Satellite Radar Altimetry for Determination of Geoid Undulations and Gravity Anomalies

Doktorsavhandling, 1996

The sea level changes both temporally (due to dynamic effects) and spatially (due to sea bottom topography and inhomogeneities in the Earth's crust). Satellite radar altimetry is a remote sensing technique which can be used for e.g. determination of the instantaneous and mean sea level, and gravity anomalies at sea. A radar pulse is transmitted vertically to the Earth's (sea) surface and reflected to the satellite's antenna. Thus, the distance between the satellite and the surface can be determined with centimetre-accuracy. Various applications of altimetry exist among the disciplines of oceanography, geodesy, geophysics and geology. An interesting commercial application is the determination of gravity anomalies associated with oil and gas deposits in the upper crust of the Earth. This thesis describes methods for averaging repeated measurements and to improve precision and resolution of the measured sea level undulations. It demonstrates ways of processing the altimeter data to obtain geoid undulations and gravity anomalies, and discuss some of the error sources in altimeter measurements. It is shown that the time invariant sea level can be determined with an along-track spatial wavelength resolution of 19 km, if the polynomial adjusted averaging method is used with 0.7 km spaced Geosat data. The radial precision is estimated to be 1.5 cm. The along-track resolution of Geos-3 is improved by 50% to 40 km for a data set with almost repeated tracks using cross-track averaging. The method is applicable for any data set with few or no exact repeats but with many adjacent and parallel passes. One-dimensional projections of the gravity anomaly field obtained from altimetry are shown to correlate well with accurate in situ gravity data. Altimetry is shown to contribute valuable information to data sets with both altimetry and ship-board or other in situ gravity measurements, also in cases where the latter are more accurate than altimetry. Local gravity fields are determined with combined data sets in different areas and a characterization of the gravity anomaly field in the Gulf of Bothnia is presented. One of the most serious error sources in the altimetric signal is tropospheric water vapour. The impact of water vapour in a stable humid region is analyzed and quantified in terms of artificially created (erroneous) geoid undulations. Such anomalies can occur if the altimeter data are not corrected for the water vapour.