Why my DEM made by S1A data have no elevation value?

[xydf_1992]

I created a DEM using S1A data.
The steps are below:
1 Open the Products
2 Coregister the images
Radar-Coregistration-S1 TOPS Coregistration
The tab of Read is the Master Image and the tab of Read(2) is the Slave Image.
all my Polarisations are VV
I am very confused as the orginal data is presented as 9: a lage rectangle
well after Coregistraton, the data is presented as 8: a rectangle much smaller
3 Form the Interferogram
4 TOPS Deburst
5 Phase Filtering
6 Export to SNAPHU
7 Unwrap Interferogram with SNAPHU
8 Open the files in S1TBX
9 Create the DEM -Convert Phase to Elevation
10 Geocode the DEM
well after all these steps, my result is:

all the district is black, with no elevation value.
Could someone tell me, what’s wrong with my steps? Thank you very much.

[xydf_1992]

I am very confused as the orginal data is presented as 9: a lage rectangle

well after Coregistraton, the data is presented as 8: a rectangle much smaller

[ABraun]

TOPS Coregistration only supports the processing of single swaths. You selected IW3 in your screenshot (under ‘Subswath’).
You have to repeat coregistration for all three subswaths and then use the TOPS Merge operator.

[ABraun]

About the black output: Carefully check the result of every step after you processed it. Otherwise it is not possible to tell at which step your information was lost.

[xydf_1992]

I deburst IW1 to IW3, then I use TOPS Merge, but it reports an error

[xydf_1992]

After Deburst, the result is like this: all the district is purple and I can’t see any valueable information.

I suspect that there is a problem with the DEM used by Back Geocoding when coregister

[xydf_1992]

After Coregister, there is nothing in Slave Intensity Image, is it normal?
Thank you very much.

[ABraun]

if your data is simply purple, there must be an error before the interferogram generation step already.

[xydf_1992]

yes, after coregister, the slave image have nothing. Could you tell me what’s wrong with the slave image, I tried to do this on another computer, but it doesn’t work.

[xydf_1992]

I tried many times, every time after my S1 TOPS Coregistration, the Intensity_IW_slv image presents black.
No matter how I replace SAR data, the result is slave image black.

[ABraun]

please name the products you use for image 1 and image 2 of the TOPS Coregistration

[xydf_1992]

S1A_IW_SLC__1SDV_20180907T100450_20180907T100517_023591_0291E9_13F0
S1A_IW_SLC__1SDV_20180919T100451_20180919T100518_023766_02978B_4C8C
S1A_IW_SLC__1SDV_20181001T100451_20181001T100518_023941_029D3D_1192
all these, when it use as master image is ok, but the slave image is always black.

[ABraun]

I made a test with the first two scenes.
I used S1 TOPS Coregistration with ESD

• Subswath IW3
• VV Polarization
• Bursts 1-3
• SRTM 1Sec (AutoDownload) for the BackGeocoding

and left all other parameters as standard.

Worked out well.

[xydf_1992]

I tried S1 TOPS Coregistration with ESD，but Intensity_IW3_VV_slv1 still empty, the RGB only presents Intensity_IW3_VV_mst1

[ABraun]

I can’t reproduce this error, sorry.

[xydf_1992]

I just solved the problem, I changed the Back-Geocoding DEM, I used my local DEM for Back-Geocoding. I think maybe my error is caused by the Great Firewall of China. Thank you very much!

[xydf_1992]

The Unwrapping result is as fellow:

After Phase to Elevation:

It’s black again.

[ABraun]

Phase to elevation also uses a DEM.
Change it to SRTM 3Sec or ACE30, for example

[xydf_1992]

I finally made a DEM .

But the elevation ranges from 4000 to -4900 , that doesn’t match reality.

[ABraun]

The extreme values don’t matter. They result from unwrapping errors and atmospheric disturbances of the phase signal. Have a look at your coherence layer. It tells you where the quality of the DEM is trustful.
If there is vegetation in your area, it is unlikely that you will receive a perfect DEM from repeat-pass InSAR because of temporal decorrelation.