A global grid of high-resolution gravity anomalies based on Geosat and ERS-1 altimetry

A global high-resolution marine gravity field was derived by processing
upgraded altimeter data from the Geodetic Missions of Geosat (April 1985 to
September 1986) and ERS-1 (April 1994 to March 1995). After the generation of
crossover points, along-track sea surface derivatives were calculated at each 1-Hz
altimeter point. The long-wavelength part of the gravity field GFZ95A was first
removed from the sea surface heights. The first derivatives of the sea surface profiles
in direction of the crossing tracks were interpolated at the altimeter points by using
the slope information from the crossover points. The residual sea surface slopes
were converted to east and north vertical deflections, which were then interpolated
onto a regular grid with a 2' by 2' resolution. By means of the 2-dimensional Fast-
Fourier Transformation (FFT) the deflections were loss-pass filtered and the residual
gravity anomalies were calculated in subsquares. After the inverse FFT of each
subsquare, the global anomaly grid was merged and finally, the long-wavelength
part was added to recover the complete marine gravity field. Quality tests were
performed by comparing the altimetry derived anomalies with anomalies from the
gravity maps of several authors and in addition, with validated and adjusted
shipboard gravity data.