The Marine Geoid and Satellite Altimetry

Abstract

The marine geoid defines the hydrostatic equilibrium shape that sea level would take in the absence of tides, currents and winds. The geoid is not directly observable, but its height above a reference ellipsoid may be calculated from a model of the Earth’s gravity field. Satellite altimeters measure the instantaneous sea surface height, which is the sum of the geoid plus the dynamic topography associated with the ocean’s responses to tidal and atmospheric forcings. In the early days of altimetric oceanography, knowledge of the geoid was poor and so altimeters are usually placed in exact repeat orbits to facilitate observation of the temporal variations in sea surface height. Recent gravity field models have geoid height errors around 5 cm and geoid slope errors around 1.5 μrad. Errors in the models are primarily at full wavelengths shorter than 20 km, so that some dynamic topography signals may now be directly observed. While satellite gravimetry from missions such as GRACE has improved the long wavelength geoid, there is considerable geoid power at wavelengths too short to be measured by gravimetry at orbital altitude. Determination of the geoid at mesoscale and shorter wavelengths has come from altimetry. Careful filtering has allowed to separate the geoidal height and ocean dynamics signals in the altimeter data, and thereby to refine the marine geoid. Verification of this comes from comparison of marine gravity field models against ship gravimetry. Further improvements in the accuracy of the marine geoid will require a satellite altimeter mission with improved signal-to-noise ratio and a spatially dense (less than 5 km) network of groud tracks. One may hope that CryoSat-2 will offer some improvement.