An analysis of the potentials of L-band SAR satellites for measuring azimuth motion

Azimuth or along-track (approximately north-south) motion is critical in constructing three-dimensional ground motion with synthetic aperture radar (SAR) satellites orbiting the Earth in sun-synchronous polar orbit. The main problem of measuring azimuth motion with short-wavelength SAR data is decorrelation. A fleet of newly launched and upcoming long-wavelength L-band SAR satellites bring new opportunities for measuring azimuth motion. However, azimuth motion measured with L-band SAR data often contains large azimuth shifts caused by the Earth's ionosphere. We outline the framework of separating the azimuth motion and ionospheric azimuth shift from an analysis of the ionospheric effects on SAR images and SAR measurement precisions. We demonstrate three methods, among which one is newly proposed, can separate the azimuth motion and ionospheric azimuth shift with higher precisions. We evaluate the performances of the three methods by simulations using parameters of several selected L-band SAR satellites. The results show that, at kilometer resolutions, the azimuth motion measured by multiple-aperture SAR interferometry (MAI) can achieve centimeter precision, while the ionospheric azimuth shifts can be estimated with decimeter precision. Based on these results, a strategy for obtaining corrected azimuth motion is subsequently suggested, which achieves at least a first-order ionospheric correction of the original higher resolution MAI result. The three methods were also compared by real data processing examples. Furthermore, using real and simulated data of selected L-band SAR satellites, we present the first L-band MAI time series analysis result that measures subtle ground motion, as illustrated by the example of the postseismic deformation after the 2016 Kumamoto earthquakes in Japan. The performance is expected to be further improved with future L-band SAR missions that have much higher duty cycles. Some geophysical applications, in particular, those associated with the Earth's tectonic processes, can thus benefit from the azimuth motion measured by L-band SAR data.