Speaker
Description
Active remote sensing techniques based on LIght Detection And Ranging (lidar) and Synthetic Aperture Radar (SAR) can provide high resolution three-dimensional (3-D) information about forest volumes. Full waveform lidars transmit a pulse and record the continuous distribution of the laser energy (i.e., the full waveform) returned to the sensor after being reflected by the vegetation elements within the illuminated footprint. On the other hand, SAR acquisitions in tomographic or interferometric modes allow the reconstruction of the (vertical) profiles of the backscattered power (i.e., radar reflectivity) and / or the estimation of a parameterization of them. Both lidar waveforms and SAR reflectivity profiles and interferometric coherences depend on the 3-D distribution of the vegetation elements, and their potential to contribute uniquely to the observation and quantitative characterization of forest structure has been recognized in several experiments.
Therefore, the optimization of observation strategies and combination algorithms for the generation of unified and enhanced forest structure products for future space borne missions has become a crucial issue. With this in mind, two frameworks have been proposed in which synergies and complementarities between lidar and SAR measurements are being investigated:
1) In the model-based framework, lidar waveforms (and derived parameters) are used to initialize polarimetric-interferometric (Pol-InSAR) backscattering models. The applicability of this framework has been investigated mostly for forest height estimation and proved its effectiveness for instance in those cases in which the Pol-InSAR observation space is too small to allow a balanced model inversion and / or the SAR wavelength does not allow penetration until the ground. However, this framework may reach its limit when it comes to SAR reflectivity profiles. Depending on the SAR frequency, look angle and resolution, differences in propagation make lidar and SAR pulses interact differently with the physical vegetation elements, and the parameterization of the SAR models provided by lidar waveforms can lose significance.
2) The structure-based framework, which is in a very early stage of development, consists in establishing a correspondence between lidar waveforms and SAR profiles 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. The main challenge here is the definition of appropriate structure indices from remote sensing observables that can express physical structure characteristics, remembering that current and future space borne lidar and SAR implementations do not possess spatial resolution capabilities to allow to extract information down to the single trees.
The purpose of this work is to discuss the link between lidar and SAR measurements as characterized by the model-based and the structure-based frameworks. Experimental results obtained by processing data collected in recent airborne campaigns will be presented, with particular reference to the NASA’s Global Ecosystem Dynamics Investigation (GEDI) full waveform lidar mission. Indeed, towards enhanced products, GEDI is expected to use data of present and future space borne synthetic aperture radar (SAR) missions for instance to fill the gaps in its coverage, increase its product resolution, and gain from different ways to calibrate and estimate biomass. In this frame, the possibility of fusing GEDI waveforms and TanDEM-X single-pass X-band interferometric (InSAR) coherences is currently being investigated.
