Evaluation of GEOSAT data and application to variability of the northeast Pacific Ocean

Abstract

Part of the N.E. Pacific was studied to evaluate and use altimetric data from the Navy Geodetic Satellite GEOSAT. The zero-order accuracy of the major GEOSAT geophysical data record (GDR) channels was verified, and occasional gaps in the altimeter coverage were noted. GEOSAT's 17-day repeat orbit allowed use of collinear-track processing to create profiles of the difference between the sea surface height along a given satellite repeat, and the mean sea surface height along that repeat's groundtrack. Detrending of sea surface bias and tilt on each repeat reduced orbit and other long wavelength errors in the difference profiles. Corrections provided on the GEOSAT GDR were examined for their effects on the difference profiles of three test arcs. It was found that only the ocean tide, electromagnetic bias, and inverted barometer corrections varied enough over the arc lengths (approx. 4400 km) to have any noticeable effect on the difference profiles. Only the ocean tide correction was accurate enough to warrant using it to adjust the sea surface heights. Recommended processing of GEOSAT data for the area included making the ocean tide correction, three-point block averaging successive sea surface heights, and forming the mean height profiles from 18 repeat cycles (to reduce aliasing of the M2 tidal component), A set of difference profiles for one GEOSAT arc indicated that a reasonable estimate of GEOSAT's system precision was approx. 4.5 cm (RMS). The mid wavelength range (100-500 km) of these profiles was found to be the only range in which oceanic mesoscale features could be separated from altimeter errors