Oceanwide Prediction of Gravity Anomalies and Sea Surface Heights Using Geos-3, Seasat, and Geosat Altimeter Data and ETOPO5U Bathymetric Data
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Date
1992-02
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Ohio State University. Division of Geodetic Science
Abstract
Satellite altimeter data (Geos-3, Seasat and Geosat) and bathymetric data have been used to calculate an ocean wide set of free-air gravity anomalies and geoid undulations. An averaged, over one year ( 1986/1987), Geosat sea surface height track, based on improved, over GEM-T2, orbits was used as the master altimeter frame. The Geosat track was based on the averaging of 22 ERMs and taking into account geoid gradients (Wang and Rapp, 1992). In the computation the altimeter sea surface heights were reduced to geoid undulations using an Ohio State long wavelength spherical harmonic representation of sea surface topography (dynamic heights). The altimeter data in a region, usually a 8° x 8° area, was subjected to a bias alone crossover adjustment with a fitting to the geoid undulation implied by the OSU91 potential coefficient model. The average Geosat crossover discrepancies after this adjustment were ± 4 cm with poorer agreement with Seasat data (± 10 cm) and Geos-3 data (± 46 cm). The anomalies and undulations were predicted in a 0°.5 cell using a two component remove/restore technique. The first part was the use of the OSU91A potential coefficient model to degree 360 as a reference while the second part involved the computation of the residual terrain model (RTM) effect on both anomalies and geoid undulations using the 5' x 5' ETOPO5U data set and a Fourier computation program from Forsberg. The incorporation of the bathymetric data introduces a strength to the solution, especially in areas where the altimeter coverage is sparse. The initial predictions were subjected to several editing steps. Contour plots of the undulations were examined to identify obviously poor prediction. The predictions were also examined relative to magnitude and predicted standard deviations so that unacceptable predictions could be removed. A second edit was carried out by comparing predicted along track gradients along a Geosat track for comparison with observed values. Such comparison revealed significant discrepancies in some areas. The first edit predictions were then modified to correct most, but not all, of the problems. The final data set contained 2,312,964 geoid undulation and 2,325,669 gravity anomalies, on a 0°.125 grid, with standard deviations for each point. A mean sea surface can be created by applying a sea surface topography correction to the predicted geoid undulation, and taking into account a proper tide reference system. The predicted point values were used to create 30' x 30', 1 ° x 1 °, and 5° x 5° mean gravity anomalies and geoid undulations. The 1° x 1 ° anomalies were compared to terrestrial anomaly data with an agreement of ± 9.2 mgals which is 14% less than with the previous (Hwang, 1989) prediction. A detailed description of a number of different data sets developed in this report are described in an appendix.