We introduce ultra-wideband synthetic aperture radar (SAR) interferome
try as a new technique for topographic height retrieval. It is based o
n using a SAR system with large relative bandwidth that acquires data
along two parallel tracks with a separation of the same order of magni
tude as the Eight altitude. The complex SAR image data are resampled o
nto a common reference surface, filtered, and followed by a Hermitian
multiplication. The resulting interferogram is shown to have a finite
depth-of-focus (DOF) in terms of phase coherence. The achieved height
precision is controlled by the ambiguity height, which is shown to sca
le to the DOF as the relative bandwidth, This means that only one frin
ge is within the DOF as the resolution approaches the fundamental wave
length limit; i.e., the phase is unambiguously related to topographic
height. The topography may thus be determined by changing the referenc
e surface and retrieving the height at each step. The technique is suc
cessfully demonstrated to generate fringes based on VHF-band data acqu
ired by the CARABAS airborne SAR system. Temporal decorrelation is not
a problem due to the long wavelengths nor is the effect of tropospher
ic delay on the retrieved height.