Feasibility of ALOS2 PALSAR2 Offset-Based Phase Unwrapping of SAR Interferogram in Large and Complex Surface Deformations

The interferometric synthetic aperture radar (InSAR) is a well-known tool for mapping ground surface deformations with centimeter precision, and is much better than that used in the offset tracking method. However, the InSAR method has difficulties in precisely measuring large deformation signals because the phase unwrapping can include severe errors in the case of large and complex deformations. To overcome this drawback, an offset-based unwrapping strategy was proposed. However, this strategy has almost never been used for the unwrapping approach because the accuracy of offset measurement is much lower than that of InSAR measurement. As the spatial resolution of the SAR image becomes more precise, the offset measurement accuracy increases. Specifically, the L-band ALOS2 PALSAR2 sensor has much higher offset measurement accuracy due to its higher spatial resolution. In this paper, the feasibility of the unwrapping of a PALSAR-2 interferometric phase with large surface deformation signals, using an offset map, is proved. This validation of offset-based unwrapping was performed using two ALOS2 PALSAR2 co-seismic pairs in the 2016 Kumamoto earthquake where large deformations of more than 200 cm occurred. The offset-based unwrapping was more successful than traditional unwrapping. The root-mean square errors of offset-based unwrapping using GPS in situ data were about 1.96 and 1.90 cm in the ascending and descending pairs, respectively. The validation results showed that the ALOS2 PALSAR2 offset-based unwrapping enables us to measure complex and large deformation signals accurately. Precise measurements in high-gradient and high-decorrelated deformation areas would be helpful to better understand geological mechanisms.