logical development. These designs offer high resolution at lower cost than other systems described in the past, due to improved technologies and the fact that very low orbits and expensive "drag-free" designs are no longer called for.

Two broad categories of mission designs, gravity gradiometry and satellite-to-satellite tracking, were considered. Both of the categories were subdivided into "generic" missions, based on the technology used and the mission duration. Gravity gradiometry, which measures the differences in acceleration of two masses within the same spacecraft, was divided into two missions:

• Spaceborne Gravity Gradiometry (SGG). An SGG mission would yield a significant improvement over current results to degree 155 (250 km wavelength) at an orbital height of 400 km and to degree 215 (180 km wavelength) at 300 km. However, currently estimated accuracies are poorer than for the satellite-to-satellite tracking missions for degrees less than 25 (1600 km wavelength). The launch vehicle allowable within a reasonable cost cap limits the size of the dewar for current gradiometer technology to one sufficient for only a year lifetime. Hence the SGG mission is of limited value for the study of temporal variability.

• Extended Spaceborne Gravity Gradiometry (SGGE). A mission using a larger launch vehicle and/or more miniaturized gradiometer could extend the lifetime to five years, and thus would permit the detection of temporal gravity variability on seasonal and interannual time scales. However, the accuracy would have to be improved to be competitive with the satellite-to-satellite tracking missions.

  Satellite-to-satellite tracking utilizes differential tracking of two satellites and thereby measures orbital perturbations; accelerometers are required to remove atmospheric drag effects. Three missions were considered:

• High-Low Microwave Tracking (GPS). In the immediate future, such a mission would depend on the Global Positioning System (GPS) for the high satellite. A mission flown at an altitude of 400-500 km would yield significant improvements over the best current Earth-gravity models at harmonic degrees less than 25 (1600 km wavelength), whereas a mission flown at 300 km would be useful for degrees up to 30 (1300 km wavelength). A system much more accurate than GPS would be of great value to geodesy and gravimetry and is technically feasible, but probably much more expensive than tolerable for these applications.

• Low-Low Satellite-to-Satellite Microwave Tracking (SST). The SST mission is highly accurate at long and moderate wavelengths (10,000 to 1600 km), at which it produces more accurate geoid heights and gravity anomalies than the SGG, SGGE, and GPS missions. Also, the mission lifetime (estimated to be five years) permits the effective determination of many important time-varying effects.

• Low-Low Satellite-to-Satellite Laser Interferometry (SSI). The anticipated results from the SSI mission are the best of the five scenarios studied. They are an order of magnitude better than the SST results at all wavelengths considered  and  two  orders of magnitude better than SGG at long wavelengths. However, both SST and SGG involve mature technologies, whereas SSI requires additional development (e.g., order of magnitude improvements are needed in accelerometer accuracy and laser-cavity thermal noise at low frequencies) and proof-of-concept, which would delay a mission some years compared with the other techniques.

• The SGG, SST, and SSI missions at an orbital altitude of 400 km offer dramatic improvements in the knowledge of the absolute dynamic topography and surface circulation that can be obtained from satellite altimetry. The most significant improvement will come at basin scales (300-3000 km), at which the geoid determination will be virtually eliminated as an error source. The orbital height needed to resolve smaller-scale phenomena, such as western boundary currents, would need to be impractically low. In situ techniques, such as airborne or shipborne gravimetry, both with GPS positioning and controlled acceleration environment, appears more practical. Studies in ocean regions with a strong barotropic component will benefit from knowledge gained from the static geoid. These include the recirculation cells in the subtropical gyres of the western Atlantic, the Kuroshio Current, the Agulhas Current, and the Antarctic Circumpolar Current.

• Most of what is known about the ocean occurs in the upper 500 meters. Studies suggest that uncertainties in the deep circulation and heat and mass transport will be reduced by a factor of two or more in oceanographic regions that are data sparse. Part of this reduction comes from an improvement in estimates of