My-Linh Truong-Loi

Estimation of Soil Moisture and Faraday Rotation From Bare Surfaces Using Compact Polarimetry

The potential of compact polarimetry (CP) mode at longer wavelengths in a space environment for surface parameter estimation is investigated. CP consists of transmitting a single polarization while receiving two polarizations. At longer wavelengths, one of the main challenges associated with CP from space is Faraday rotation (FR) estimation and correction. In this paper, an estimation procedure for FR is presented, which relies on the scattering properties of bare surfaces. The selection of the bare surfaces is based on a new parameter, the conformity coefficient computed from CP measurements. This parameter is shown to be FR invariant. Once estimated, the FR can be corrected over the whole image. A simple approximation to sigmaHH o and sigmaVV o based on CP measurements over bare soil surfaces is presented, from which soil moisture can be estimated using the 1995 Dubois algorithm. The results obtained using CP are shown to be in good agreement with those obtained from the standard Dubois algorithm using fully polarimetric data. This implies that, for soil moisture, CP can be used instead of HH and VV dual-polarized measurements.

The conformity coefficient or how to explore the scattering behaviour from compact polarimetry mode

In a previous paper, we have investigated the potential of the compact polarimetry mode at longer wavelengths in a space environment for soil moisture estimation. At longer wavelengths, one of the main challenges in dealing with compact polarimetry is the Faraday rotation estimation and correction. One proposed technique relies on bare surface scattering properties. Identifying the bare surfaces can be done using the conformity coefficient. This new coefficient can also be used to discriminate between surface, volume and double bounce scattering. It provides similar results to the Freeman-Durden classification in discriminating bare surfaces.

Potentials of a compact polarimetric SAR system

The goal of this study is to show the potential of a compact-pol SAR system for vegetation applications. Compact-pol concept has been suggested to minimize the system design while maximize the information and is declined as the π/4, π/2 and hybrid modes. In this paper, the applications such as classification, Faraday rotation and soil moisture estimates are first reminded, then, biomass estimation from CP data is presented and a calibration procedure using external targets is proposed. Finally, the interferometry concept is added to assess the capabilities of a compact-pol system to retrieve vegetation height.

Compact Polarimetry Mode for a Low Frequency SAR in Space

In spaceborne SAR, a single-polarisation on transmit has, over full polarisation, the vast advantage of a double swath due to SAR system design issues. Without getting into the technical details deserving by themselves a full paper, we can just mention the swath characteristics of ALOS PALSAR, reducing from 70km for dual-pol mode to 30 km for full-polarisation mode. The options chosen up to now for dual-pol system designs (or single-polarisation on transmit) rely on a linear polarisation on transmit, with two orthogonal polarisations on receive. In an earlier paper, we proposed a more pertinent alternative for the selection of the transmit polarisation leading to a better characterisation of the scattering mechanisms [1,2]. This mode is called the iquest/2 CP mode, in reference to the transmit polarisation being circular. At low frequency, where the ionosphere has a significant effect, the circular transmit polarisation is the only sensible option as it provides an effective constant polarisation as seen by the scattering surface. This is an essential condition for a meaningful multi-temporal analysis In this paper, the analysis is pursued in more depth by including the effect of the ionosphere on the wave propagation, exploring both the POLSAR and PolInSAR applications and investigating procedure to estimate the Faraday rotation from the CP data.

Polinsar at Low Frequency and Ionospheric Effects

Global warming is now known to be the major environmental issue mankind will have to face in the next decade. Monitoring of vegetation and biomass is clearly an essential piece of information required at all levels ranging from the scientific studies to understand and forecast, to the political actors and government leaders responsible for drafting remediation policies and evaluating their impact. Microwave remote sensing with the low-frequency SAR technique can provide a useful characterization of forest (spatial coverage, species, density, height...) at a global scale, relying on the all-weather imaging capabilities of SAR linked with the significant penetration of the low-frequency EM wave in the canopy. The published techniques for forest characterization from low frequency SAR data include radiometry inversion, polarimetric inversion based on the anisotropy parameters and PolInSAR Random Volume Over Ground inversion. In this paper, we will more specifically concentrate on the PolInSAR technique and the impact of ionospheric effect on this inversion. PolInSAR at low frequency can be envisioned with two radar platforms flying in formation or as a repeat pass mission. The second alternative is more plausible given the cost and the size of a low frequency SAR instrument. However the two cases will be discussed in the paper.