Refinement step and GCP collection for SARscape Interferometry

 

The SARscape interferometry chain includes a step called Refinement and Re-flattening. 

This step consists of refining the orbits (i.e. to correct possible baseline inaccuracies and to remove orbital fringes), to calculate the phase offset (i.e. to get the absolute phase values before phase conversion to elevation or displacement values) and to remove possible phase ramps.

 

When should the refinement step be completed?

The refinement step is mandatory for DEM generation and for displacement map generation. This step can be completed during the interferometry workflow or as a single step if you are not using a predefined workflow:

This step is available as a single step from the following SARscape menu:

              /SARscape/Interferometry/Phase Processing/4 -Refinement and Re-flattening

This step will require input GCPs.

How to select GCPs for the refinement step?

The GCPs for the refinement step should be selected (using the Generate Ground Control Points tool of SARscape) based on the following recommendations:

      - The GCPs should be located on the residual orbital fringes if present

      - The GCPs should not be located on topographic fringes or displacement fringes

      - The GCPs should be located on high coherence area

      - The GCPs should be well distributed on the area of interest

The best approach is then:

      -  to select GCPs on the differential interferogram (_dint or _fint) so the user can easily identify the topographic and displacement fringes and avoid these areas when selecting GCPs

      -  to display the coherence map (_cc) at the same time as the differential interferogram so the user can identify high coherence areas and select GCPs in these areas.

Note that GCPs located on NaN values will be automatically discarded during the refinement process.

If an input DEM (_srdem) had been used for the interferogram flattening the relevant map coordinates and elevation of each GCP will be automatically extracted from this input DEM during the refinement: the user would not need to manually enter the GCP map coordinates when creating the GCP file.

Which refinement method should be selected?

The refinement method can be selected from the Parameters tab in the Refinement and Re-flattening interface. By default the Automatic refinement method is selected. It is recommended to keep this configuration.

This configuration allows SARscape to automatically switch from the orbital refinement method to the polynomial residual phase evaluation method when

-      the number of valid GCP is less than 7

-      the baseline is too short to obtain a relevant result using orbital refinement

How to check if the output of the refinement method is relevant?

The first check a user should complete to evaluate the quality of the refinement is to analyze the RMS displayed in the output window. This RMS should be higher than 1 and less than 10.

An RMS higher than 10 can be considered as valid for very high resolution images.

An output RMS between1 and 10 means that the user can pursue the interferometry process to generate DEM or displacement maps.

The user can also analyze the re-flattened outputs, especially the _reflat_dint,_reflat_fint and _reflat_upha files: they should not show orbital fringes or phase ramps anymore if those were present before the refinement step.

 

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Written by BC on 10/28/2014,  Reviewed by PS on 10/28/2014