Altimeter methods in satellite geodesy

The application of satellite-borne radar altimeters to the field of satellite geodesy and satellite orbital motion has been investigated. The altimeter measurement, the height of the satellite above the mean ocean surface, can be used directly or by differencing the measurements at the points where the orbit ground tracks intersect. The latter case is referred to as a "crossover measurement." The methods of utilizing these measurements in the estimation of orbit and geodetic parameters have been applied to data obtained by the oceanographic satellite, Seasat, during the 18-day period from 28 July to 14 August, 1978. Because of the near 17-day repeat cycle of the orbit ground track, this data set constitutes a reasonably uniform global distribution over the ocean surface. Supplementing the altimeter data with laser range data to provide overland coverage, a single continuous ephemeris spanning the 18-day interval resulted in a laser range residual RMS of 1.06 m and a crossover residual RMS of 1.04 m using the Goddard Space Flight Center PGS-S4 gravity field. This ephemeris was used as a reference to evaluate the time tag accuracy of the altimeter measurements, as well as to estimate geodetic parameters including the spherical harmonic coefficients which represent the gravity field of the earth. Using laser range, altimeter and crossover measurements, a batch estimation procedure based on a square root formulation of the least squares problem using Givens rotations was used to estimate two sets of spherical harmonic coefficients of degree and order 14 and 26. The 18-day orbit computed with the solved degree and order 26 gravity field has an ephemeris accuracy in terms of laser range residual RMS of 0.89 m, altimeter residual RMS of 1.36 m, and altimeter crossover residual RMS of 0.48 m, respectively. The crossover residuals reflect the influence of remaining radial orbit errors and temporal sea surface topography effects, whereas the altimeter residuals reflect the remaining geoid errors and short wave length ocean topography effects which cannot be accommodated in the estimated gravity field. The estimated orbit and geodetic parameters have been used to compute a mean ocean surface model which is compared to other similarly determined surfaces. Also obtained is the earth's mean equatorial radius and its flattening