Synthetic Aperture Radar (SAR) tomography uses the principle of the azimuth synthetic aperture in the elevation direction. Instead of using different positions of the radar sensor along the flight path in order to increase the aperture length, SAR tomography uses multiple passes of the radar sensor over the same area at different elevation positions, i.e. orthogonal to the azimuth-range plane, on different orbits. Similar to the synthetic aperture in azimuth direction, a larger aperture in cross-range elevation direction allows increasing the resolution in the elevation direction. Therefore, the echoes are focused in the whole 3D space (azimuth, range and elevation), and scattering contributions can be separated at different heights, even if they are situated in the same azimuth-range cell.
SAR tomography exploits therefore these multiple passes of the radar sensor at different orbit positions (orbits heights) in order to retrieve 3D information about volumetric targets, where the 2D SAR signals often overlaps due to the typical side-looking geometry.
The result of tomographic processing is a tomogram, i.e. it is a hologram of a specific area of interest, usually represented as a tomographic profile along a particular direction. Using polarimetric data, the different scattering mechanisms happening at different heights can be represented in the profile, allowing a full understanding of the volumetric information and backscattering processes.
Unlike the azimuthal aperture, the tomographic aperture is achieved by repeat-pass acquisitions, the antenna having to come back over the area. An important parameter is therefore the target coherence, that may decrease by longer repeat-pass cycles. In general, a 1-4 day revisit cycle is preferred for tomographic applications.
SAR tomography finds applications in the imaging and monitoring of cities and single buildings, as well as in height and biomass estimation of forest stands. The use of longer wavelength that guaranty the penetration into canopy volumes allows a better retrieval of the complete forest structure and its undergrowth.