MUZUBI – Advanced phase unwrapping using split-band interferometry

Absolute phase retrieval is a mandatory pre-requisite for accurate topographic measurements from SAR interferometric data. Because they use the phase of a starting point as a reference level, classical phase unwrapping techniques usually fail to connect regions separated by non-coherent areas and require additional corrections to produce continuous phase maps which are consistent with the ground truth. A technique commonly used for that purpose is split-band interferometry, also known as multi chromatic analysis (MCA), which exploits the large range bandwidth of SAR sensors such as TerraSAR-X / TanDEM-X. The idea behind this technique is that the absolute phase is proportional to the intercept and slope of the spectrally-dispersed phase across individual sub-bands. Previous MCA analysis generally focused on the search of carefully-chosen targets, called frequency-persistent scatterers, which are used to estimate the global phase offset of the entire disconnected region. In this study, we take another approach and argue that all pixels in each independently-unwrapped region carry useful information that can be used to improve the precision on the absolute phase. In the context of the MUZUBI project, we have applied this idea to TerraSAR-X / TanDEM-X and SENTINEL images and investigated the effect of sensor bandwidth, mean coherence across the region, and region size on the precision of the computed absolute phase.