Characterization of Forest Structure by Means of Multi-Frequency Multi-Baseline SAR Measurements: Status and Challenges

Abstract

The wavelength of synthetic aperture radar (SAR) waves affects the sensitivity to scatterers depending on their size and the penetration through forest volumes. Therefore, forest backscattering at different wavelengths (from P- up to X-band) contains different structural information about the geometry of vegetation elements and their distribution in space. On the other hand, the acquisition of more than one image in polarimetric interferometric (Pol-InSAR) or tomographic (TomoSAR) multi-baseline modes enables the estimation of structure parameters and 3-D reflectivity profiles, which in turn allow to access directly 3-D physical structure information. An enhanced and accurate characterization of forest structure is expected to be achieved if Pol-InSAR and/or TomoSAR structure measurements at multiple frequencies are combined together based on their complementarity of information content. One way to understand structural complementarities and (possibly) develop combination algorithms is to refer to a “structure-based” framework. Although in a very early stage of development, it consists in establishing a correspondence between measurements at different frequencies by means of their capability in reflecting physical 3-D forest structure. This can be accomplished for instance by defining two complementary structure indices expressing heterogeneity in the horizontal and in the vertical direction with the aim to reflect the behavior of analogous indices based on ground inventory single-tree measurements. In this presentation we first discuss and compare the information content of TomoSAR vertical profiles at multiple frequencies for different forest types. Then, the structure-based framework is reviewed and validated against experimental data (including Lidar and ground inventory measurements) acquired over different temperate and tropical forest conditions. The potentials and challenges of such framework to link structure measurements at multiple frequencies are addressed as well. Results achieved in the framework of actual multi-frequency multi-baseline airborne SAR campaigns (DLR F-SAR platform) in temperate and tropical forest campaigns in different configurations are reported and critically discussed.