I’am working with SNAP V5.0 on Sentinel-1 data for some weeks now on a small Area of Interest (AoI).
I’ve download several S1A_IW_SLC products encompassing the AoI and have done the following processing with SNAP:
- find the swath of the AoI and split to this Swath
- deburst the splitted product
- apply the orbit-file
- apply the single speckle filter Gamma-Map 3x3
- collocate the results of 3. and 4.
- build the quotient of Intensities_VH before and after despeckling (looking for mean and sigma in the statistics) with band-maths
The results of the calculated band for an ascending (AoI_S1A_IW_SLC__1SDV_20161225_IW2) and a descending passage (AoI_S1A_IW_SLC__1SDV_20161229_IW3) together with the coordinates of the AoI are shown:
(Since I couldn’t upload two pictures as a new user I describe the “descending picture”: in the upper half of the image three smaller ellipsoids (in comparison to the picture above) show up)
Due to the regularity I think the ellipsoids are artefacts. But of what kind?
- I’ve found no hint about possible deficiencies of sentinel-1-data
- Speckle-Filtering works locally so (imho) can’t produce such big regular feature
- Importing the artefacts by the other processing steps with SNAP looks very improbable
But in the meantime I found the following
- The regular pattern were seen in the whole sub-swath of a product so the problem is independent of the AoI.
- (Mis-)Using the stack-Operator of SNAP to collacate the intensities before and after despeckling and building the quotient shows no regular structure.
- Using the Scatter-Operator of SNAP to compare the despeckled intensity with the collocated one shows heavy aberration from the expected line (in contrary to the same procedure with the stacked products).
- Using bilinear interpolation resampling (instead of nearest neighbor resampling) produces comparable but less contrasted images for the quotient.
Did I miss some preconditions for using the collocation-operator in SNAP?
Is there some unexpected behavior of the collocation algorithm?
Since I heavily use the collocation for comparisons I am very interested in an answer.