Monitoring the TOPEX and Jason-1 Microwave Radiometers with GPS and VLBI Wet Zenith Path Delays

Monitoring of altimeter microwave radiometer measurements is necessary in order to identify radiometer drifts or offsets that if uncorrected will introduce systematic errors into ocean height measurements. To examine TOPEX Microwave Radiometer (TMR) and Jason-1 Microwave Radiometer (JMR) behavior, we have used coincident wet zenith delay estimates from Very Long Baseline Interferometry (VLBI) and Global Positioning System (GPS) geodetic sites near altimeter ground tracks. We derived a TMR path delay drift rate of -1.1 +/- 0.1 mm/yr using GPS data for the period from 1993.0-1999.0 and -1.2 +/- 0.5 mm/yr using VLBI data. Thereafter, the drift appears to have leveled off. Already after 2.3 years (82 cycles) of the Jason-1 mission, it is clear that there have been significant systematic errors in the JMR path delay measurements. From comparison with GPS wet delays, there is an offset of -5.2 +/- 0.6 mm at about cycle 30 and a more abrupt offset of -11.5 +/- 0.8 mm at cycle 69. If we look at the behavior of the JMR coldest brightness temperatures, we see that the offsets near cycle 30 and cycle 69 are mainly caused by corresponding offsets in the 23.8 GHz channel of -0.49 +/- 0.12 K and -1.18 +/- 0.13 K, although there is a small 34.0 GHz offset at cycle 69 of 0.75 +/- 0.22 K. Drifts in the 18.0 and 34.0 GHz channels produce a small path delay drift of 0.3 +/- 0.5 mm/yr.