Virtual bandwidth SAR (VB-SAR) for centimetric-scale sub-surface imaging from space

Abstract

Virtual bandwidth synthetic aperture radar (VB-SAR) is a radical new technique that promises, for the first time, sub-surface imaging at large stand-off distances applicable to airborne and spaceborne platforms. The scheme relies on understanding and exploiting the way a radar wave interacts with a soil volume. A wave passing through a soil to a buried object is delayed relative to the equivalent free-space path due to the higher relative permittivity of a soil, which is measured as a phase shift. This can equivalently be thought of as a shift in the real radar frequency to a virtual frequency. Temporal variations in soil moisture across a temporal stack of differential interferometric SAR (DInSAR) images can transport the real radar frequency at a pixel across a set of virtual frequencies to create a virtual bandwidth. Even small changes in moisture can produce large virtual bandwidths promising very high, centimetric-scale vertical resolutions: 1.6 cm at X-band and 8.6 cm at L-band for a 10% moisture change in a sandy soil. To produce a sub-surface depth profile at an image pixel, a Fourier transform is performed on the complex data stack collected across the DInSAR set. Retrieving the depth profile at each pixel across the 2D image provides a 3D map of the sub-surface backscattering through the scene. A description of the model is presented, and representative modelling results are presented to assess performance and application across different soil types, moisture regimes, and SAR platforms.