Reducing Ionospheric Decorrelation Effects in Insar Data Using Accurate Coregistration

Interferometric synthetic aperture radar (InSAR) is now routinely used to generate topographic data and to study geophysical phenomena, such as crustal deformation, ice motion and structure, and vegetation canopy depths [1]. In this study, we show that ionospheric disturbances can distort InSAR phase and correlation maps, and, further, that accurate image coregistration can compensate for ionospheric propagation variations and significantly improve the interferometric coherence. Specifically, spaceborne synthetic aperture radar (SAR) instruments illuminate the Earth's surface with a sequence of narrowband microwave pulses and receive the backscattered echoes from these pulses. Both the transmitted and received signals propagate through the ionosphere, which causes the phase to be advanced by an amount proportional to the total electron content (TEC) along the propagation path [2]. Therefore, an azimuth gradient in the TEC 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. Because of temporal variations in the ionosphere, these offsets are different in the two SAR images of an interferometric pair. As a result, when the offset between the two images is described using a low-order polynomial function of range and azimuth position, there are regions where the two images will not be correctly coregistered. These regions often form “azimuth streaks” which can be particularly salient in coherence images [3].