High-Accuracy, High-Resolution Gravity Profiles from 2 Years of the Geosat Exact Repeat Mission

Sandwell, DT, McAdoo DC.  1990.  High-Accuracy, High-Resolution Gravity Profiles from 2 Years of the Geosat Exact Repeat Mission. Journal of Geophysical Research-Oceans. 95:3049-3060.

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Satellite altimeter data from the first 44 repeat cycles (2 years) of the Geosat Exact Repeat Mission (Geosat ERM) were averaged to improve accuracy, resolution and coverage of the marine gravity field. Individual 17-day repeat cycles (two points per second) were first edited and differentiated resulting in alongtrack vertical deflection (i.e., alongtrack gravity disturbance). To increase the signal to noise ratio, 44 of these cycles were then averaged to form a single, highly accurate vertical deflection profile. The largest contributions to the vertical deflection error is short-wavelength altimeter noise and longer-wavelength oceanographic variability; the combined noise level is typically 6 μrad. Both types of noise are reduced by averaging many repeat cycles. Over most ocean areas the uncertainly of the average profile is less than 1 μrad (0.206 arcsec) which corresponds to 1 mgal of alongtrack gravity disturbance. However, in areas of seasonal ice coverage, its uncertainty can exceed 5 μrad. To assess the resolution of individual and average Geosat gravity profiles, the cross-spectral analysis technique was applied to repeat profiles. Individual Geosat repeat cycles are coherent (>0.5) for wavelengths greater than about 30 km and become increasingly incoherent at shorter wavelengths. This Emit of resolution is governed by the signal-to-noise ratio. Thus when many Geosat repeat profiles are averaged together, the resolution limit typically improves to about 20 km. Except in shallow water areas, further improvements in resolution will be increasingly difficult to achieve because the short-wavelength components are attenuated by upward continuation from the seafloor to the sea surface. These results suggest that the marine gravity field can be completely mapped to an accuracy of 2 mgal and a half-wavelength resolution of 12 km by a 4.5-year satellite altimeter mapping mission.