Calibration validation for the GEOS 3 altimeter

An absolute calibration of the altitude data is needed for some applications of the large quantity of altimeter data taken by the GEOS 3 intensive mode altimeter. The considered calibration technique is based on the use of a gravimetric geoid model and satellite passes which are nearly overhead at island laser tracking sites. Near overhead passes of GEOS 3 were achieved at Bermuda. Two of these passes were tracked by the Bermuda laser. The reported investigation has the objective to obtain a best estimate of the GEOS 3 altimeter calibration bias, using the two available passes and reconciling differences between them. Assuming that orbits with accurate altitudes over Bermuda are obtained, discrepancies between the two passes could be due to incorrect time tagging of the altimeter data. To assist in the resolution of the timing question, additional GEOS 3 passes through the calibration area were selected. On the basis of the obtained results, it is recommended to add 10.24 ms to GEOS 3 altimeter time tags before using the data.