InSAR and Closure Phase Errors: Two New Mechanisms

With advancements in and increased standardization of SAR hardware and processing algorithms, major InSAR errors have been largely mitigated, leading to substantial improvements in measurement precision and accuracy. The identification of new error sources will further enhance InSAR measurements and promote new and emerging InSAR applications. In this talk, we present two new mechanisms that cause InSAR and closure phase errors.
The first mechanism is associated with range misregistration. High-quality InSAR measurements require precise and accurate range coregistration both between the reference and secondary SAR images, and between the reference SAR image and the DEM used for computing the topographic phase. However, this requirement is not always satisfied due to range misregistration arising from various sources that have been largely overlooked. The range misregistration can occur between the reference image and the DEM, between the reference and secondary images, or due to the inhomogeneity within the range resolution cell. Our analysis reveals that, apart from decorrelation, the effects of all three types of misregistration ultimately reduce to that of an equivalent misregistration between the reference image and the DEM, which manifests as a phase error. Moreover, closure phase errors can be induced by InSAR phase errors arising from range misregistration between the reference and secondary images, as well as from the inhomogeneity within the range resolution cell. These InSAR and closure phase errors are confirmed by simulations and experiments with real data.
The second mechanism is associated with along-track ionospheric variations within the synthetic aperture. To analyze their effects, we decompose the along-track Total Electron Content (TEC) using a Taylor series expansion. An analysis of the matched filtering process reveals that odd-order components shift the target peak position, while even-order components introduce a phase error. Moreover, all components, except the linear one, can cause target defocusing. These effects can lead to non-negligible InSAR and closure phase errors. The resulting InSAR and closure phase errors are also confirmed by simulations and experiments with real data. These errors are particularly significant in the current golden age of L-band satellite SAR missions.