Synthetic Aperture Radar Interferometry (InSAR) has shown its potential on seismic deformation monitoring since it can achieve the accuracy of centimeter level or even the millimeter level. However, the irregular varieties of ionosphere can induce the additional phase delay on SAR interferometry, restricting its further application in high-precision deformation monitoring. Although several methods have been proposed to correct the ionospheric phase delay on SAR interferometry, the performances of them haven’t been evaluated and compared. In this study, three commonly used methods, including polynomial fitting, azimuth offset and split-spectrum are applied to L’Aquila Earthquake to correct the ionospheric phase delay on two Phased Array type L-band Synthetic Aperture Radar (PALSAR) onboard the Advanced Land Observing Satellite-1 (ALOS-1) images. The result indicates that these three methods can effectively correct the ionospheric phase delay error for SAR interferometry, where the standard deviations of the ionosphere-corrected results have decreased by almost a factor of 1.8 times for polynomial fitting method, 4.2 times for azimuth offset method and 2.5 times for split-spectrum method, compared to those of the original phase. Furthermore, the result of the sliding distribution inversion of the seismic fault shows the best performance for split-spectrum method.

An increasing interest in the use of low frequency Synthetic Aperture Radar (SAR) systems, e.g., L- and P-bands, makes the research of the ionospheric effects on SAR interferograms become urgent and significant. As the most pronounced signature in interferograms, the ionosphere-induced azimuth streak was thoroughly investigated in this study through processing of the 19 L-band Advanced Land-Observing Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR)images over the Chongqing City, China. The investigations show that the visible ionosphere-induced stripe-shape azimuth shifts with the invariable direction of 26°E, 113°N are observed in some interferometric pairs. Relating these anomalous azimuth shifts to the International GNSS Service (IGS) final ionospheric products shows that the detected ionosphere-contaminated SAR images display the relatively large ionospheric variation with time during SAR satellite travelled through the study area, indicating a somewhat correlation between them. After detecting the ionosphere-contaminated interferograms, we estimated the Ionospheric Phase Streak (IPS) based on an approximate linear relationship between IPS and azimuth shift, and then removed them from the original interferograms. The corrected results show that ionospheric phase patterns are largely removed from the ionosphere-contaminated interferograms. The investigation indicates that the direction of the IPS keeps approximately constant in space and time, which provides the potential chance to develop methods to correct the ionospheric effect. Furthermore, this study once more proves that the ionospheric effect on SAR interferogram can be detected, estimated and corrected from azimuth shifts.

Interferometric synthetic aperture radar (InSAR) has been widely used to measure ground displacements related to geophysical and anthropic activities over the past three decades. Satellite SAR systems use microwave signals that interact with the ionosphere when they travel through it during the imaging processes. In this context, ionospheric variations can significantly contaminate SAR imagery, which in turn affects spaceborne InSAR measurements. This bias also leads to a decrease in the coherence and accuracy of InSAR measurements, especially for the low-frequency SAR systems. In this paper, we give an overview of the latest methods for mitigating the ionospheric contributions in InSAR, including Faraday rotation method, azimuth shift method, and range split-spectrum method, and only focus on the single pair of InSAR interferograms. The current challenges and future perspectives are outlined at the end of this paper.