An evaluation of Pol-InSAR complementarities between L- and S-band in forest structure observation

Abstract

There is a common agreement on the relevance of L-band wavelengths (around 20 cm) for forest observation. For instance, the larger backscatter dynamic range increases the sensitivity to larger biomass gradients with respect to C- and X-band. Furthermore, L-band allows the penetration into and through dense forest canopies in all forest ecosystems, remaining sensitive to canopy structure elements, and possibly leading to more relevant structure estimates than P-band. Finally, the larger temporal stability of the scatterers may allow the implementation of polarimetric interferometric (Pol-InSAR) acquisitions in repeat pass modes. In contrast, S-band applications in the forest domain are rather unexplored. The interest in S-band is being raised by its implementation on a number of space borne platforms, e.g. HJ-1C (China), NovaSAR-S (United Kingdom) and NISAR (United States and India). Clear potentials of S-band polarimetry have been found for instance for land classification, while S-band interferometry has been shown to lead to accurate estimates of digital elevation models for instance in a dual-frequency framework with X-band. Differently from L-band, the shorter wavelength allows the realization of flexible single-pass implementations with relevant interferometric sensitivity even on airborne platforms. Despite the first airborne experiments, the S-band performance in terms of forest structure characterization from polarimetric interferometric (Pol-InSAR) data has not been systematically evaluated yet. After the first S-band Pol-InSAR data acquisition, larger scale campaigns have been performed by the DLR’s F-SAR airborne platform on a number of test sites. In this context, the purpose of this work is to further investigate the complementarity of L- and S-band for forest structure observation as a consequence of the difference in penetration and sensitivity to sizes of vegetation elements, starting from multibaseline Pol-InSAR data. In particular, the role and importance of polarimetry is compared for the two frequencies. First of all, the spectrum of the ground-to-volume ratios is estimated under the Random-Volume-over-Ground assumptions, and its variability is evaluated as function of terrain slopes and incidence angle. Then, the distributions of the interferometric coherences within the Pol-InSAR coherence region are considered in different forest stands. Finally, the analysis is extended to vertical reflectivity profiles estimated by tomographic techniques. Experiments are carried out by using multibaseline Pol-InSAR data sets at L- and S-band acquired simultaneously by the F-SAR platform over the forest site of Traunstein (South of Germany) with (nominally) uniformly distributed baselines. In the S-band case, single-pass Pol-InSAR acquisitions are available as well, allowing to assess directly temporal decorrelation effects. Field inventory data collected continuously in space over 25 ha at single-tree level and fine-beam airborne lidar data are used as a reference.