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
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 launc-
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