Topographic phase recovery from stacked ERS interferometry and a low‐resolution digital elevation model

A hybrid approach to topographic recovery from ERS interferometry is developed and assessed. Tropospheric/ionospheric artifacts, imprecise orbital information, and layover are key issues in recovering topography and surface deformation from repeat‐pass interferometry. Previously, we developed a phase gradient approach to stacking interferograms to reduce these errors and also to reduce the short‐wavelength phase noise (see Sandwell and Price [1998] and Appendix A). Here the method is extended to use a low‐resolution digital elevation model to constrain long‐wavelength phase errors and an iteration scheme to minimize errors in the computation of phase gradient. We demonstrate the topographic phase recovery on 16‐m postings using 25 ERS synthetic aperture radar images from an area of southern California containing 2700 m of relief. On the basis of a comparison with 81 GPS monuments, the ERS‐ derived topography has a typical absolute accuracy of better than 10 m except in areas of layover. The resulting topographic phase enables accurate two‐pass, real‐time interferometry even in mountainous areas where traditional phase unwrapping schemes fail. As an example, we form a topography‐free (127‐m perpendicular baseline) interferogram spanning 7.5 years; fringes from two major earthquakes and aseismic slip on the San Andreas Fault are clearly isolated.