIELD DETERMINATION AND CHARACTERISTICS - RETROSPECTIVE AND PROSPECTIVE

Gravimetry has had a long history, using pendulums, torsion balances, and static spring gravimeters. Relative accuracy adequate for many geo physical problems was already attained by 1900, but it took another ha lf century to build readily portable gravimeters. Calibration and datu m definition remained problems until the 1970s when free-fall absolute gravimeters were developed that now have a precision of 10(-3) mGal. The problems of geographic inaccessibility and field party costs (nota bly in areas of greatest tectonic interest) are now being overcome by airborne gravimetry that has already achieved accuracies of 1-3 mGal w ith resolutions of 10 to 20 km. Satellite techniques are the best way to determine the long-wavelength variations of the gravity field. The resolution of the models has steadily improved with the number of sate llites and the precision of the observations. The best current model i ncludes tracking data from more than 30 satellites, satellite altimetr y, and surface gravimetry and has a resolution of about 290 km (harmon ic degree 70) with the most recent improvements coming from Doppler or bitography and radiopositioning integrated by satellite (DORIS) tracki ng of the SPOT 2 satellite and satellite laser ranging (SLR), DORIS, a nd Global Positioning System (GPS) tracking of the TOPEX/POSEIDON sate llite. Meanwhile, radar altimetry has become the dominant technique to infer the marine geoid with a resolution of tens of kilometers or sho rter. Similarly, the gravity fields of the Moon, Venus, and Mars have been determined to harmonic degrees 70, 75, and 50, respectively, alth ough tracking limitations result in variations of spatial resolution. Modeling Earth's gravity field from the abundance of precise data has become an increasingly complex task, with which the development of com puter capacity has kept pace. Contemporary solutions now entail about 10,000 parameters, half of them for effects other than the fixed gravi ty field of Earth. Temporal variations arising from tides have long be en well modeled, and nontidal changes are now being identified. The im provement in gravitational models engendered corresponding advances in geophysical interpretation. Isostatic models were refined and expande d to account for regional thermal and tectonic histories. Interpretati on of the long-wavelength gravity field determined by satellite techni ques has been mainly in terms of plate tectonics as a manifestation of mantle convection. Gravity has been significant in inferring that the re must be a large increase in viscosity with depth (most strongly, fr om the apparent slow sinking of subducted slabs). The prospects for in creasing accuracy and resolution in the determination of Earth's gravi ty field rest primarily with the development of new measurement system s. Airborne gravimetry is taking promising new steps using GPS, but si gnificant global model improvement awaits a dedicated satellite gravim etry system, and future satellite altimeter missions will do more for ocean dynamics studies than geoid improvement. Advances in interpretat ion will occur through the development of other data, such as seismic tomography, and larger-scale computer modeling of tectonics and convec tion.