Phase gradient approach to stacking interferograms (original) (raw)

1998, Journal of Geophysical Research

The phase gradient approach is used to construct averages and differences of interferograms without phase unwrapping. Our objectives for change detection are to increase fringe clarity and decrease errors due to tropospheric and ionospheric delay by averaging many interferograms. The standard approach requires phase unwrapping, scaling the phase according to the ratio of the perpendicular baseline, and finally forming the average or difference; however, unique phase unwrapping is usually not possible. Since the phase gradient due to topography is proportional to the perpendicular baseline, phase unwrapping is unnecessary prior to averaging or differencing. Phase unwrapping may be needed to interpret the results, but it is delayed until all of the largest topographic signals are removed. We demonstrate the method by averaging and differencing six interferograms having a suite of perpendicular baselines ranging from 18 to 406 m. Cross-spectral analysis of the difference between two Tandem interferograms provides estimates of spatial resolution, which are used to design prestack filters. A wide range of perpendicular baselines provides the best topographic recovery in terms of accuracy and coverage. Outside of mountainous areas the topography has a relative accuracy of better than 2 m. Residual interferograms (single interferogram minus stack) have tilts across the unwrapped phase that are typically 50 mm in both range and azimuth, reflecting both orbit error and atmospheric delay. Smaller-scale waves with amplitudes of 15 mm are interpreted as atmospheric lee waves. A few Global Positioning System (GPS) control points within a frame could increase the precision to -20 mm for a single interferogram; further improvements may be achieved by stacking residual interferograms. 1993; Massonnet et al., 1993; Zebker et al., 1994a; Massonnet and Feigl, 1995a; Dixon et al., 1993; Meade and Sandwell, 1996]. Sums of interferograms could be used to generate high-resolution topographic maps [Zebker and Goldstein, 1986; Werner et al., 1992; Madsen et al., 1993; Zebker et al., 1994b, 1997], while differences may reveal tectonic deformations and atmospheric-ionospheric disturbances [Afraimovich et al.We present a new approach to the analysis of interferograms based on the gradient of the phase rather than the phase itself. Because this method is largely untested, we attempt to address the following questions: What is the best mathematical model for relating phase and phase gradients given uncertainties in the data? What are the main limitations of InSAR measurements derived from ERS data for both line of sight (LOS) accuracy and horizontal resolution? How can InSAR data be improved for both topographic recovery and change detection? What is the best design for an InSAR processing system in order to achieve near optimal results and be efficient? Of course, many Paper number 1998JB900008. 0 ! 48-0227/98/! 998 JB900008509.00 of these questions have been adequately addressed in previous publications, and there already exist tested and efficient InSAR codes. Nevertheless, we hope our answers will help clarify the literature in several areas.