ALOS L-Band Interferometry

We report on our experiences making interferograms using the L-band synthetic aperture radar aboard the ALOS spacecraft launched by JAXA in January, 2006. As part of our calibration and validation effort, we have begun interferometric processing of repeat passes over radar corner reflectors at Pinon Flat Observatory. The main advantage of L-band (23 cm wavelength) over C-band (5.8 cm wavelength) is that deeper penetration of vegetated areas results in less temporal decorrelation enabling interferograms having longer time separation. This will enable the study shallow creep along the San Andreas Fault System. The longer wavelength, combined with a 2 times increase in range resolution, also requires some modifications to standard InSAR processing codes. Improvements are needed in three areas: 1) Proper focus of the SAR image is very sensitive to the accuracy of the Doppler rate parameter. The precise orbital information, along with a simple polynomial-fitting algorithm, is needed to estimate this parameter. 2) The pulse repetition frequency (PRF) of the radar changes incrementally along the track to accommodate variations in spacecraft height and velocity. A PRF interpolator will be needed. 3) The longer wavelength, combined with the higher bandwidth, permits the use of long baseline pairs (1-2 km) for refining the topographic phase correction. In areas of significant overall topography (> 2 km), the range change between the top and base of a mountain can exceed a range resolution cell (4.6 m). We have implemented a topographic, image-shift algorithm that significantly improves coherence at extreme elevations. We find that flattened interferograms have less than 1 fringe (12 cm) of residual phase suggesting that ALOS orbits are quite accurate. Phase coherence is excellent in areas of rigged terrain but is somewhat lower in flat areas.