Terrain Flattening Correction

Hello I am Diego, I’m working with Sentinel-1B GRD IW products. I have already done the calibration of my image in this order:

  • Thermal noise removal
  • Apply precise ephemerides
  • Border noise removal (default settings)
  • Calibration to obtain Betta0
  • Terrain Flattening Correction (Gamma0 with the SRTM 1’)
  • Speckle filtering (Gamma-MAP/kernel 7x7)
  • Range Doppler Terrain Correction (use of the SRTM 1’)

Finally, when I get my product the VH band doesn’t have any information, it is empty. I only have a result in the VV band. I repeated the process in order to find an error, and I saw that before doing the terrain flattening I have information in both bands but after doing TFC all the information just disappear.

If anyone can help, I will really appreciate their advices.

So the image is empty after Terrain Flattening?
Have you selected the Auto Download DEM? If no elevation data is found the operator will produce incorrect outputs.
Maybe your data lies outside the SRTM coverage?
Is your data near the coast or an island?

Speckle filtering should be done before terrain flattening that changes the statistics of the image. Now that I think of it I’m not sure if speckle filters automatically convert between sigma0/beta0 as needed? @jun_lu

This is a bug and it has already been fixed. The fix should be available in the next release.


Thank you for your quick response. In answer to your questions, after Terrain Flattening I just have information in the VV band and nothing in the VH band.
I have selected the auto download DEM I’m sure of that. And finally, the observation about the area of interest is really good because I’m working with an island, but I doubt about it because in my results I have information in my VH band or could I have that problem you say just in my VV band.?


Thanks for your response and advice, I will try to do the correction of my image in that order to see the results.

Hi, thanks for your response and observation. But I have already checked the updates and the SNAP is up to date.

The update was not released yet, but the fix is included in the next one.

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Same error over here, thank you very much for your answers.
Any clue about the release that will solve the bug?
approximate date, version number … Anyway to know when this process will work could be great!
Otherwise some of us will be testing periodically to see if it works.

Thank you again!

Hi Javier,
I recommend you to run each tool by each polaration image. I first correct my VV image and then I worked with the VH polarization and that was a solution for me. I hope to help you with your analysis.


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Hi Diego,
that’s a good tip, thanks. In this case we have to process several dates, with and without merging frames and with some band math to calculate ratios and indexes (it’s an automatic classification of crops routine).

We can wait some weeks for the updates before changing the workflow, but if needed, we’ll follow your recommendation. I tested it and it works as you said: each polarization in a different process.



Hi Andreas,
do you know when this update will be released?

I am working on an area with orography, and I expect my results to improve a lot with terrain flattering correction.
At the moment I am triying a couple of tricks to filter those echoes out.

Thanks, Sabrina

Actually you can use the current terrain flattening operator if you just want the TF result for one polarization. In case you want TF result for both polarization, you have to run the operator twice with each time one TF result for one polarization

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Thanks for this trick, Jun Lu!

Hi there again.
I just wanted to share that, after having updated SNAP, the TF tool is still not working properly.
It just - still - works on one band. :worried:

Isn’t this highly inefficient as the area correction factor is identical for both polarisations? Surely being able to correct both polarisations at once should be the preferred method?

I think this is fixed with SNAP 8, if I correctly remember testing it.

@ABraun is correct. SNAP 8 now supports terrain flattening for dual pol data. I tested this with ALOS-2 PALSAR-2 L 1.1. (SM3) dual-pol imagery.

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