Interferometric Coherence of Bistatic Radar Observations and Spatial Resolution

The coherence of Interferometric Synthetic Aperture Radar (InSAR) observations has many applications, including monitoring of surface topography changes, soil moisture, forest biomass and snow water equivalent. The InSAR phase measurement accuracy depends on the interferometric coherence of radar signals from two repeat passes. The interferometric coherence can be influenced by spatial resolution and geometry offset in observations in addition to temporal variabilities of land cover. The formulation of interferometric coherence for nominal backscattering geometry has been well established but has not yet been presented for bistatic forward radar scattering. In this article, we present the theoretical expression of interferometric coherence for bistatic geometry along the forward scattering direction. We model the natural variability by bivariate Gaussian distributions of the phase changes of repeat pass radar observations, which can be characterized by spatial correlation and standard deviations of terrain cover changes. We have completed a set of parametric analyses to assess the dependence on satellite baseline offset, spatial resolution, and cross-track distance. Specifically, we applied the theoretical expression to examine the impact of spatial variability of snow depths on the interferometric coherence for two SAR measurement geometries: monostatic backscattering SAR and bistatic forward looking Signals of Opportunity SAR and illustrated their differences.