Potentials for Multi-Frequency 3D Forest Structure Mapping With BIOMASS, TanDEM-X and GEDI

Abstract

The combination of polarimetric interferometric (Pol-InSAR) and / or tomographic (TomoSAR) SAR measurements at multiple frequencies and lidar measurements is widely recognized to be a critical step for an improved characterization of 3D forest structure, intended as size, location and arrangement of trees, trunks and branches in a stand. However, the realization of this combination strongly depends on the ability to establish a (physical or statistical) link between them at the different scales / resolutions performed [1]. Beside statistical regressions, such combination can be realized more systematically by means of (1) scattering models or (2) common structure-dependent indices. Within the first category, many experiments so far focused on using lidar measurement to enable Pol-InSAR parameter inversions, see e.g. [2]-[4]. More recently, it has been shown that lidar GEDI waveforms can be parameterized to define directly the relationship between X-band interferometric coherence and forest height [3], enabling large-scale forest height mapping at high spatial resolution with TanDEM-X data. At the same time, GEDI waveforms within a scene can be used to optimize a function basis for reconstructing an approximation of an X-band vertical profile of the scattered intensity [4]. Within the second category, the horizontal-vertical structure framework introduced in [5], [6] has been demonstrated to accommodate consistently structure measurements from lidar and SAR configurations at different frequencies [5], [7]. At the increase of the frequency, the penetration of transmitted electromagnetic pulses decreases, and the sensitivity to structure gradients becomes driven more and more by vegetation components close to the canopy top. The objective of this work is to evaluate potentials of the combination of P-band BIOMASS, lidar GEDI and X-band TanDEM-X measurements for mapping structure gradients at large scale. In particular, complementarities in information content determined by penetration and sensitivity to canopy elements, but also in spatial resolution and measurement types (profiles or interferometric coherences) will be explored for addressing (and probably solving at least in part) the ambiguities in structure mapping arising from the use of measurements of single sensors alone. This evaluation will be carried out by means of the horizontal-vertical structure framework of [5] by calculating appropriate, yet defined, structure metrics expressing heterogeneity (i.e., complexity). This framework is directly applicable to BIOMASS tomographic measurement, as it is defined in terms of tomographic reconstructions of vertical reflectivity profiles. In the TanDEM-X case, no tomographic acquisitions are available, hence the reconstruction will be performed using the (polarization) coherence tomography concept as in [4]. The profile is expressed as a linear combination of basis functions, and a meaningful approximation can be obtained from just one interferometric coherence [4]. The basis function is derived from GEDI waveforms within the TanDEM-X scene. Conclusions will be drawn based on real data experiment on selected test site with significant structural spatial gradients and suitable BIOMASS, TanDEM-X and GEDI data, in correspondence of ground measurements or (continuous) airborne lidar data to be used as reference.

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