Estimation of Residual Motion Errors and Phase Ambiguity for Repeat-Pass In-CSAR Without External DEMs

For airborne repeat-pass synthetic aperture radar interferometry (InSAR), residual motion errors (RMEs) exist between the true and measured trajectories due to the inaccuracy of current navigation systems. In addition, a global unknown phase ambiguity (PA) exists in the unwrapped phase due to the error of absolute phase estimation. The RME and PA are significant error sources in the repeat-pass InSAR, which can cause phase errors in the final interferograms. In general, an external digital elevation model (DEM) can be used to estimate the RME and PA. However, it is hard to find a high-precision external DEM matching with the airborne InSAR data. Compared with the traditional InSAR technique, the 360° aperture gives circular InSAR (In-CSAR) the capability to estimate the RME and PA without external DEMs. Unlike previous approaches, in this article, a multiangle observation model is established to estimate RME and PA simultaneously. This model registers the DEM images obtained from different observation angles. In the field of image registration, the Demons algorithm is often used for nonrigid registration. We use, for the first time, the Demons algorithm to register multiangle false DEM images containing height errors and obtain the reference DEMs. Due to the gradient of the DEM image being destroyed by the RME and PA, a two-step preprocessing is proposed to achieve subpixel registration of multiangle false DEM images. After obtaining the reference DEM, the least square (LS) or robust mixed integer linear programming (RMILP) can estimate the unknown RME and PA. The results show that, with respect to the true values, the root-mean-square error (RMSE) of the calibrated DEMs is 0.37 m for the simulated dataset and 1.17 m for the real dataset. After correcting the RME-induced and PA-induced height errors, the RMSE of the DEM is improved by at least 92.6% for the simulated dataset and by at least 70.8% for the real dataset.