Multi-level Phase Unwrapping Method by Combining the Branch-cut and Minimum Cost Flow

The complexity of the mountainous terrain leads to the large phase gradients of the interferogram, especially in the area with the dense vegetaion. And the resulting low coherence brings difficulty to phase unwrapping. To overcome the difficulty of phase unwrapping in these areas and the weakness of a single method, this paper proposes a novel multi-level phase unwrapping method combining the branch-cut and minimum cost flow algorithm. The proposed method is inspired by the principle of "control first then fragments" in surveying, specifically, the unwrapping task is progressively performed from the high-coherence regions to the low-coherence regions by taking the coherence as an indicator. Firstly, the unwrapped phase result obtained from the minimum cost flow method in the high-coherence region is seen as the "control" part, i.e. the initial phases. Secondly, the "fragments" phases are computed using the branch-cut algorithm. The process is repeated several times until the interferom of the whole image is done, and each time a part of the remaining low-coherence region is determined by setting an appropriate coherence threshold then unwrapped based on the previous unwrapping phases. The experimental result using the TanDEM-X Bistatic mode demonstrates the new method has higher efficiency and more reliability than the branch-cut method and the minimum cost flow method.