Tomographic profiling: a technique for multi-incidence-angle retrieval of the vertical SAR backscattering profiles of biogeophysical targets

Tomographic profiling (TP) is a new imaging technique designed to provide vertical backscatter profiles through biophysical and geophysical target volumes, such as snow, ice, vegetation, and forest canopies. Data is collected as for normal synthetic aperture radar (SAR) imaging, but with the antennas aligned along the scan or along-track direction. The real antenna provides a wide beam in the along-track direction, which is sharpened by the addition of elemental measurements across a subaperture using a SAR-like processing scheme. A novelty of the scheme is the ability to produce an image transect in which the incidence angle is constant at every point. This is accomplished by incrementally sliding the subaperture across the full aperture, and utilizing the appropriate subaperture to provide the necessary viewing geometry at each pixel. This is in contrast to the SAR case, in which the angle of incidence varies across a scene. By suitable phasing between the subaperture elements, the synthesized beam can be steered in angle within the wide angular extent of the real beam, allowing post-measurement retrieval of the backscattering properties of the scene over a continuous range of incidence angles from a single scan. In the across-track direction, a narrow real beam is required to maintain good vertical resolution and limit the size of the horizontal footprint. Example TP experimental fieldwork results are provided for a 42-cm-deep snowpack, collected with a ground-based SAR system. Although the scheme was developed for ground-based applications, its application to the airborne and satellite cases is also discussed