An image reconstruction and calibration approach for the SMOS aperture synthesis radiometer

Abstract

The SMOS mission (Soil Moisture and Ocean Salinity) has the objective to observe two crucial variables, soil moisture over land and ocean salinity over sea. Both variables are used in predictive atmospheric, oceanographic and hydrologic models and may be important for extreme event forecasting. SMOS is based on an innovative 2-D L-band (1.4GHz), Y-shaped aperture synthesis radiometer to observe globally and with a revisit time of 3 days the brightness temperatures in H and V polarisation with a spatial resolution better than 50km and with an angular coverage ranging from 0°-50°. A sun-synchronous (6 a.m. ascending), circular, 757 km orbit was proposed to ensure the requisite sampling, and at the same time to minimize signal perturbing effects such as Faraday rotation, sun glint, and thermal differences in soils and vegetation. This paper outlines an image reconstruction approach as being developed and investigated for the SMOS instrument. It is based on the in-space measurements of well-known calibration targets like point sources in a low-power background as it could be provided by the Sun or the Moon. The use of multiple scenes, where the point-source is located on different positions within the usable field of view, allows the determination of a G-matrix describing the whole imperfect system. Once determined, the matrix can be used to compute efficiently the corrected and calibrated brightness temperature map of the actually observed scene. Simulation results to assess the performance of the approach are shown and discussed.