Contribution to spatial bias mitigation in interferometric radiometers devoted to Earth observation : application to the SMOS mission

Abstract

This work has been undertaken within the frame of the FPI scholarship BES-2009-028505 of 30/07/2009, by the "Secretario de Estado de Investigación del Ministerio de Ciencia e Innovación", related to the project TEC2008-06764-C02-01 (Universitat Politècnica de Catalunya) titled "Advanced concepts on active and passive microwave remote sensing: technology and applications". In a more general scope, this thesis is related to the Remote Sensing Laboratory (Signal Theory & Communication Department, UPC) on-going activities, within the SMOS (Soil Moisture and Ocean Salinity) mission by the European Space Agency (ESA). These activities have been organized to provide original advances in the following four main topics: 1) SMOS system performance assessment. SMOS commissioning phase finalized in May 2010 providing preliminary performance results. Therefore, as a first step in this work, radiometric sensitivity and spatial bias (systematic spatial errors) have been reviewed and assessed. To achieve this, new techniques have been developed to better compare SMOS images to ground truth targets. SMOS ocean views have been selected as the best option since provide smooth brightness temperature distributions for which accurate models are available. 2) Spatial bias mitigation techniques. Spatial bias was a major issue in SMOS first images since it was larger than expected. In order to identify the dominant sources of spatial bias, each potential source of error, at calibration level,has been simulatedand compared tomeasured SMOS error patterns. 3) Spatial bias in the EAF-FoV. Whereas SMOS performance in the alias free FoV was well within mission requirements, this issue was especially important for the four polarimetric temperatures in the extended regions. This issuehas been addressed in two ways. First a new approach has been developed to systematically analyze spatial bias by splitting SMOS EAF-FoV into different regions of interest. Also, the pixels have been arranged per angle of incidence. Secondly, the impact in each region of calibration errors and different image reconstruction has been addressed. 4) Spatial bias in full-pol measurement.The main objective of this work has been achieved by developing, implementing and validating a SMOS full-pol G-matrix image reconstruction procedure that resulted into significantly improved polarimetric images. This tool allowed a comprehensive assessment of the two error contributions that have been identified as the main sources of residual spatial bias: the so-called floor error and antenna pattern uncertainty. Finally, as a validation of the quality of SMOS Level 1 fully polarimetric data that has been achieved, the last chapter of this work has been devoted to provide operational Faraday rotation retrievals in a per snap-shot basis.