Long term stability of ERS-2 and TOPEX microwave radiometer in-flight calibration.
Eymard, L. ; Obligis, E. ; Tran, N. ; Karbou, F. ; Pilon, A. ; Dedieu, M.
Année de publication
2005
The microwave radiometers on altimeter missions are specified to provide<br> the "wet" troposphere path delay with an uncertainty of 1 cm or lower, <br>at the location of the altimeter footprint. The constraints on the <br>calibration and stability of these instruments are therefore <br>particularly stringent. The paper addresses the questions of long-term <br>stability and absolute calibration of the National Aeronautics and Space<br> Administration Topography Experiment (TOPEX) and European Space Agency <br>European Remote Sensing 2 (ERS-2) radiometers over the entire range of <br>brightness temperatures. Selecting the coldest measurements over ocean <br>from the two radiometers, the drift of the TOPEX radiometer 18-GHz <br>channel is confirmed to be about 0.2 K/year over the seven first years <br>of the mission, and the one of the ERS-2 radiometer 23.8-GHz channel to <br>be -0.2 K/year. The good stability of the other channels is confirmed <br>(drift less than 0.04 K/year). The use of continental targets for <br>analyzing the long-term drift is evaluated: the natural interannual <br>variability prevents one from directly monitoring the drift of each <br>channel, but the relative variation between two channels of the same <br>instrument is found reliable. Over cold areas (Antarctic and Greenland <br>plateau), results are consistent with the "cold ocean" analysis. <br>Intercomparison of radiometer absolute calibrations is performed over <br>the same continental area, leading to an anomalously high difference <br>between channels 36.5 and 37 GHz of the ERS-2 and TOPEX radiometers, <br>respectively, over "hot" targets (Sahara desert and Amazon forest). To <br>quantify and analyze this difference, other radiometer measurements are <br>analyzed over the Amazon forest, from the Special Sensor Microwave <br>Imager (SSM/I) and the Advanced Microwave Sounding Unit (AMSU). Biases <br>are confirmed for both TOPEX and ERS-2 radiometers by comparing <br>brightness temperatures and derived surface emissivities: the TOPEX <br>radiometer channels exhibit a negative bias with respect to SSM/I and <br>AMSU-A, whereas the ERS-2 radiometer 36.5-GHz channel is positively <br>biased, by several kelvin in brightness temperature in both cases. The <br>method presented here could be used for controlling the in-flight <br>calibration of any radiometer, and correct for remaining calibration <br>errors after launc-<br>h.</div>
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