Maps of Altimetric Gravity Based on ERS-1 Geodetic Phase Data

Abstract

Maps of high-resolution gravity anomalies are derived from vertical deflection grids, which are computed in a first solution from a high-density ERS-1 mean sea surface height model. For generation of high-quality sea surface heights, GFZ/D-PAF has reprocessed the ESA standard altimeter product (ERS.ALT.OPRO2). This process includes improved orbit calculation, the replacement of standard geophysical corrections and the application of additional range corrections. A second solution is based on the computation of vertical deflections derived from gradients of the 1-Hz altimeter measurements in order to keep the high frequency signal of the original data.

The computation of gravity anomalies from deflections of the vertical is performed by the 2-dimensional Fast-Fourier transformation (FFT), which is based on the plane approximation of the earth. The error due to this assumption can be minimized by first subtracting a long wavelength spherical harmonic geoid model from the gridded sea surface heights, before generating the vertical deflection grids. The gravity anomalies now result from addition and multiplication operations of the Fourier transformed vertical deflections, followed by the inverse FFT. A piecewise application of this method combined with the elimination of the edges of each computation area suppresses truncation effects. To obtain the final result, the long wavelength part is added back to the computed residual gravity anomalies.

A global map of gravity anomalies was generated which served as one of the input data sets for computing the high-resolution gravity model GFZ96. Quality tests are performed for an area along the Mid-Atlantic Ridge (30.5°–36.5°S), where the altimetric derived anomalies from both solutions are compared with anomalies from the gravity maps of Marks et al. [1993] and of Smith & Sandwell [1995], and additional with validated shipboard gravity data [Neumann et al. 1993].