Strategy to remove velocity jumps during the SARscape PS (Persistent Scatterers) processing

 

1. Example

When processing SAR data using the PS processing you may observe velocity jumps between groups of PS after the geocoding step.

See the example below with a blue group of PS in the middle of a green group of PS.

Such jumps are well visible after the geocoding step but could also be detected after the first inversion, in slant range geometry.

 

2. What is causing this effect?

This effect is related to the PS process that is dividing the whole area of interest in sub-areas, during the first inversion, if the area of interest is large enough - more than 25 square km by default.

You can check this threshold in the parameters of the first inversion: the parameter is called Subarea for Single Reference Point. Its default value is 25 sqkm.

During the first inversion step, one reference point per subarea is automatically set. This GCP usually corresponds to the highest coherence and the lowest velocity point in the given sub area. The PS velocity in each subarea will then be computed based on its own reference point, which may cause jumps in the final output.

 

3. How to visualize subareas and reference points

The reference point of each subarea can be displayed using the below approach

• Load the Velocity_first file in ENVI. This file is located in the output \work\work_first_inversion\ directory of the PS process
• Load the SubAreas.shp files to display the limits of subareas defined during the first inversion. This file is located in the output \work\work_first_inversion\ directory of the PS process
• Load the Ref_GCP.shp file to display the reference GCP for each subarea. This file is located in the output \work\work_first_inversion\ directory of the PS process
• The ID of each subarea is defined in the attribute table of the SubAreas.shp file: it corresponds to the polygon ID

 

4. Strategy to remove velocity jumps

Option 1: increase the minimum size to create subarea by increasing the value of the parameter called Subarea for Single Reference Point at the first inversion step. This will remove or reduce the number of subareas used for the computation, and so on limit the velocity jumps.

However, it may have side effects

• the decreasing of number of detected PS
• a less efficient atmospheric correction which will result in errors in velocity estimation for areas far from the reference point

 

Option 2:  select new reference points in the overlapped areas between subareas

This can be done using the tool called

 /SARscape/Interferometric Stacking/Stacking Tools/PS Compute Specific Sub-Area

After selecting the ID of the subarea(s) you need to update, select a new GCP in the overlapped area of subareas.

You would then need to re-run the first inversion and second inversion, as well as the geocoding step afterwards. A merging step needs to be performed during the first inversion and requires setting to true the SubArea Merging Only flag in the “Other Parameters” of the First Inversion. 

You can then check if the GCP update is valid by displaying the velocity map and/or the velocity first which should no longer show velocity jumps

 

 

If you need help when implementing the strategy to remove velocity jumps, please contact Tech Support.

 

 

 

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Created by BC on 11/10/2021

Reviewed by SARMAP and TS