Compensation of Ionospheric Azimuth Shifts Using Azimuth Sub-Apertures Interferometry

Abstract

L-band synthetic aperture radar interferograms have the potential to measure geophysical processes such as earthquakes, volcanoes, landslides, and glacier and tectonics movements. The coherence of L-band interferograms is usually higher than that of C- or X-band interferograms thanks to the higher penetration rate of L-band signals, improving the measurements. On the other hand however, lower frequencies are also more sensible to the ionospheric influence. The ionosphere is a layer of the atmosphere where the electron density of ionized gases is high enough to affect the propagation of radiowaves. The effects on L-band signals are a phase advance and a time delay which, converted to meters, can consist of even several meters. Variations of the differential ionospheric total electron content (TEC) between the acquisitions are measured from the interferogram; they are superimposed to topography and ground deformation signals, hindering the measurement of geophysical processes. The ground movements, between acquisitions, in the along-track direction can normally be measured by cross-correlating the SAR images. However, a second ionospheric effect to SAR signals, in the form of azimuth shifts, is generated by the variations of the TEC along the azimuth direction. For this reasons, even if L-band data are more suitable to measure ground movements with respect to C- or X-band data, in particular in difficult conditions like vegetated areas, they are more susceptible to errors due to ionospheric variations. These ionosphere-induced errors have therefore to be removed to enable thorough modelling of geophysical processes.  In this work we propose a method to estimate and compensate ionospheric effects to interferograms. The method is based on the combination of two ionosphere estimation techniques, with the aim to take advantage of the strengths of each technique, and to obtain a precise and robust result. The two techniques are the split-spectrum method and the azimuth-shift method. The split-spectrum method is based on the dispersive nature of the ionosphere and separates the ionospheric component of the interferometric phase from the nondispersive component related to topography, ground motion, and tropospheric path delay. The split-spectrum method ensures accurate estimation over long wavelengths and can recover range variations, but cannot recover rapid changes of the ionosphere. It is therefore not precise enough to estimate the small-scale ionospheric variations and consequently the ionosphere-induced azimuth shifts. The azimuth-shift method exploits the proportional relation between differential azimuth shift and the azimuth derivative of the differential ionosphere. Being sensitive to local azimuth variations of the ionosphere, the azimuth shifts can be used to estimate the high-frequency components of the ionosphere spectrum but are prone to an increasing error in the long distance and are insensitive to range variations. The proposed method uses three azimuth sub-bands to estimate the azimuth shifts and to separate ground motion from the ionosphere-induced shifts. A Bayesian inverse problem is used to combine the two techniques.  The results of applying the proposed method to ALOS PALSAR images will be presented in this work. Earthquakes often show distinctive ground motion signals, L-band interferograms of earthquakes which also show ionospheric disturbances are used to demonstrate the potentials of the combined method with respect to the single split-spectrum technique.