First Three Years Of The Microwave Radiometer Aboard Envisat: In-Flight Calibration, Processing and Validation of the Geophysical Products.
Estelle Obligis(1) , Laurence Eymard(2) , Ngan Tran(1) , and Sylvie LABROUE(1)
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In order to correct the altimeter range for water vapor path delay over ocean, a nadir-looking microwave radiometer has been embarked on the spacecraft. As any error in the wet tropospheric correction directly impacts the sea level determination, the constraints on the quality and stability of the radiometer products are particularly stringent. The uncertainty on the wet tropospheric correction is today around 1 cm rms (Ruf et al, 1994, Eymard et al, 1996) but remains a significant contribution in the global uncertainty on the sea level estimation (around 4 cm rms, Fu and Cazenave, 2001). In this paper, we present the improvements performed in 1) the processing, 2) the retrieval algorithms, 3) the calibration/validation and 4) the long-term survey of the ENVISAT Microwave Radiometer with respect to other altimetry missions (ERS2, TOPEX, Jason), to minimize this error.
1) The particular position of the microwave radiometer on the ENVISAT platform, induces strong contamination of the main measurement by the side lobes. An algorithm dedicated to the correction of this contamination has been proposed and represents actually an improvement with respect to the classical side lobe correction algorithm implemented in the processing of other altimetry missions.
2) Retrieval algorithms for wet tropospheric correction are developed using a database of geophysical parameters (from ECMWF fields) and corresponding simulated brightness temperatures by a radiative transfer model. This database is used to determine statistically the relation between the brightness temperatures and the wet tropospheric correction. The quality of the retrieval algorithm depends therefore on the representativity of the database, the accuracy of the radiative transfer model used for the simulations, and finally on the quality of the inversion model. The development of the retrieval algorithms for the ENVISAT/MWR follows the methodology used for the ERS2 mission, but has benefit from a higher quality meteorological model, a more accurate radiative transfer model and particularly of neural network techniques to take into account the non-linearities in the relation between the brightness temperatures and the wet tropospheric correction.
3) After launch, the major difficulty lies in performing the in-flight calibration of the microwave radiometer, because there is no natural blackbody target, which could help to control the measured brightness temperatures. The method we used to calibrate the ENVISAT/MWR is therefore a combination of comparison with its predecessor ERS2/MWR flying on the same orbit with a time lag of about half an hour, and simulations over sea using ECMWF fields and a radiative transfer model. The use of the same tools (ECMWF analyses, radiative transfer model) to formulate the retrieval algorithm and to perform the in-flight calibration ensures the consistency in the processing to provide at the end a high quality product.
4) The sea level rise is estimated at the mm/year level, so an accurate survey in time is necessary to provide stable products. The methods developed these last years (survey over hot and cold continental targets, survey of the coldest ocean brightness temperatures, comparison with other instruments) allows the detection and the evaluation of weak instrumental drifts, and therefore to propose suitable corrections to users.