Some explanations about concepts of fringes of interferogram and coherence

I have one question . If we take two sentinel images, one ascending and one descending then will it effect the interferogram in any aspect?

I can’t imagine that this will give you a proper interferogram as the looking direction is different. But it’s worth a try.

Some guys tried it with ERS data:
http://earth.esa.int/workshops/ers97/papers/carrasco/

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That is not possible, the result will be total decorrelation and no useful phase-signal at all. The interfering images must have the same geometry (same orbit track & perpendicular baseline less than the critical baseline).

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Fringes are generated in a somewhat intermediate step when you have a debursted, topographic phase removed and filtered interferrogram, at this stage the interferrogram is in wrapped phase form. you can get preliminary information by counting the number of fringes between two points of your interest and multiply this number by the wavelength of S1 (which is 5.546 cm).
To get the deformation spatially you have to unwrap the phase in SNAPHU and get the result of deformation in meter or cm

I am working with topsar over a large area of mixed coherence values. I am at the unwrapping stage and its not going so well.
I am looking for deformation but I guess what confuses me slightly is that deformation implies a lack of coherence.
I know insar can determine deformation quite accurately in some circumstances but if lack of coherence is a problem how is it used for landslides or large movements due to earthquakes. Surely this leads to a lack of coherence?

@marjanmarbouti
@ABraun
My master thesis is about atmospheric correction, it can be accessed here:
http://www.dongiatti.com/wp-content/uploads/2016/10/Ph_Ott_Masters_Thesis_GEO511.pdf

I asked myself the same exact question and I managed to do an atmospheric correction based on ECMWF weather data. I processed the S1A data with SNAP and did a DEM based interpolation of the atmospheric values. At the end I corrected the relative unwrapped phase values with the measured path delay.

Have a look
Kind regards

Philippe

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very interesting - thank you for sharing!

Hello Philippe, is the required weather data freely available globally? If yes it would be possible to develop a tool to SNAP that would perform this correction…

Marcus

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@mengdahl
Sure! That was I aiming for “on click-option”…that was the purpose…so this could be implemented and done in future with “one click” in SNAP…but I also think, research and DEV team of SNAP is already doing the research and set up for this task.

The data are globally available, but the only drawback is the data comes to public with 3-month delay. This means the whole month of January 2017 is the newest dataset right now. Sure thing, there is an option that the weather data can be available earlier (1-2 days of delay) if you have an official billing account.

Here the link, just create an account and download some sets. I always went with the Net-CDF output:
http://apps.ecmwf.int/datasets/data/interim-full-daily/levtype=sfc/

I also check on my study site the a meteostation and the offsets are pretty small, so the modelled weather data of ECMWF are credible.

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Hello , thank you for sharing your theis it very interseting i have a question howa did you extract site of interest in snap ? what software are you use to do dress your map? I need that :slight_smile:

Hi annakisara

The image processing is all done in the SNAP software.
After image processing and befor you geocode to the geoprojection (eg. WGS 84) you create a subset (right click on the image, select the rectangle you want to focus on)

North arrow, scale bar and the grid can be applied in SNAP, via Layer Manager (right windows-side, click on the circled plus symbol, mapping tools and there you select your preference)

Kind regrads

Don
sorry for the late answer

Thanks for sharing…I look at it:slight_smile:

hello,

As ABraun mentioned, one phase cycle difference between acquisitions, this can be interpreted as a half of wavelength change on the line of sight.

Looking at the image (produced using SNAP) we see the color progression from green to red. Is this one cycle change ? which translates to 2.8cm. So we can say that in red areas we expect a subsidence or uplift of 2.8cm
is that right?

thanks

no. An interferogram gives you the difference of phase between both images.
But as this is given in +PI to +PI (one full cycle of the phase) you need to unwrap it to get absolute values.

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One phase-cycle is red-to-red = 2pi

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I want to thank @ABraun @mengdahl for your answers.

My research focuses on subsidence analysis for an urban environment south in Vietnam. I have downloaded 12 images, processed them, stacked them and finally obtained the subsidence map. Doing a subsidence analysis in that area is challenging because humidity and pressure is very high (almost all year round) as well as high temperature which affect the signal a lot. So, in order to improve my results, i tried to investigate each individual interferogram trying to detect any artifacts or high phase values( caused from atmospheric errors) and remove them from my processing. I am going to need to help to see whether my interpretation is correct or not. So, i would like to show you two of the interferograms created for the same area of interest as shown below.

Fig1: interferogram formed for month August and September

fig 2: Interferogram formed for the months Septermber October.

First of all, we know from previous studies that the displacement (per year) in this area is between 0 and -6. The intereferograms i am displaying here formed for a period of one month. Hence, we do not expect high displacement to occur, in other words, we expect the phase to be close to zero (I hope i am right on that).

Looking a the interferogram on the top (August,September), we see that the phase is constantly high across the image. I do not see a smooth color progression (from red to blue), instead it goes from red straight to blue. Is this happening due to atmospheric errors? is this a ‘phase jump’ or it looks normal?

On the other hand, for the next interferogram (for the same area of interest), we see a color progression from blue (bottom-up) to blue. Is this interpreted as a change of 2.8 cm? I see this color progression happening when elevation changes . In my case, tpo-phase was removed and the area is flat. So, i guess this is due to atmospheric errors.

I have few other interferomas that look like the ones i have shown you above, and some other that phase approaches to zero (which seems more normal to me).
To me, the interferogram in figure 2 does not look good. I cannot explain all these fringes formed, so i guess thats atmospheric noise. So, i decided not include in my process.

Is my interpretation close to reality or i am completely off?
thank you in advance

It is a good start to sort out those interferograms which make no sense or have too much noise.
But you should definitively have a look at coherence of your image pairs. It is a spatial indicator for the quality of your interferogram. Not every part of the image can be trusted, especially in areas with strong atmospheric effects, as you say.

You should not directly relate colour cycles to absolute displacements. Please have a look at this site - it shortly describes the difference between a wrapped and an unwrapped interferogram:
http://vldb.gsi.go.jp/sokuchi/sar/mechanism/mechanism04-e.html

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Hi Phill, I’m Muhammad Budi, I have read your master thesis and I’m so interested with it. Can I get the processing steps in atmospheric correction? Because I am now doing my thesis too about DInSAR. I can learn all methods about atmospheric but I still don’t understand some.
Thank You

Hi, Ihave one ask


this is one of my result, but I want know if are the values positives are elevation of terrain and if are the values negatives are subcidence. or if there is other interpretation.
I found one document (Interferometria radar de apertura sintética aplicada al estudio del movimiento en laderas aledañas al volcan Calbuco con ayuda de imágenes Sentinel 1A. David Emmanuel Hermosilla Diaz, Santiago, 2016) where it say that the sequency of colors determines if it is a subcidence or a lifting.

my question is, what determines the type of movement of the land, the colors or values ​​that throws the results palette or other evidence in the result image?


Hello @dongiatti

I am happy I came across your work which was referred by @falahfakhri for one of my queries.
At the moment I am looking at some interferograms generated with S1 pair (Atmospheric Fringes) and I would like to clarify if the fringes are atmospheric or result of actual displacement. I would like to try out your method to mitigate the atmospheric elements if any.

Regards
Arnab