External calibration in L-Band 2D synthetic aperture radiometers: application to sea surface salinity retrieval

The SMOS(Soil Moisture and Ocean Salinity) Mission was selected in May 1999 by the European Space Agency to provide global and frequent soil moisture and sea surface salinity maps. SMOS´ single payload is MIRAS(Microwave Imaging Radiometer by Aperture Synthesis), an L-band 2D aperture synthesis interferometric radiometer with multiangular observation capabilities. To achieve the ultimate goal of 0.1 psu error in ocean salinity retrievals, most studies assume the independence of measurements both in time and space so that the standard deviation of the retrieval errors decreases with the inverse of square root of the number of measurements being averaged. However, biases in the brightness temperature images originated from errors in the noise injection radiometers. Sun contributions to the antenna temperature, and imaging under aliasing conditions need to be compensated before attempting to retrieve the sea surface salinity. This work presents an external calibration suitable to perform salinity retrievals with an acceptable error. This study has been performed using the SMOS End-to-end Performance Simulator(SEPS) including thermal noise, all instrumental error sources, current error-correction and image reconstruction algorithms, and correction of atmospheric and sky noises. Simulation results show a retrieved salinity error of ∼1 psu in one overpass over open ocean, which can be reduced afterwards by spatio-temporal averaging, much better than the -5 psu error that appears when image biases are not corrected.