Extension of the Clean Technique To the Microwave Imaging of Continuous Thermal Sources By Means of Aperture Synthesis Radiometers - Abstract

Abstract

The derivation of soil moisture and ocean salinity contents can be performed by means of passive observations at L-band. The global coverage, the low revisit time (1-3 days), the radiometric resolution (1 K) and the spatial resolution required (10-30 Km) can be achieved by means of low orbit interferometric radiometers. Interferometric radiometers synthesize a thin beam by correlating the outputs of a sparse array of small antennas. The imaging algorithms used in radioastronomy can not be directly applied to Earth observation because of the large number of baselines, the proximity between the antennas, which increases coupling effects, and the antennas' wide beams required to cope with the large field-of-view (FOV), about 120°. Since most receiver errors and drifts can be hardware or software calibrated, inversion algorithms are focused mainly in the correction of the aberrations introduced by antenna voltage pattern mismatches and antenna positioning errors. This paper presents an extension of the CLEAN technique used in radioastronomy to map quasi-point thermal sources, i.e., stars, to the generation of brightness temperature maps of extended thermal sources, i.e., the Earth. The iterative approach used allows to account for antenna voltage pattern mismatches within the large FOV. The proposed algorithm has been optimized to process hexagonally sampled visibilities given by Y-shaped arrays, as it is the case of the MIRAS instrument (Microwave Imaging Radiometer by Aperture Synthesis), an interferometric radiometer currently under study at the European Space Agency.