Long-term stability of ERS-2 and TOPEX microwave radiometer in-flight calibration

The microwave radiometers on altimeter missions are specified to provide the "wet" troposphere path delay with an uncertainty of 1 cm or lower, at the location of the altimeter footprint. The constraints on the calibration and stability of these instruments are therefore particularly stringent. The paper addresses the questions of long-term stability and absolute calibration of the National Aeronautics and Space Administration Topography Experiment (TOPEX) and European Space Agency European Remote Sensing 2 (ERS-2) radiometers over the entire range of brightness temperatures. Selecting the coldest measurements over ocean from the two radiometers, the drift of the TOPEX radiometer 18-GHz channel is confirmed to be about 0.2 K/year over the seven first years of the mission, and the one of the ERS-2 radiometer 23.8-GHz channel to be -0.2 K/year. The good stability of the other channels is confirmed (drift less than 0.04 K/year). The use of continental targets for analyzing the long-term drift is evaluated: the natural interannual variability prevents one from directly monitoring the drift of each channel, but the relative variation between two channels of the same instrument is found reliable. Over cold areas (Antarctic and Greenland plateau), results are consistent with the "cold ocean" analysis. Intercomparison of radiometer absolute calibrations is performed over the same continental area, leading to an anomalously high difference between channels 36.5 and 37 GHz of the ERS-2 and TOPEX radiometers, respectively, over "hot" targets (Sahara desert and Amazon forest). To quantify and analyze this difference, other radiometer measurements are analyzed over the Amazon forest, from the Special Sensor Microwave Imager (SSM/I) and the Advanced Microwave Sounding Unit (AMSU). Biases are confirmed for both TOPEX and ERS-2 radiometers by comparing brightness temperatures and derived surface emissivities: the TOPEX radiometer channels exhibit a negative bias with respect to SSM/I and AMSU-A, whereas the ERS-2 radiometer 36.5-GHz channel is positively biased, by several kelvin in brightness temperature in both cases. The method presented here could be used for controlling the in-flight calibration of any radiometer, and correct for remaining calibration errors after launch.