Help Content > Interferometry Module > Phase Processing - Interferometry Single Steps > 3

Purpose

The phase of the interferogram can only be modulo 2π; hence anytime the phase change becomes larger than 2π the phase starts again and the cycle repeats itself. Phase Unwrapping is the process that resolves this 2π ambiguity. Several algorithms (such as the branch-cuts, region growing, minimum cost flow, minimum least squares, multi-baseline, etc.) have been developed; in essence, none of these is perfect and different or combined approaches should be applied on a case by case basis to get optimal results.

Depending on specific data characteristics a further phase editing could be required in order to correct errors in the unwrapped interferogram.

Technical Note

Two methods are implemented:

Region Growing -> This is the default unwrapping algorithm. If this method is selected, it is suggested to avoid setting a high coherence threshold (good values are typically between 0.15 and 0.2) in order to leave enough freedom during the growing process; this shall limit the possibility to introduce erroneous phase jumps - "unwrapping islands" - in the output unwrapped phase image.

Minimum Cost Flow -> This method should be adopted when the unwrapping process becomes difficult due to the presence of large areas of low coherence or other growing limiting factors; in such cases the Minimum Cost Flow algorithm enables to obtain better results than using the Region Growing method. This approach considers a square grid all over the image pixels. All pixels whose coherence is lower than the "Unwrapping Coherence Threshold" are masked out.

Delaunay MCF (Minimum Cost Flow) -> It is the same approach of the previous method, with the only difference that the grid does not necessarily covers all image pixels, but only those above the "Unwrapping Coherence Threshold"; moreover it adopts the Delaunay triangular grid instead of square one. As result only the points with good coherence are unwrapped, without any influence from the low coherence pixels. The exploitation of the Delaunay triangulation is especially useful when there are several areas of low coherence (water bodies, densely vegetated areas, etc.) distributed throughout the image; in such case the others unwrapping approaches would eventually produce phase islands/jumps, while the Delaunay approach is able to minimize these jumps.

Decomposition Levels

The process is normally executed with the original pixel sampling (i.e. 0) or with the minimum decomposition level (i.e. 1). The use of the decomposition is intended to multilook and undersample the data in an iterative way: the interferogram is unwrapped at the lower resolution and then reconstructed back at the original resolution.

The use of the decomposition can help to reduce unwrapping errors (e.g. in case of distributed low coherence areas) and it reduces the processing time and it limits the use of computer resources.

The user can specify the number of iterations (decompositions) to be executed; each iteration corresponds to an undersampling factor of 3. We suggest to avoid setting this value higher than 2. In case of very large displacements or very steep topography (fast phase/dense fringe distribution) the use of the decomposition can cause aliasing effects. In this case the decomposition process should be avoided by setting its value to 0.

Tandem-X Data

When these data are acquired in bistatic mode, the unwrapped phase (_upha) is automatically reflattened (_reflat_upha). Due to this the Refinement and Re-flattening step has not to be performed.

Input Files

Coherence file

File name of the coherence (_cc). This file is mandatory.

Interferogram file

File name of the flattened - and possibly filtered - interferogram (_fint). This file is mandatory.

Parameters - Principal Parameters

Unwrapping Method Type

The choice is given between the following unwrapping methods:

•Region Growing, the Region Growing unwrapping method is used;

•Minimum Cost Flow, the Minimum Cost Flow (square grid) unwrapping method is used;

•Delaunay MCF, the Minimum Cost Flow (triangular grid) unwrapping method is used.

Unwrapping Decomposition Level

The number of under sampling levels to be applied to the complex interferogram before performing the phase unwrapping can be specified (refer to the Technical Note).

Unwrapping Coherence Threshold

Pixels with coherence values smaller than this threshold are not unwrapped.

Parameters - Global

It brings to the general section of the Preferences parameters. Any modified value will be used and stored for further processing sessions.

Parameters - Phase Unwrapping

It brings to the interferometry section of the Preferences parameters. Any modified value will be used and stored for further processing sessions.

Parameters - Other Parameters

It brings to the general section of the Preferences parameters. Any modified value will be used and stored for further processing sessions.

Output Files

Unwrapped Phase file

File name of the output unwrapped phase. This file is mandatory.

_upha

Unwrapped phase with the associated header files (.sml, .hdr).

_reflat_upha

Re-flattened unwrapped phase with the associated header files (.sml, .hdr). This file is generated only for Tandem-X bistatic Data.

Details specific to the Units of Measure and Nomenclature of the output products can be found in the Data Format section.

General Functions

Exec

The processing step is executed.

Store Batch

The processing step is stored in the batch list. The Batch Browser button allows to load the batch processing list.

The window will be closed.

Help

Specific help document section.

Specific Function(s)

None.

Interferometry - Phase processing - 3 - Phase Unwrapping

Interferometry - Phase processing - 3 - Phase Unwrapping