A Comparative Study of Ionospheric Correction on SAR Interferometry—A Case Study of L’Aquila Earthquake

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.

Detection, Estimation and Compensation of Ionospheric Effect on SAR Interferogram Using Azimuth Shift

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.

Ionosphere correction algorithm for spaceborne SAR imaging

For spaceborne synthetic aperture radar （SAR） imaging,the dispersive ionosphere has significant effects on the propagationof the low frequency （especially P-band） radar signal. Theionospheric effects can be a significant source of the phase error inthe radar signal, which causes a degeneration of the image qualityin spaceborne SAR imaging system. The background ionosphericeffects on spaceborne SAR through modeling and simulation areanalyzed, and the qualitative and quantitative analysis based onthe spatio-temporal variability of the ionosphere is given. A novelionosphere correction algorithm （ICA） is proposed to deal with theionospheric effects on the low frequency spaceborne SAR radarsignal. With the proposed algorithm, the degradation of the imagequality caused by the ionosphere is corrected. The simulation resultsshow the effectiveness of the proposed algorithm.