Relative Vertical Positioning Using Ground-Level Transponders with the ERS-1 Altimeter

The microwave altimeters flown on the GEOS-3 and Seasat satellites have demonstrated the remarkable potential of the instrument for making global measurements of ocean currents and sea state. In this paper a technique is proposed for using the altimeter to be carried on the ESA ERS-1 satellite (launch date 1989) to measure the relative vertical position of land-based transponders and the ocean geoid on 2000-km-long segments of the satellite ground track. The absolute position accuracy will be similar to that of the laser tracking stations, relative accuracy achievable may be of the order of a few centimeters, and measurements of changes in transponder elevation during the three-year satellite life may be at the subcentimeter level. The opportunity for such measurements is provided by exploiting together, very accurate range measurements made to ground-based transponders and the demonstrated constancy of form of satellite, short-arc orbits repeated over an identical ground track. Altimetric vertical position measurements of this accuracy would make possible new techniques for altimeter calibration, sea-state bias measurements, orbit measurement, and seismic/geodetic study.