Crossed orbit interferometry: theory and experimental results from SIR-B

Abstract In a conventional imaging radar interferometer, two receiving antennas separated slightly in the cross-track direction view the same scene and altimetry information is deduced from the phase differences between the corresponding pixels in each image. It is possible to perform the same measurements with only one antenna by making two images of the scene on two separate passes; for SIR-B this involved imaging on two separate orbits. If the two orbits are parallel and separated in the cross-track direction, altitude information is derived exactly as in the two-antenna interferometer and the baseline is determined by the orbit separation. For the available SIR-B data, however, the orbits were skewed, which substantially increases the difficulty of finding altitudes. Since the orbits were not exactly parallel, the antenna on one orbit was pointed slightly forward or backward compared to the other; this was compensated for in the azimuth processing. Furthermore, the skew gave the images a cross-track displacement that increased linearly with azimuth distance, which was removed by resampling and stretching one of the images. Because of the skewing, the baseline increased with distance from the crossing point and theory indicates that altitude variation appears as a change in the along-track position of a given pixel. This is borne out in the experimental data which is then used to construct a coarse altitude map. Additionally, the phase difference of each overlaid pixel is related to the altitude in a complicated way. An algorithm was developed to invert this relationship, the results of which were then used to construct a more refined altitude map.