Reducing Ionospheric Effects in InSAR Data Using Accurate Coregistration

Interferometric synthetic aperture radar (InSAR) is a valuable tool for the study of geophysical phenomena such as crustal deformation, ice motion and structure, and vegetation canopy depths, but it is adversely affected by uncharacterized inhomogeneities in ionospheric propagation delay. Ionospheric disturbances distort both InSAR phase and correlation maps. Here, we present a method to compensate ionospheric propagation variations using accurate image coregistration. This significantly improves both the interferometric coherence and phase accuracy. An azimuth gradient in the total electron content (TEC) from a spatially variable ionosphere results in a range-dependent azimuth phase gradient being added to the phase histories of the pixels being imaged. These phase gradients are equivalent to Doppler shifts, and thus they cause azimuth offsets between the actual and imaged positions of the pixels. Measuring these offsets accurately permits estimation of the gradient and correction of the interferograms for much of the phase distortion, resulting in more accurate estimates of coherence. We show an example over Greenland where the TEC variation causes the correlation to drop from about 0.7 to about 0.2 in one region if spatially varying offsets are not accounted for; it also adds an estimated 4.4 radians of interferometric phase over an 80 km InSAR scene. After applying our algorithm, we find that the correlation in regions affected by the ionospheric inhomogeneity becomes comparable to correlation in the rest of the image. In a more challenging example over Iceland, we show that our method improves the correlation from 0.15 to 0.25 in some areas.