Forest Structure Retrieval from Multi-Baseline SARs
Remote Sensing of Biomass - Principles and Applications
where λ is the carrier wavelength, L s is the length of the synthetic aperture, and r is the stand-off distance between the sensor and the target. Slant range resolution is obtained according to eq. (1). A further description of SAR image formation is beyond the scopes of this chapter. For further details, the reader is referred to (1), (2), (3), (4). 3D SAR imaging Given the definitions above, a SAR SLC image represents the scene as projected onto the slant range, azimuth coordinates. This
... ils the complete loss of the information about the third dimension, i.e.: about the vertical structure of the targets within the imaged scene. The key to the recovery of the third dimension is to enhance the acquisition space so as to form a further synthetic aperture. The easiest way to do this is to employ a multi-baseline SAR system, where several SAR sensors, flown along parallel tracks, image the scene from different points of view, see figure (2). Such a system offers the possibility to gather the backscattered echoes not only along the azimuth direction, but also along the cross-range direction, defined by the axis orthogonal to the Line Of Sight (LOS) and to the orbital track. Accordingly, the backscattered echoes can be focused not only in the slant range, azimuth plane, but in the whole 3D space. Therefore, the exploitation of multi-baseline acquisitions allows to create a fully 3D imaging system, where the size of the 3D resolution cell is determined by the pulse bandwidth along the slant range direction, and by the lengths of the synthetic apertures, according to eq. (2), in the azimuth and cross range directions, see Fig. (3) . Both 2D and 3D SAR imaging data may be regarded as specializations to the SAR case of the more general concept of Diffraction Tomography, widely exploited in seismic processing (5). As such, 3D focusing is analogous to the basic formulation of SAR Tomography (T-SAR), in which no particular assumption is adopted to describe the imaged scene (6).