A New Approach for PolInSAR Forest Parameters Inversion: Results Using the ESA ALOS-PALSAR Prototype Processor

Marco Lavalle, Eric Pottier, Domenico Solimini, Yves-Louis Desnos, Nuno Miranda, Betlem Rosich,
and Mirko Santuari

Abstract

In the last two years, ALOS-PALSAR has played a key role for the on-orbit demonstration of PolSAR and PolInSAR applications. PALSAR SAR sensor operates at L-band, offers full polarimetric capabilities, a spatial resolution of 10 meters and a revisit time of 45 days. Due to these characteristics, PALSAR is suitable for agriculture and forestry. In particular, the retrival of bio-physical parameters from forests, such as the tree height, has been a challenging objective using polarimetric InSAR technique. The basic idea is to discriminate between the ground and the canopy response by using different polarization states and to derive the canopy height using phase information. In recent works, vegetation height has been estimated using the Random Volume over Ground (RVoG) model, which, for each polarization state, yields the polarimetric/interferometric degree of coherence in terms of four variables: topographic phase, canopy height, average canopy extinction, and ground-to-volume ratio.

Despite the well known limitations due to the temporal effects, which are more evident for vegetated area, in this paper we consider the complete end-to-end PoInSAR processing chain using the ESA ALOS-PALSAR Prototype Processor for the basic SAR processing. Higher-level processing steps are performed using the ESA Toolbox PolSARPro and two new algorithms that we have developed. First, we have improved the coherence optimisation algorithm for the best polarisation selection: the difference between the scattering phase center of the ground and of the top of the canopy is optimized through the maximization of the difference between the coherence magnitudes at different polarisations. Second, we proposed an alternative inversion approach for the retrieval of tree height that compares the optimized coherence with the output of a POLINSAR coherent scattering model that simulates a real realization of a forest.

The results are shown for each step of the processing chain. We start from a pair of PALSAR L1.0 products and focus them by using the ESA SAR Processor; then the calibration of the SLC products according the Jaxa distortion matrices is applied and the Faraday rotation, estimated from TEC data or from the SLC products, is compensated. The co-registration of the InSAR image pair is based on the amplitude correlation and the range spectral filtering. We remove the phase contribution of the flat Earth and optionally reduce the speckle using a refined Lee filter that generates the PolInSAR coherency matrix. The next step is the estimation of the complex degree of coherence and the flattened interferogram, and the selection of the ground and canopy scattering centers through the phase optimisation algorithm. This estimation is corrected using the terrain slope information from an external digital elevation model (DEM). Finally, we test the alternative height inversion approach by comparing the optimised interferograms with the model output and compare it with the classical approach of the RVoG inversion

Index Terms—PolInSAR. Interferometry. Tree height. Forest. ALOS. PalSAR.

POLinSAR 2009, the 4th International Workshop on Science and Applications of SAR Polarimetry and Polarimetric Interferometry, ESA/ESRIN, Frascati, Italy, 26-30 January 2009.

© Authors, 2009