Wrong linear and continuous subsidence measured by S-1A

Microwave Toolbox Interferometry
1

Dear S-1 InSAR community,

I have been building ground deformation maps of the same area of around 100 km^2, for a duration of 1 to 2 years using S1A products (sampling time of 12 days, Vertical polarisation, ascending orbit and band 2).
In order to work with SAR products as coherent as possible, the strategy consists in building one deformation map with every pair of consecutive images. For instance, with 50 consecutive products, acquired every 12 days, 49 deformation maps can be built.
If for each pair of products used to build a deformation map the Master (oldest) / Slave (newest) order is respected, the global deformation map for the entire time period can be obtained by simply adding all the individual deformation maps.

I used the standard vertical displacement operator offered by SNAP and executed the standard set of successive operators needed to build InSARs and deformation maps. SNAP default parameters were kept as much as possible.

The result looks pretty good, but I can observe a continuous and linear subsidence of the whole area of interest that has never been reported before. Here we are speaking about 2 mm of subsidence per day! It really seems to be an error: too big, too linear, not known and the same effect has been observed in two different areas when using the same algorithms.

  • Has anybody else observed the same problem?
  • Is it a known problem?
  • Are there extra calibration parameters to be applied?

My strategy for building ground deformation maps is supposed to be as standard as possible and is solely based on SNAP and SNAPHU.
Any help or clarification would be more than welcome.

KR, JB

2

Could you provide more information regarding your processing chain (e.g. operators and parameters etc.) so that we can reproduce the problem? Thank you.
A JIRA ticket (Jira) has been created to track the issue. We will look into it.

3

Depending on the surface characteristics of the area you are studying, the phase closure bias might be affecting your velocity estimates. Using only adjacent-in-time interferograms can cause an ongoing bias in the phase. See
Zheng, Y., H. Fattahi, P. Agram, M. Simons, and P. Rosen (2022), On Closure Phase and Systematic Bias in Multilooked SAR Interferometry, IEEE Transactions on Geoscience and Remote Sensing, 60, 1–11, doi:10.1109/tgrs.2022.3167648.

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4

Dear EJFielding,
Indeed, it seems I did what you are describing. Again, the idea is to work with pairs of images as coherent as possible, i.e. close in time. Therefore, I need to find another strategy.
I will try to find the article.
Many thanks for your support.
KR, JB

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