Split-Band Interferometry-Assisted Phase Unwrapping for the Phase Ambiguities Correction

Ludivine Libert,Christian Barbier,Dominique Derauw,Nicolas D’Oreye,Anne Orban

doi:10.3390/rs9090879

Abstract

Split-Band Interferometry (SBInSAR) exploits the large range bandwidth of the new generation of synthetic aperture radar (SAR) sensors to process images at subrange bandwidth. Its application to an interferometric pair leads to several lower resolution interferograms of the same scene with slightly shifted central frequencies. When SBInSAR is applied to frequency-persistent scatterers, the linear trend of the phase through the stack of interferograms can be used to perform absolute and spatially independent phase unwrapping. While the height computation has been the main concern of studies on SBInSAR so far, we propose instead to use it to assist conventional phase unwrapping. During phase unwrapping, phase ambiguities are introduced when parts of the interferogram are separately unwrapped. The proposed method reduces the phase ambiguities so that the phase can be connected between separately unwrapped regions. The approach is tested on a pair of TerraSAR-X spotlight images of Copahue volcano, Argentina. In this framework, we propose two new criteria for the frequency-persistent scatterers detection, based respectively on the standard deviation of the slope of the linear regression and on the phase variance stability, and we compare them to the multifrequency phase error. Both new criteria appear to be more suited to our approach than the multifrequency phase error. We validate the SBInSAR-assisted phase unwrapping method by artificially splitting a continuous phase region into disconnected subzones. Despite the decorrelation and the steep topography affecting the volcanic test region, the expected phase ambiguities are successfully recovered whatever the chosen criterion to detect the frequency-persistent scatterers. Comparing the aspect ratio of the distributions of the computed phase ambiguities, the analysis shows that the phase variance stability is the most efficient criterion to select stable targets and the slope standard deviation gives satisfactory results.

Highlights

Over the years, performances of Synthetic Aperture Radar (SAR) sensors have been improved to reach the metric resolution by combining the synthetic aperture principle in the azimuth direction with an increase of the radar signal bandwidth in the range direction
Split-Band Interferometry is a three-step process derived from classical SAR Interferometry
The SBInSAR-assisted phase unwrapping is applied on the four artificially disconnected areas of the Copahue test case in order to validate the approach for the phase ambiguities correction and determine which detection criterion is the best

Summary

Introduction

Performances of Synthetic Aperture Radar (SAR) sensors have been improved to reach the metric resolution by combining the synthetic aperture principle in the azimuth direction with an increase of the radar signal bandwidth in the range direction Using such data, the well-known SAR Interferometry (InSAR) solves the relationship between the phase and the optical path difference to retrieve the topography. It applies InSAR to subrange images obtained by splitting the large available range bandwidth of recent sensors and explores the phase trend through the partial interferograms in order to provide pointwise absolute phase measurements This process is equivalent to an absolute and spatially independent phase unwrapping, as long as it is performed on scatterers with a stable behaviour across the spectral domain. The practical feasibility of topographic measurements has been reported in [3] for airborne data in X-band with a total bandwidth of 400 MHz

Methods

Results

Conclusion