Improved Image Reconstruction Algorithms for Aperture Synthesis Radiometers

The SMOS (soil moisture and ocean salinity) mission is an earth explorer opportunity mission from the European space agency that will be launched in September 2007. It aims at producing global and frequent maps of soil moisture and ocean salinity to improve the climate modeling. SMOS\´ single payload is a new type of radiometer called MIRAS (microwave imaging radiometer by aperture synthesis) operating at L-band in which brightness temperature images are formed by a Fourier synthesis technique. Prior to the image reconstruction process, it was first proposed by Camps in 1996 to decompose the calibrated visibilities in several terms: a contribution from the sky, computed from the L-band brightness temperature map of the sky, a contribution from a constant brightness temperature within the Earth at "T_Earth", and the differential visibilities (AV), whose value at the origin is zero, and therefore only account for deviations with respect to the constant brightness temperature value within the Earth. This decomposition significantly reduced the Gibbs phenomenon (ringing) near the Earth aliases and accelerated the convergence towards the solution of iterative methods. However, despite these improvements, the retrieved brightness temperature images still present a scene-dependent bias which is found to be higher for high contrast brightness temperature scenes (coastlines), and lower for homogeneous scenes (all ocean or all land), which will ultimately limit the achievable accuracy of the retrieved salinity. This work presents an improvement of the image reconstruction algorithm for 2D aperture synthesis radiometers to mitigate these biases, among other advantages.