Sensitivity of SAR Tomographic Reconstructions to Forest Structure Changes

Abstract

Low frequency (P- or L-band) Synthetic Aperture Radar (SAR) pulses can penetrate through even dense vegetation layers until the ground, and thus interact with vegetation elements (depending on frequency, polarization and dielectric properties) located at different heights. For this, a set of SAR images acquired under slightly different angular directions along displaced tracks or orbits is required to reconstruct the 3D distribution of the backscattered power, also called reflectivity, and constitutes the measurement principle of interferometric and tomographic SAR (TomoSAR) measurements. The application of TomoSAR techniques enables the reconstruction of the 3D reflectivity [1]. Despite first general investigations [2], a systematic understanding of the potentials of TomoSAR reconstructions to reflect 3D reflectivity changes induced by changes of 3D physical structure (growth, management, logging, mortality, disturbance etc.), and their parametrization at different spatial and temporal scales are missing critical aspects. The objective of this presentation is to discuss the sensitivity of TomoSAR reconstructions to specific forest structural changes in time by means of real TomoSAR data. In this analysis, forest structure changes are first detected and characterized by analyzing the 3D distributions of fine-beam lidar returns in acquisitions performed close in time to the SAR acquisitions. Then, the appearance of the individuated changes in the available TomoSAR reconstructions are addressed at profile level, as well as in terms of TomoSAR-derived structure indices aggregating multiple adjacent TomoSAR profiles within a cell [2]. The outcomes are expected to provide conclusions not only about the characteristics (especially in terms of spatial resolution) of TomoSAR acquisitions able to detect these types of changes, but also about ambiguities with reflectivity changes corresponding to weather/seasonal dielectric changes occurring at the same time [3]. The discussion is supported by processing multiple L-band TomoSAR and lidar data sets acquired over the forest of Traunstein (south of Germany) in a 10-year time span (2013-2023). The TomoSAR data acquisitions were carried out by means of the DLR’s F-SAR airborne platform, and realized time intervals from hours up to months and years with underlying daily, weekly and seasonal dielectric-induced reflectivity changes. References: [1] A. Moreira, P. Prats-Iraola, M. Younis, G. Krieger, I. Hajnsek, K. P. Papathanassiou, “A Tutorial on Synthetic Aperture Radar," IEEE Geoscience and Remote Sensing Magazine, vol. 1, no. 1, pp. 6-43, March 2013. [2] V. Cazcarra-Bes, M. Tello-Alonso, R. Fischer, M. Heym, and K. Papathanassiou, “Monitoring of Forest Structure Dynamics by Means of L-Band SAR Tomography,” Remote Sensing, vol. 9, no. 12, p. 1229, Nov. 2017 [3] M. Pardini, K. P. Papathanassiou, F. Lombardini, “Impact of Dielectric Changes on L-Band 3-D SAR Reflectivity Profiles of Forest Volumes,” IEEE Transactions on Geoscience and Remote Sensing, vol. 56, no. 12, pp. 7324-7337, Dec. 2018.