The most significant recent development in the area of gravity measurement techniques has been the use of the Global Positioning System (GPS) tracking data for precisely measuring gravitational perturbations on near-Earth spacecraft and for precisely positioning aircraft/spacecraft carrying gravimeters and gravity gradiometers. GPS provides nearlycontinuous tracking measurements between the receiver and each visible GPS satellite simultaneously [ Blewitt, 1993]. In addition, a vast permanent network of ground GPS receivers supports this effort by directly tracking the GPS satellites themselves. The advancement of airborne geophysics using GPS is reviewed by Brozena [1991] and it Bell [1995, this issue], and has been demonstrated over Greenland [ Forsberg and Brozena, 1993; Brozena and Peters, 1994]. While a number of medium accuracy GPS receivers have been flown on satellites in the past, the development of a precise space-based GPS geodetic receiver was finally demonstrated with the receiver carried on the TOPEX/POSEIDON altimeter mission [ Melbourne et al., 1994; Yunck et al., 1994]. The radial accuracy of TOPEX/POSEIDON orbits computed using the GPS data has been estimated at 2-3 cm root-mean-square (RMS) [ Bertiger et al., 1994], which is similar in accuracy to the precision orbits computed using satellite laser ranging (SLR) and Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) tracking data [ Tapley et al., 1994b]. The GPS data, through ``reduced dynamic'' analysis strategies [ Yunck et al., 1994] can be analyzed using a method which is far less sensitive to satellite orbital force modeling errors, such as those arising from errors in the gravity model. While the TOPEX/POSEIDON radial orbit errors caused by the latest gravity models are small (2 cm RMS [ Nerem et al., 1994b]), further reduction of these errors using the GPS data has recently been demonstrated [ Schutz et al., 1994; Tapley et al., 1994a] by incorporating the data in the models.
Although a mission has not yet flown, there continues to be a number of studies of dedicated missions for measuring the gravity field [ Antreasian et al., 1991; Bettadpur et al., 1992; Gleason, 1991; Jekeli, 1993]. 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 unsuccessful gravity missions that have been proposed over the last few decades. There are several proposed satellite missions (Gravity Probe B (GP-B) and 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. However, due to their satellite altitudes, 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.
Despite the potential that GPS has brought to gravity field determination, some limitations have developed due to the implementation of the security features called Selective Availability (S/A) and Anti-Spoofing (A/S) by the U.S. Department of Defense (DoD). While scientists have largely overcome the effects of S/A, the effects of A/S have proven to be more difficult to circumvent, especially for kinematic applications such as aircraft positioning. Undoubtedly, this will require geodesists to acquire ``Y-code capable'' receivers for precise gravity applications, which removes the effects of A/S, but also introduces restrictions associated with the classification of the Y-code. The next several years will certainly see increased use of Y-code receivers for satellite and aircraft missions where the highest precision is required. Codeless receivers will also provide the means for making precise geodetic measurements in the presence of the security features.
SLR has historically provided the bulk of the tracking data used for determining the long wavelengths of the gravity field [ Lerch et al., 1993b], however its relatively poor spatial and temporal distribution has limited its utility for measuring the shorter wavelengths. In addition to GPS, two additional tracking systems have become available during the last four years which can provide improvements to the gravity field. One system that has been very successful in the 1990s is the French DORIS tracking system [ Nouel et al., 1988]. DORIS provides nearly continuous dual-frequency rangerate measurements between the satellite and a network of more than 50 ground stations with a precision of 0.4 mm/sec [ Watkins et al., 1992]. The tracking data are collected on-board the satellite and downlinked to a single ground station. Substantial improvement in the gravity field has been obtained using DORIS tracking of the SPOT-2 [ Nerem et al., 1994a; Tapley et al., 1991] and TOPEX/POSEIDON [ Nerem et al., 1994b] satellites. The primary limitation of the current DORIS system is that the satellite can track only a single ground station at a time. The use of DORIS on a lower altitude mission may provide significant gravity modeling improvements if non-gravitational forces can be properly modeled. The German Precise Range and Range-rate Equipment (PRARE) system [ Wilmes and Reigber, 1989] will provide 2-way range and range-rate measurements at the S (2.2 Ghz) and X (8.5 GHz) band frequencies from a soon to be deployed global network of tracking stations. PRARE also has the capability to track multiple stations simultaneously. Unfortunately, the initial implementation of PRARE on the ERS-1 satellite was unsuccessful due to a hardware failure, so geodetic demonstration of the system will be delayed until the launch of ERS-2, although limited tracking of the Russian Meteor satellite is currently being demonstrated with a limited set of ground stations.
Satellite altimetry is the principal technique used to define the short wavelengths of the marine gravity field in current models [ Rapp et al., 1991; Rapp and Basic, 1992]. ERS-1 altimetry and the release of Geosat Geodetic Mission (GM) altimetry south of 30¡S latitude have led to significant advances in our knowledge of the short wavelengths of the marine gravity field [ McAdoo and Marks, 1992a; 1992b; Sandwell, 1992; Small and Sandwell, 1992; Livermore et al., 1994], especially south of 30¡S latitude. The model of the gravity field in the Arctic Ocean has been significantly improved using ERS-1 altimeter measurements over sea ice in an innovative paper by Laxon and McAdoo [1994] . The failure of the DoD to release Geosat GM altimetry north of 30S latitude has been a major disappointment for marine geophysicists and is currently under review. Fortunately, the 168 day repeat orbit being flown by ERS-1 is already eclipsing the unique need for the Geosat GM data [ Sandwell et al., 1994].
The last several years have also seen improvements in
absolute gravimeters with accuracies of several microgals (1 gal =
1 cm/sec
) being demonstrated [ Klopping et al., 1991;
Sasagawa and Zumberge, 1991; Niebauer and Faller, 1992;
Peter et al., 1993; Moody and Paik, 1993]. The primary goal
of these studies is the precise measurement of vertical crustal
motions, which along with geometric measurements provided by GPS, Very
Long Baseline Interferometry (VLBI), and SLR, will allow the
discrimination of different phenomena causing the motion. The
recent development of the commercially available ``FG5'' absolute
gravity meter [ Carter et al., 1994] should provide
measurement accuracies approaching 1 microgal, greatly improving the use
of this method for crustal studies.