Calibration of SMOS geolocation biases

The Soil Moisture and Ocean Salinity (SMOS) mission aims at observing two variables critical for a large scientific community, from biosphere dynamics to climate monitoring. The mission should also provide information on root zone soil moisture and vegetation and contribute to significant research in the field of the cryosphere. The original design, 2D interferometric radiometer at L-band, and principle of measurement makes SMOS a challenge at various technical levels. Moreover, stringent requirements on the estimated variables make the complete processing of SMOS data even more challenging. One of these requirements is concerned with the ability to accurately localize all the footprints of the instrument on the surface of the earth. Based on simulation and sensitivity studies with respect to the final retrieval of soil moisture, this accuracy requirement has been established so that the localization error on each footprint presents a zero mean and a standard deviation of 400 m. This high accuracy is mainly due to the need for knowledge of open water within a footprint, not to bias soil moisture estimation. This accuracy is highly challenging and unprecedented for sensors of this class and resolution. The on board devices that will help characterize the geolocation of the SMOS products include stellar sensor and gyroscopes, which can achieve an accuracy consistent with the requirements in terms of standard deviation. But the overall localization budget is also contaminated by an important bias, due to the mechanical deployment of the instrument antenna arms after launch, and to the launch shift that impacts all the alignments on the satellite (mechanical shift of stellar sensor due to shocks and vibrations, moisture desorption in mechanical brackets...). The purpose of this study is to characterize these biases, obviously inaccessible to on ground measurement and expected not to evolve once in orbit, so that they can be accounted for in the ground processing prior t- o initiate the soil moisture retrieval.