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Potentiels d'un système RSO basse fréquence opérant en polarimétrie compacte. (Potentials of a low frequency SAR system operating in compact polarimetry)

The aim of this dissertation is to assess the potential of a Synthetic Aperture Radar (SAR) system operating in compact polarimetry mode at low frequency for polarimetric as well as polarimetryinterferometric applications. This study is in line with space-borne missions as BIOMASS for European Space Agency which suggests putting into orbit a SAR system operating at P-band in order to monitor the dynamics of the terrestrial biomass on a global scale, and other future space-borne missions whose purpose is to study global environmental changes such as Deformation, Ecosystem Structure and Dynamics of Ice (DESDynI), Soil Moisture Active and Passive (SMAP) for Jet Propulsion Laboratory, SAtélite Argentino de Observación COn Microondas (Saocom) for Argentine Space Agency. The compact polarimetry mode is a special case of dual polarimetry. In fact, it consists in a unique polarization in transmission and two orthogonal polarizations in reception. The word compact comes from the design system which is minimized (i.e. a single transmission) while maximizing the polarimetric information. Three configurations have been suggested: the π/4 mode which is characterized by a transmission at 45° and two linear polarizations, the π/2 mode with a circular transmission and two orthogonal and independent polarizations, and the hybrid polarity (a special case of the π/2 mode) with a circular transmission and two linear polarizations in reception. It is important to note that it is a specific dual-pol mode where the relative phase between the two receive channels is preserved. The first part of this dissertation introduces the basic concepts of SAR system, polarimetry, interferometry, polarimetry-interferometry SAR, compact-polarimetry and the motivations for implementing compact-polarimetry. The low frequencies allow applications such as vegetation parameters estimation and inversion of surface parameters. However, in a spatial context at low frequency, some external effects can disturb the microwaves propagation for example during the ionosphere crossing. The propagation plane of large wavelengths is rotated while crossing the ionosphere and this rotation is called the Faraday rotation. So, the second part of this dissertation deals with the estimation and correction of the Faraday rotation which relies on the scattering properties of bare surfaces. The selection of the bare surfaces is based on a new parameter, the conformity coefficient from compact-pol measurements. This parameter is shown to be Faraday rotation invariant and allows to differentiate three main types of scattering (i.e. volume, surface and double-bounce). This new coefficient is validated against Cloude-Pottier classification, also known as Entropy/alpha, and the Freeman-Durden model. Once bare surfaces are selected with the conformity coefficient, three methods using compact-pol data to estimate the Faraday rotation are suggested. These techniques are assessed over airborne (RAMSES) and space-borne (PALSAR) data and compared to Bickel & Bates and Freeman methods. One of these processes allows to estimate the Faraday rotation modulo π, a real improvement over existing models and a great interest for systems operating at low frequencies. In addition, we note that the estimation of the Faraday rotation angle from the compact-pol measurements appears straightforward over water bodies, where both the reflection symmetry and the zero phase difference hypotheses are verified. This could open new perspectives in total electron content (TEC) mapping over the ocean. Once Faraday rotation is estimated over bare surfaces, it is possible to correct for the whole scene. The third part of this dissertation focuses on the possible inversion of soil moisture and estimation of biomass from compact-pol data. The co-polarized measurements can closely be approximated by the compact-pol measurements over bare surfaces. The standard 1995 Dubois et al. algorithm, applied directly to the compact-pol data, provides soil moisture estimates that are very close to those computed from the full-pol data and in situ measurements, over RAMSES and AIRSAR data at Lband. Then, estimation of forest biomass from compact-pol measurements is tested and compared to the equivalent algorithm based on full-pol data. Over the Landes forest the results show to be almost equivalent. The calibration of a compact-pol system is considered in the next part. After introducing the calibration, some current methods (i.e. Quegan and Freeman methods) to calibrate full-pol data are presented to better understand which process is possible and available in compact-polarimetry. Freeman suggests to consider that the emission is not really circular and points up a method to calibrate such a system ignoring system gain, noise and cross-talks but submitted to Faraday rotation. In fact, the calibration of a compact-pol system is more challenging because only one polarization is transmitted implying that a correction of the imperfect transmission polarization is impossible. Therefore, we restricted our study to the receive channel calibration. A process to calibrate a compact-pol system in an airborne case is first presented and uses two gridded trihedrals and a 45°-dihedral. Secondly, in a spatial case where Faraday rotation is also present and needs to be corrected apart from cross-talk and channel imbalance, the outlined method uses a dihedral oriented at 0°, another one oriented at 45° and a trihedral. To complete this study, in the last part, the interferometry concept is added to the compact polarimetry to assess the potential of a compact-PolInSAR system to retrieve the vegetation height. With compactPolInSAR data, the RVoG model can be directly applied but in some cases, the canopy height estimation can be degraded compared to the results obtained by using full PolInSAR data. Actually, the data space is reduced in compact-polarimetry resulting in a smaller coherence region included in the full polarimetric coherence region. An alternative could be to reconstruct a “pseudo” full-PolInSAR matrix but this method requires some assumptions. A method using Tabb algorithm is explained, tested over RAMSES P-band data and compared to full-pol inversion with RVoG model and to ground truth measurements. Most of the analysis was performed on simulated compact-pol data obtained from full-pol data. However, equivalence between compact-pol data simulated from full-pol data and compact-pol data processed from raw data as such was established, comforting the linearity property of a SAR processor.