A COMPARISON OF THE BRIGHTNESS TEMPERATURES AND WATER-VAPOR PATH DELAYS MEASURED BY THE TOPEX, ERS-1, AND ERS-2 MICROWAVE RADIOMETERS

Nadir-looking microwave radiometers are flown on altimeter missions to correct the altimeter range for water vapor path delay. This paper de scribes a technique to intercalibrate the brightness temperatures of t he ERS-1 and ERS-2 microwave radiomrters by using the TOPEX radiometer as a common reference. The technique is based on the analysis of radi ometer measurements made at crossover points between TOPEX, ERS-1, and ERS-2 orbits with less than 1-h time lags. This provides about 1000 c omparison points within 4 months and covers most atmospheric and ocean ic states. It is shown first that the ERS-2 radiometer brightness temp eratures need to be corrected to make them consistent with ERS-1 and t hus to ensure the homogeneity of the series of ERS altimeter data. The accuracy of the method is then estimated from the results of a 3-yr T OPEXIERS-1 data comparison and is shown to be about 0.2 K for the ERS- 2 23.8-GHz channel and about 0.4 K for the 36.5-GHz channel. This 3-yr comparison also shows a -1 mm yr(-1) drift in the (TOPEX/ERS-1) water vapor path delay difference, which needs to be confirmed with a longe r time series. This technique could be used for the calibration of fut ure radiometers placed in similar orbital configurations, such as the one to be flown on ENVISAT (relative to ERS-2 and using TOPEX or JASON as a reference).