SAR Tomography for Forest Structure Classification and Monitoring: Actual Status and Perspectives

Abstract

Microwaves can propagate through vegetation layers, allowing the radar signal to interact with the different physical forest structure elements. Exploiting this property, SAR Tomography techniques provide a 3D profile of the backscattered radar power providing a proxy to the physical forest structure. This capability has been demonstrated by means of several (long wavelength) airborne campaigns, which have triggered the development of a new generation of forest structure monitoring applications by means of SAR remote sensing. At the same time, the progress in SAR technology makes possible the implementation and operation of a new generation of spaceborne SAR configurations able to realize these applications on a global scale with a high spatial and temporal resolution. The objective of this work is to present the status and to address potentials and challenges related to applications of SAR Tomography for 3D forest structure characterization and monitoring. A number of issues will be addressed, in particular: 1.the performance of several reflectivity estimators (model-based and not) with respect to acquisition configuration (e.g. geometry and number of baselines, availability of multiple polarization channels, frequency, etc.), and different seasonal and environmental conditions (leaves on/off, dry/wet acquisition days, etc.); 2.the interpretation of the reflectivity profiles, by means of a comparison with ground measurements, lidar profiles and forest simulations. In this, a key role is played by the variability of the profiles with seasons and environmental conditions; 3.the possibility and the methodologies to distinguish between different physical forest structure types by using the reflectivity profiles. In the presentation, several experimental results will be presented. Airborne (L-band polarimetric repeat-pass interferometric) SAR data will be used. Data have been collected under different environmental and seasonal conditions systematically over a period of 12 years (2003-2015) over the Traunstein test site by means of the DLR’s airborne platforms E-SAR and F-SAR. Traunstein is a heterogeneous temperate forest located in southern Germany, with a wide range of stand compositions, stand heights (from 10 to 40m) and stand biomass (mean biomass level of 200 Mg/ha and stands up to 500 Mg/ha). Radar results have been validated by means of extensive ground measurements and lidar profiles.