ALOS 2 Orthorectification

Microwave Toolbox
#1

How to perform Terrain correction of ALOS2 data in SNAP? I followed the steps given for ALOS1, and it works perfectly for ALOS 1 CEOS but when I apply same procedure to ALOS2 data, it wont work.

#2

Don’t deskew for ALOS-2

1 Like
#3

Thanks Iveci for support. am also working with ALOS 2 using ALOS 1 guide. Launched Multilooking after Calibration but when i input the _Cal data and head to define parameters i find the Azimuth and Ground Range predefined as -1 and infinity respectively, pls see attached

#4

Yes, It happens with me too, but on ALOS1 data. I think -1 to 1 range problem occurs if the data is not properly calibrated. If you carryout multi-looking of the product with other factor it will work but you won’t be able to apply terrain correction as given in the manual for ALOS1 data. SAR simulated terrain correction won’t work on such product showing -1 to 1 scale.

#5

Hello,
I am also having the same problem as Chege. If it is due to improper calibration then how can we fix this ?
Can S1TBX perform terrain correction of ALOS2 data?
Thanks
Frances

#6

Hi,
I have two ALOS-2 images (one in SM2 mode and other in SM3 mode) with different incidence angles with a common overlapping area. I want to extract and compare the sigma-naughts of that common area. But since sigma-naught is a function of incidence angle, I cannot compare them.
How do I bring both the sigma-naught values to a common ref level for comparison??

Please help.

Thanks !!

#7

Your objective is to compare the sigma nought of two images having different incidence angle. Since the sigma nought is function of topography, you can’t compare them from two images having different incidence angle. What I suggest is

1] You calculate the local incidence angle using external DEM in SNAP for one image at a time. which will require terrain correction.
2] Do the same to image 2.
3] Refer to the book ‘Introduction to Microwave Remote Sensing’, Page no 108-109. There you will find that, when you want to compare two images with different incidence angle the gamma is good measure (gamma= singma nought / cos (inci_angle)).
4] I don’t have ALOS-2 data now. But, you just try to orthorectify (Terrain correction) one image. And utilize the local incidence angle to convert sigma nought into gamma and then compare the values.

Note: There are some issues with the calibration of ALOS-2 data. So, calibrate the ALSO-2 data correctly before applying the terrain correction.

#8

Hi @lveci,
I’m using your tutorial (ALOS PALSAR Orthorectification Tutorial - 2016) to ortortify images PALSAR-2 (1.1; CEOS). I do the same steps except Deskewing. My question is about the parameters you used in the “simulation terrain correction” step (Figures below). Can I use the same parameters for PALSAR-2 images?


Thank you for your attention!

#9

I would say yes. If you are happy with the result, I see nothing wrong here.

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#10

Hi,
I am processing ALOS-2 data over ALASKA, for which SRTM 30sec (Auto download) option is not available.
I have tried with the ACE2-5min (auto download), ACE30 (Auto download) and ASTER 1sec DEM. The process runs but the files generated after SAR Simulated Terrain Correction have ‘NaN’ values.
I have even tried downloading external ASTER DEM and process the same. It also ends with the same result.
Do you know to resolve the issue?
Any help is appreciated.

Thanks

#11

Have you tried Range Doppler Terrain Correction with ACE 30 as well? SAR Simulation Terrain Correction depends on topographic features which match the patterns in the image. Probably that was not the case with the coarse-scaled DEMs.

#12

Yes, I tried it now and it does work.
Thank you.
However, the resolution of the local incidence angle map is very coarse.
Can this resolution be improved?

#13

Try the Range Doppler Terrain Correction with the downloaded ASTER DEM. Just select “External DEM” and navigate to the location. It should be a TIF file (mosaic if it consists of various parts) in WGS84.

#14

This does work with the external DEM.
The local incidence angle resolution is so much improved.

Last thing:
Is terrain correction advised before or after co-registration?

Thanks again.

#15

First coregistration.