An alternative procedure for the estimation of the altimeter bias for the Jason-1 satellite using the dedicated calibration site at Gavdos

The dedicated calibration site for satellite radar altimeters in Gavdos has been operational since 2004. The small island of Gavdos is located along a repeating ground track of Jason satellites (crossover point No.109 ascending and No.18 descending pass and adjacent to Envisat), and additionally where the altimeter and radiometer footprints do not experience significant land intrusion. The purpose of such permanent Cal/Val facility is to calibrate the sea-surface height and ancillary measurements made by the satellite as it passes overhead, by using observations from tide gauges, GPS, DORIS and other sensors directly placed under the satellite ground tracks. Up to now, altimeter calibration at Gavdos has been performed by averaging gridded sea-level anomalies as produced by the satellite altimeter measurements and then comparing the result with the tide gauge observations. The absolute altimeter bias has thus been previously estimated to be +121 mm ±10 mm for Jason-1 satellite. In this work, the absolute altimeter bias of Jason-1 has been determined using (1) approximately one year of sea-surface height observations; (2) the GDR-B altimeter record and the seasonal effects of sea level; and (3) an alternative approach for calibration by interpolating each individual altimetric correction given by the satellite at its point of closest approach over the corresponding value of the tide gauge observation.