Forest Structure Retrieval from Multi-Baseline SARs

Abstract

The vertical structure of the forested areas has been recognized by the scientific community as one major element in the assessment of forest biomass. Dealing with a volumetric object, the remote sensing techniques that are best suited to infer information about the forest structure are those that guarantee under foliage penetration capabilities. One important, widely popular technology used to investigate the forest vertical structure from the above (typically on board aircrafts) is given by high resolution LIDAR (Light Detection and Ranging) sensors, whose signals penetrate down to the ground trough the gaps within the vegetation layer. In the recent years, however, the attention of the scientific community has been drawn by the use of SAR (Synthetic Aperture Radar) techniques. As opposed to LIDARs, for which high resolution is crucial, the signals emitted by SARs propagate down to the ground by virtue of the under foliage penetration capabilities of microwaves. This different way of sensing the volumetric structure of the imaged objects determines many peculiarities of SAR imaging with respect to LIDAR, some of which are advantages and other drawbacks. The most remarkable advantage is perhaps the one due to the ability of microwaves to penetrate into semi-transparent media, which makes lower wavelength (L-Band and P-Band, typically) SARs capable of acquiring data almost independently of weather conditions and vegetation density. Conversely, the most relevant drawback is that the three dimensional (3D) reconstruction of the imaged objects requires the exploitation of multiple (at least two, but preferable many) images acquired from different points of view, and hence multiple passes over the scene to be investigated. The aim of this chapter is to discuss relevant topics associated with the employment of a multi-baseline SAR system for the reconstruction of the 3D structure of the imaged targets, with particular attention to the case of forested areas. The first topic considered is the design of a multi-baseline SAR system for 3D reconstruction in the framework of Fourier Tomography (FT), also referred to as 3D focusing. Even though seldom used in practical applications due to poor imaging quality, FT allows to discuss quite easily the design and overall features of a SAR tomographic system, and represents the basis for all of the developments presented in the remainder of this chapter. The next part of this chapter will focus on operative methods the generation of high-resolution tomographic imaging from sparse data-sets. This is the case of interest in practical applications, due to the costs associated with flying a high number of passes and platform trajectory accuracy. T-SAR will be cast here in terms of an estimation problem, considering both non-parametric and parametric, or model based, approaches. Non-parametric 2