Phase Unwrapping in NEST

Introduction

The principal observation in radar interferometry, is the two-dimensional relative phase signal, which is the 2pi-modulus of the (unknown) absolute phase signal. The forward problem, the wrapping of the absolute phase to the [-pi,pi) interval is straightforward and trivial. The inverse problem, the so-called phase unwrapping, due to inherent non-uniqueness and non-linearity, is one of the main difficulties and challenges in the application of radar interferometry.

There are many proposed techniques to deal with the phase unwrapping problem. The variable phase noise, as well as the geometric problems, i.e., foreshortening and layover, are the main causes why many of the proposed techniques do not perform as desired. Furthermore, any of the given phase unwrapping techniques will not give a unique solution, and without additional a-priori information, or strong assumptions on the data behaviour, it is impossible to assess the reliability of the solution.

Phase Unwrapping in NEST

The phase unwrapping functionality is not implemented in NEST. The dedicated phase unwrapping modules for NEST are in the development process, and are expected to be released by late 2012.

Interface to 3rd party software: SNAPHU

To obtain the unwrapped interferogram, a 3rd party software should be used. The recommended tool to perform the phase unwrapping is the "Statistical-Cost, Network-Flow Algorithm for Phase Unwrapping" (SNAPHU), that can be downloaded from SNAPHU project web-page. The restricted distribution license of this software prevented a direct integration in NEST, so from the user it is expected to download and install this, or other, 3rd party software for the phase unwrapping individually.

SNAPHU support tools

To make integration between NEST and SNAPHU as easy as possible, we developed a set of tools for exporting and importing data to/from SNAPHU. It should be noted that these tools are developed for the user convenience and that the same functionality can be achieved with chaining Product Generation Tools of NEST (eg. Band Arithmetic, Replace Metadata, and other operations), and manual construction of SNAPHU configuration files.

SNAPHU export
The graph for exporting NEST InSAR data to processing with SNAPHU, building SNAPHU configuration file, and creating a "phase" product. The phase product serves as a container an interface with SNAPHU. In the phase product the wrapped phase is saved, with the corresponding metadata.
SNAPHU import
The importing (ingestion) of data in previously created "phase unwrapping" container product. With importing the unwrapped phase, the existing wrapped phase data in the phase product is replaced with unwrapped phase, while preserving the metadata of the unwrapped product, or the phase product is extended with the unwrapped phase band.

Further information

Phase unwrapping: For a general reference on phase unwrapping see book of Ghiglia and Pritt, Two-Dimensional phase unwrapping: theory, algorithms, and software.

Building and running SNAPHU: A good starting point for obtaining further information on SNAPHU software and algorithms is the project web page. SNAPHU is software developed for UNIX environment and as such building it on Linux and MacOS systems is straightforward. On Microsoft Windows operating systems, SNAPHU can be built and executed on any of the Unix like environments and command-line interface for Windows (Cygwin, MiniGW, etc.)