Split band interferometry versus absolute ranging with wideband SAR systems

Estimation of differential shift of image elements between two SAR images is the basis for many applications, like DEM generation or ground motion mapping. The shift measurement can be done non-ambiguously on the macro scale at the accuracy of the range resolution of the system or on the micro scale by employing interferometric methods. The latter suffers from phase cycle ambiguities and requires phase unwrapping. Modern wideband high resolution SAR systems boast resolutions as small as a few tens of a wavelength. If sufficiently many samples are used for shift estimation, the accuracy can be increased to a small fraction of a resolution cell and even in the order of a wavelength. Then accurate absolute ranging becomes precise enough to support phase unwrapping or even make it obsolete. The paper gives equations and simulation results for the achievable shift estimation accuracy for several algorithms: coherent speckle correlation, incoherent speckle correlation, split band interferometry, and correlation of point scatterers in clutter. It is shown that the performance of split band interferometry is close to the one of optimum MLE. Special Ak systems are proposed to best take advantage of the available bandwidth.