Making interferogram for TANDEM-X bistatic mode

Hi
I have two tandem-x images (one pair in 29.05.2015) in bistatic mode;
Temporal Coverage :
Start Date : 2015-05-29T03:19:01,357
End Date : 2015-05-29T03:19:07,383
1)TDX1_SAR__SSC_BRX2_SM_D_SRA_20150529T031901_20150529T031907
2)TSX1_SAR__SSC_BTX1_SM_D_SRA_20150529T031901_20150529T031907

I also have another pair 11 days before this, I mean one pair in 18.05.2015.


Figure 1. A TDX pair
I plan to see any topography events between both images, that’s why I have to make interferogram for any pair and then subtract them from each other.
I did below steps but I do not know it is ok or not? Would you please guide me?

  1. Radar……coregistration….coregistration (I did not use S1 tops coregistration)

    Figure2.

    figure3.
  2. Radar….interferometric….products……interferogram formation

    figure4.
  3. Radar…… interferometric……filtering……goldstein phase filtering
  4. Radar….geometric….Terrain correction…….Range-doppler-terrain-correction

    If they are ok, then I think I should find a constant place near coast or a island and then subtract its phase from all phase , then subtract both phases from each other. Am I right?

Cheers

I don’t know if it makes more sense, but I would skip the coregistration and directly process inteferograms from both image pairs. Once they are unwrapped and converted to elevations, you can apply Range Dopper Terrain Correction and compare their abolute heights.

I tried it first but it gave me error and ask me to do corregistration stack.

how did you open the TDX data? There is a XML file in the top folder of each bistatic product which opens the already coregistered image pair.

Dear @ABraun

You are right. If I open XML file, then I do not need to do co-registration but something is wired for me:

If you open TanDEM-X file, you will see below folders:


Figure1.

First of all, I used;

TDX1_SAR__SSC_BRX2_SM_D_SRA_20150529T031901_20150529T031907

TSX1_SAR__SSC_BTX1_SM_D_SRA_20150529T031901_20150529T031907

And I did coregistration and then continued other processes (figure2) but my result was difference from when I used XML file and then continued other processes (Figure 3).

You can see both results in below.

Figure2.

I do not know why?

sorry, what exactly was done to create each of these images?

Dear @ABraun
For figure 2:

  1. I used below two images_if you work with TanDEM-X images, then you can find two folders that include two below images are_ (look at figure 1):
    TDX1_SAR__SSC_BRX2_SM_D_SRA_20150529T031901_20150529T031907

TSX1_SAR__SSC_BTX1_SM_D_SRA_20150529T031901_20150529T031907
2. Radar……coregistration….coregistration (I did not use S1 tops coregistration)
3. Radar….interferometric….products……interferogram formation
4. Radar…… interferometric……filtering……goldstein phase filtering
5. Radar….geometric….Terrain correction…….Range-doppler-terrain-correction
For figure 3:

  1. I used XML file
  2. Radar….interferometric….products……interferogram formation
  3. Radar…… interferometric……filtering……goldstein phase filtering
  4. Radar….geometric….Terrain correction…….Range-doppler-terrain-correction

Dear @ABraun
Another question;

As I explained before;

I have two TanDEM-X pair, I mean two XML file:

First XML file (18.05.2015):

Second XML file (29.05.2015):

I want to make interferogram from any of them, for example:

  • Interferogram by XML file (18.05.2015) in VV mode
  • Interferogram by XML file (18.05.2015) in HH mode
  • Interferogram by XML file (29.05.2015) in VV mode
  • Interferogram by XML file (29.05.2015) in HH mode

Then for finding topography, I want to subtract two days from each other like;

Result1= Interferogram by XML file (29.05.2015) in VV mode - Interferogram by XML file (18.05.2015) in VV mode

Result2= Interferogram by XML file (29.05.2015) in HH mode - Interferogram by XML file (18.05.2015) in HH mode

But my problem is, two XML files are not in same size (one of them is 686217802 and another one is 686217688).

I know I should use stack but I do not know when? Before making interferogram or after making interferogram?

I would first select the VV bands of each product and write them into a separate file.
Subsequently, use coregistration to bring them into one stack.

How can I bring VV bands in one folder?

there is no need to do that, simply select
Raster > Data Conversion > Band Select
and write one new file per product which only contains the VV polarization.
This makes sure that they are the only ones used in the later coregistration.

I converted them and put all VV bands in 18.05.2015 in one folder and all VV bands in 29.05.2015 in another folder:
VV bands in 18.05.2015:


Fig1.

VV bands in 25.05.2015:

band_vv_29
Fig2.
Then I used coregistration for them:
coregistration_vv_bands_18_29
Fig3.

Then I used Radar…interferometric…product…interferogram formation
and result is here:
interferogram_vv_band
Fig4.

But when I opened interferogram results, i think for 18.05.2015, it was ok:


Fig 5.

But the result was wierd for 29.05.2015 (i think it mixs between 18 and 25)???

Fig 6.

I think something is wrong with co-registration;

I used Radar……co-registration………co-registration…


Fig1.


Fig2.

I do not know why SNAP is mixing???

test

do you want to make an interferogram between 18th and 29th may?
Then simply open one of the bistatic products per date (select one polarization) and coregister them.
Otherwise you get a stack of 4 vv bands which snap obviously causes erroneous pairs.

Dear @ABraun
No, I do not want. I want to make two interferogram (one of them VV and another one HH for 18.05.2015) and then make another two interferogram (one of them VV and another one HH for 29.05.2015).

Finally, I want to subtract interferogram (29.05.2018_vv) from interferogram (18.05.2018_vv).

but snap makes error when I am doing coregistration.

When I want to subtract interferograms 29.05.2018 from interferograms 18.05.2018, then it is not possible, because they are not in same dimension.

First of all, I should make them with same dimension (coregistration), then go further. Do u have any idea?

Marjan

That doesn’t make sense to me. What would you expect from that? And from the subtraction of interferograms?
If you really want to highlight topographic changes, you have two options:

  1. Coregister VV18 with vv29, create a differential interferogram (including topographic phase removal) and convert this to displacement. To have four images (two per date) is nice but doesn’t help very much in this case I think.
  2. Create one interferogram per date (based on only one polarization), unwrap it and convert it to elevation. Then you can subtrackt the absolut elevations of both dates to search for changes.

That doesn’t make sense to me. What would you expect from that? And from the subtraction of interferograms?
My area is over sea ice, I want to see ridging events between 18 and 25 th May.
If you really want to highlight topographic changes, you have two options:

Coregister VV18 with vv29, create a differential interferogram (including topographic phase removal) and convert this to displacement. To have four images (two per date) is nice but doesn’t help very much in this case I think.

I tried to do coregistration but you saw it gives me error.
Create one interferogram per date (based on only one polarization), unwrap it and convert it to elevation. Then you can subtrackt the absolut elevations of both dates to search for changes.

what kind of error? Sorry, it’s confusing.

Create one interferogram per date (based on only one polarization), unwrap it and convert it to elevation. Then you can subtrackt the absolut elevations of both dates to search for changes.

If I creat one interferogram (for example for VV on 18.05) and then convert to elevation, and make another interferogram (for example for VV on 20.05) and then convert to elevation.

When I want to subtract both elevation from each other, again I will have problem because they are not in same dimension.

Did u get my point?