Stamps output - how to spot unwrapping and atmospheric errors

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Gav H

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Sep 4, 2021, 4:57:29 AM9/4/21
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Hello
I’ve run the SNAP-StaMPS workflow for a masters project looking at subsidence on a golf course on a coastal peninsula. I’ve manged to run this successfully thanks to really clear and comprehensive guidance from Andreas Braun, Matthias Schlögl, Andrew Hooper and the ESA RUS training kit among others, for which many thanks. I am having difficulty though accepting the final velocity output.

The results seem to be robust to changing various parameters, I could not detect any unwrapping errors (although I'm not totally confident on what these look like), and the general of LOS range seems to be within a reasonable range for a non-seismic area (generally -20 to +10 mm/yr) when referenced to a GNSS station. All good so far.

However, when looking at the velocity output, two things strike me as odd.

  1. A huge section of the interior (~100km2) seems to be subsiding at a rate of 2-5mm/yr, which is unexpected as this is a hard rock area with shallow soils. The analysis period (12 months) hopefully mitigate against any seasonal effects (but not interannual ones). There seems to be a gradual gradient from SW-NE which is particularly notable in the southwest of the image.

  2. The maps shows lots of groupings 50-200m wide at the extreme ends of the range above. Closer inspection in the area of interest shows that a lot of these are on arable fields, with evidence of tracks showing movement of heavy machinery, but not always.

The ‘vdrop-do’ plots do not show any interferograms with strong influence on the output, and the results are similar for ascending and descending passes. I attach the velocity map and the ‘w-dm’ and ‘u-dm’ plots. I wonder though if anyone provide any thoughts on the following:

  1. Is the effect in (1) an atmospheric or unwrapping error (I applied the linear correction from TRAIN, but did not run step 8)?
  2. Are the groupings in (2) likely to be a real signal or localised unwrapping errors?

I also have a couple of more general questions:

  1. How does PSI work in areas of uniform ground, e.g. short grass/bare soil, where there is not a strong single dominant scatterer, but instead the signal is made up of a contribution from lots of different scatterers from the resolution cell? To what extent can we be confident that phases changes are a reflection of the ground moving and not anything else that might affect the phase shift, e.g. vegetation growth/die back, or soil disturbance?
  2. As StaMPS takes into account neighbouring cells when unwrapping, can errors propagated through a grouping of cells similar in size to that mentioned in (2)? I want trying to get a trade off between having a sufficient density (30-100 PS/km2 over non urban errors) and noise, but in any case the output seems robust to chaing the parmeters
Parameters used for these plots: Da=0.42, max_topo=10, weed_sd=1.2, percent
_rand=20, unwrap_gs=200, unwrap_time_win=730, scla_deramp=y (procssed as single patch)

Many thanks
Gavin

u-dm.png
v-do_refNWLN.png
w-dm_120.png
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