The Compact Polarimetry Alternative for Spaceborne SAR at Low Frequency

Abstract

International audienceIn spaceborne synthetic aperture radar (SAR), a single-polarization on-transmit offers twice the swath width compared to full polarization. This is linked to SAR system design issues, and, without getting into the technical details deserving by themselves a full paper, we can just mention the swath characteristics of ALOS PALSAR (the Advanced Land Observing Satellite, Phased Array L-Band Synthetic Aperture Radar), reducing from 70 km for the dual-pol mode to 30 km for the full polarization mode. The reduced coverage in the full polarization mode has a harmful impact on the revisit time, which is always a major drive for the Earth-observing community. The options chosen up to now for dual-pol system designs (or single-polarization on-transmit) rely on a linear polarization on-transmit [either horizontal (H) or vertical (V)], with two orthogonal polarizations on-receive. Souyris and Raney in earlier papers proposed more pertinent alternatives for the selection of the transmit polarization leading to a better characterization of the scattering mechanisms. In this paper, the analysis is pursued in more depth by including the effect of the ionosphere on the wave propagation and extending the applications to polarimetric interferometry SAR (PolInSAR). A compact mode is developed where the transmit polarization is circular, whereas the only constraint on the two receiving polarizations is independence. Indeed, the choice of the polarizations of the two receive channels does not matter, as any polarization on-receive can be synthesized from these two measurements. This is, however, not the case for the unique transmit polarization. At a low frequency, where the ionosphere has a significant effect, the circular transmit polarization is the only sensible option, as it provides an effective constant polarization as seen by the scattering surface. This is an essential condition for a meaningful multitemporal analysis. Both the polarimetric SAR applications and the PolInSAR applications in the context of this compact polarimetry (CP) mode are explored. A pseudocovariance matrix can be reconstructed following Souyris' proposed approach for distributed targets and is shown to be very similar to the full polarimetric (FP) covariance matrix. The reconstruction of the cross-polarized Sigma0 is shown to be reliable and to have very low sensitivity to Faraday rotation. A PolInSAR vegetation height inversion for P-band is presented and applied to the CP data with a level of performance that is similar to the one derived from FP (a 1.2-m root-mean-square height error on the ONERA Airborne radar (RAMSES) data over the Landes Forest). A procedure is developed to correct for the ionospheric effects for the PolInSAR acquisition in the FP or CP mode and is assessed on the data simulated from an airborne acquisition. The results demonstrate that the technique is efficient and robust. The calibration of CP data is identified as an important challenge to be solved, and some clues are provided to address the problem

Compact polarimetry assessment for rice and wetland mapping

“…However, the results from the CP radar should be better than those from a single-polarized or dual-polarized SAR. Many investigations (Dubois-Fernandez et al 2008;Ainsworth, Kelly, and Lee 2009;Nord et al 2009;Charbonneau et al 2010) have confirmed these results for various applications.…”

“…Recently, compact polarimetry (CP) has been suggested as an alternative SAR configuration to maintain swath coverage while increasing information content compared to single-polarization or dual-polarization systems (Souyris and Mingot 2002;Raney 2007;Ainsworth, Kelly, and Lee 2009;Dubois-Fernandez et al 2008;Nord et al 2009). The m-delta decomposition for CP data is very similar to the Freeman-Durden decomposition (Charbonneau et al 2010).…”

A Collection of SAR Methodologies for Monitoring Wetlands

“…Moreover, when the transmission of a field has a linearly polarized component it will cause uncertainties or omission when classifying dihedral backscatter [96]. Circular polarization transmission is the best way to prevent the rotation of a linearly polarized wave as it transmits through the ionosphere, but some omission may still occur [97]. Therefore, some features will not be as visible in the simulated compact polarimetry data because they are too faint and do not exceed the noise floor.…”

Soil Moisture Retrieval from Active Spaceborne Microwave Observations: An Evaluation of Current Techniques

“…Fully polarimetric SAR (PolSAR) and also Compact Polarimetric SAR [167][168][169][170] measurements have been used to study the dependence of the polarimetric signature on land cover changes and on surface parameters such as soil moisture and surface roughness. The major advantage of using PolSAR over traditional SAR is its ability to measure all the polarisation characteristics of a surface target simultaneously.…”