Status of the geopotential

Significant improvements in the determination of the earth's potential have been achieved during the past four years. Radar altimetry data has provided a detailed geoid over most of the ocean surface while improvements and extensions of laser tracking networks and computer programs permitted increases in the accuracy and detail of the mathematical representation of the gravity field. Studies continued in methods of representation and in new data acquisition techniques. The GEOS‐3 satellite provided radar altimeter observations to the ocean surface along satellite sub‐tracks typically spaced at one degree intervals of longitude at the equator. Observations within the latitude range of the satellite of ±65° were telemetered to permanent sites of the National Aeronautics and Space Administration and to mobile sites deployed by the Department of Defense. At the permanent sites, track spacings are closer than one degree while data relay through ATS‐F satellite provided some coverage in areas out of range of sites at which mobile equipment could be deployed. The precision of the altimeter measurements is typically 60 cm at a one second data rate which permitted determinations of undulations of the geoid with wave lengths greater than about 100 miles. Since the accuracy of the computed satellite orbits was poorer than the precision of the altimeter, except in calibration areas, bias corrections to the satellite position were obtained by minimizing the differences between computed geoid heights at the intersections of a satellite sub‐tracks on different passes over the same area. Some analysts obtained these bias corrections with respect to orbits initially accurate to one to two meters radially (Anderle 1978) while others (Blaha 1977, Hadgigeorge and Trotter 1977, Rapp 1976a, Rummel 1976) eliminated dependence on orbit accuracy entirely by fitting the altimeter data to low order geoids. G. Hadgigeorge (private communication 1977) recovered the same geoid using orbit data arbitrarily biased by 100 m in position and 1 mps in velocity at an epoch time. Contours of the ocean geoid and/or gravity anomalies inferred from altimeter data have been derived by Brace (1977), Hadgigeorge (in press), Kahn et.al. (in press 1978) Rapp (1976a, 1976c, 1978), Wong (1978), Yionoulis et.al. (1978). The accuracy of the geoid is estimated to be l m while the accuracy of the mean gravity anomalies range from 3 mgals for five degree square sizes to 8 mgals for one degree square sizes. While the SEASAT‐I satellite suffered a power failure, sufficient altimetry data was acquired prior to the failure to provide ocean geoid data at a grid size somewhat finer than one degree, over a latitude range of ±72°, and with a shorter wavelength resolution of geoid undulations due to the higher precision of the SEASAT altimeter. Satellite‐to‐satellite tracking data between the GEOS‐3 or Apollo‐Soyuz and the ATS‐6 satellites (Hajela 1977, Marsh et.al. 1977, Vonbun et.al. 1978., Wells and Hahn 1978, Wong 1978) have been used to recover mean gravity anomalies to an accuracy of 5 to 7 mgals for five degree square sizes.