Contributions of Satellite Laser Ranging to Past and Future Radar Altimetry Missions

Abstract

Satellite laser ranging (SLR) has proven a very efficient method for contributing to the tracking of altimetric satellites and determining accurately their orbit although hampered by the non-worldwide coverage and the meteorological conditions. Indeed, in some cases it is the only method available to determine the satellite orbit (e.g., the orbits of the ERS-1 and Geosat-Follow-On missions). Moreover, any operational and non-weather dependent techniques, like GPS, DORIS, PRARE, can exhibit systematic errors in positioning and orbitography. A comparison with SLR results allows to evidence such errors and vice versa. For doing that, two different approaches for determining precise orbits can be considered: one based on global orbit determination, the other on a short-arc technique used to locally improve a global orbit determined by another tracking techniques, such as DORIS or GPS. We can thus validate a global orbit and achieve orbit quality control to a level of 2 to 3 centimeters at present and expect to achieve a level of 1 to 2 centimeters in the near future. Errors induced by station coordinates or by the gravity field (geographically correlated errors, for example) can be estimated from SLR tracking data. Colocation experiments with different techniques in the same geodetic site play also a key role to ensure precise relationships between the geodetic reference frames linked to each technique. In particular, the role of the SLR technique is to strengthen the vertical component (including velocity) of the positioning, which is crucial for altimetry missions. The role of SLR data in the modelling of the first terms of the gravity field has finally to be emphasized, which is of primary importance in orbitography, whatever the tracking technique used. Another application of SLR technology is the satellite altimeter calibration. Examples of past calibration and future experiments are given, including the accuracy we can expect from the Jason-1 and EnviSat space oceanography missions.