Atmospheric artifacts also show up as fringes - you should not expect that a single pair gives you an error-free measurement.
I applied the topographic phase removal then i can clearly see the fringes only on the glaciers. How do I make sure whether the fringes are due to atmosheric artifacts? I would really appreciate any help on this matter.
I also read that atmospheric artifacts are removed when correcting the orbit. Or is it not?
Orbit correction removes most of the orbital fringes (fringes resulting from errors in the flat earth correction). Atmospheric fringes still remain, but they do not follow glaciers so you should be able to tell the difference between them.
can you explain briefly the last step … i identified the stable pixel …now i want to subtract the value of it with the remaining pixels to get the subsidence results … please explain the last step briefly
Hello ABraun,
your post is very interesting. By the way I cann’t get what do you mean with “apply orbit file before your co-registration”. Could you explain me better this step? 
Thank you so much!
Piero.
From the Help file:
The orbit state vectors provided in the metadata of a SAR product are generally not accurate and can be refined with the precise orbit files which are available days-to-weeks after the generation of the product.
The orbit file provides accurate satellite position and velocity information. Based on this information, the
orbit state vectors in the abstract metadata of the product are updated.
Applying orbit files in SNAP:
Menu > Radar > Apply Orbit file
Things to keep in mind:
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Ok thank you. In effect I do “apply orbit file” processor when I run the graph for co-registration.
that is right, it’s already included in the TOPSAR graph. The checkbox prevents the process to fail if no orbit file is found.
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Hi ABraun,
You mentioned about the data displacement extraction using DINSAR and high coherence region was reliable. However here I have a question about when high coherence region be selected?
there are two options here.
(1) select unwrapped phase with high coherence, then filter it and use SNAPHU to unwrap it.
(2) select unwrapped phase without considering coherence, then filter it, use SNAPHU to unwrap it and select displacement with only high coherence.
Do you think we should follow (1) or (2) when conducting DINSAR processing?
Thanks for all your instruction and response in this forum.
Best,
Xianwei
you can unwrap the whole image without considering the coherence at first, but you should later consider it when you interpret your result, e.g. masking out low coherence areas.
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Thanks for the reply. I will try both.
Processing the whole image is time consuming. However only unwrapping phase from high coherence may cause some problem as the phase may be spatially unconnected. That is my concern.
I would be interested in your results and how they are different. I am not sure either
Once I have some results, I will post them.
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Hi,
I’m trying to create a subsidence map using the steps mentioned above.
It looks like that I have a problem. My unwphase images (always) have values which decrease from one side if an image to another. As a result Step1 image [(Unw_Phase * wavelength in mm) / (-4 * PI * cos(rad(incident_angle)))] has the same pattern of its values too. Values here fall from 65 on the north to - 20 on the south of image. I can’t find where my mistake or how to fix it.
My processing steps.
- Split
- Orbit file
- Deburst
- Coregistration
- Ifg creation
- Topophase removal
- Goldstein filter)
- Unwrap
- What ABraun mentioned in the post Sep. 8, 2016.
I tried all the precessing steps possible: with and without topo, range and azimuth filters, all the options of Unw Export, etc. Despite that I always receive these “angled” unw.phases.
Please recommend me something. Thanks!
can you please show us the interferogram before unwrapping?
What is your perpendicular baseline?
This is my phase.
Here I have a baseline of 24m since I used 2 closest passes of the same sattelite. But in another case, I have a baseline of 80 m (2 satellites used) and unw looks the same. I started to think about SNAPHU, maybe is it possible that it works wrong?
The impact of topography on your phase is too low. Try another image pair with a higher baseline:
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Hi ABraun,
Actually I could not make Snaphu Export working when using the phase after masking with coherence threshold.
I used the coherence coefficient (threshold=0.50) to create a mask and used this mask to extract related phase (wrapped). A new phase band was generated. I gave this new phase band a different name (compared with the original phase name) and added it to the original data file. In this way, Snaphu Export worked and after unwrapping process, I noticed that the new phase band I generated was not used since no data gap was found in the unwrapped phase.
Then I tried another method. Deleted the original phase and gave the new phase band the same name as the original one. However Snaphu Export did not work and the reminder is “Wrapped phase band required”.
Snaphu Export did not find the wrapped phase band which was not because of the file name of wrapped phase band. Maybe because of some other information in the original data file. However I just created the new phase band with Band Maths, I did not think I changed much head information of phase band. Maybe the developer of SNAP can help to explain this.
Best,
Xianwei
Masking and unwrapping was reported to have issues several times. Try unwrapping the original phase and mask out low coherence areas at a last step.
Would you like to tell me please, what is a vertical resolution of a subsidence map? Does it equal to a wavelength?