MONITORING THE TOPEX MICROWAVE RADIOMETER WITH GPS - STABILITY OF COLUMNAR WATER-VAPOR MEASUREMENTS

Using data from terrestrial global positioning system (GPS) receivers, we describe an anomalous drift in columnar water vapor measurements f rom the TOPEX microwave radiometer (TMR). Based on their long-occupati on histories and proximity to open-water TOPEX/POSEIDON (T/P) ground t racks, we identified four geodetic-quality GPS stations that are suita ble for calibrating the TMR. We constructed time series at each site b ased on the differences of instantaneous vertical wet tropospheric pat h delay (PDwet) derived independently from GPS and TMR data at T/P ove rflight times. The ensemble results span 1992-1997 and suggest that th e TMR measurements of PDwet are gradually drifting lower. Our nominal estimate of the drift is -1.2 +/- 0.4 mm yr(-1) (one standard error). Accounting for this would increase the estimated rate of change in glo bal mean sea level from T/P by the same amount.