Getting local incidence angle map from SAR-Imagery?

I have a dual pol TanDEM-X imagery with a bi-static acquisition mode. To calculate the vertical wavenumber, I need a local incidence angle map of the Master and Slave image. As a first step I co-registered both images and then I proceed a Multilooking. The next step was a terrain correction (with SNAP) or the geocoding and radiometric calibration procedures with SarSCAPE to get the local incidence angle maps. After I check both results (retrieved from SNAP and SarSCAPE) I found that both local incidence angle maps from the Master and Slave imagery are identical what surprised me very much. Do you have an Idea what could be the reason for this result? What should I do?

• Visually or numerically identical? Identical as in exactly zero difference, or small difference? What was the baseline between the two images? – Benjamin Aug 9 '16 at 13:00
• It was numerical identical. Is it possible that my DEM build that problem? Here is the baseline between my master and slave file: Normal Baseline (m) = -280.663 Critical Baseline min-max (m) = [-6219.758] - [6219.758] – petermailpan Aug 10 '16 at 14:25
• Given the altitude of the satellites, compared to the size of the baseline, I would expect the incidence angles to be very similar, if not identical. Is it possible that both satellites are operating with exactly the same incidence angles, and that the two images cover a slightly (few pixels) different area on the ground? – Benjamin Aug 10 '16 at 23:19
• Hello Benjamin, I apologise for my late reply. I was on a little trip. The incidence angle of my master file is 35.821914223521766 and of my slave file is 35.843642354189249. You're right. They´re very similar. Have you any suggestions how I can calculate the interferometric (vertical) wavenumber with a very similar values for the incidence angles? – petermailpan Aug 27 '16 at 11:18
• I'm not overly familiar with the vertical number, but as far as I can tell, if you have a delta theta greater than zero you should be OK? I think you are right that the incidence angle maps should not be identical, according to the numbers you've given. You could calculate your own incidence angle maps by linearly interpolating the incidence values at near range and far range, usually given in the metadata. – Benjamin Aug 27 '16 at 14:41