I have a backscatter image from TanDEM-X. Its value is between 0.1 to 844441.063. Then I converted it to db by using ‘convert band’ and now its values are between -9.8 and 59.367.
Figure 2. convert band
As I know the backscatter image value should be almost negative.
Do you know what step I miss here?
you are using amplitude/intensity values. These are just digital numbers (DN) without a clear scale. #
Normalized radar cross-section (=backscatter intensity) is achieved by the calibration module which converts it to Sigma0. Only then you can convert them to dB scale which ranges between around -40 and + 10.
I tried it but;
I did not find any option for converting to Sigma0. Options were gamma0, beta 0 and complex and there was another option ‘save in db’ (figure1). Any way I chose ‘save in db’ but then I got below error (figure 2).
Error: Can not apply calibration to co-registered product.
Please have a look in the documentation every now and then Sigma0 is the standard output.
Beta0 and Gamma0 are just optional (the first is a lesser, the latter a higher level of calibration)
I your case, calibration to Sigma0 is no longer possible because the image geometry was already altered during the coregistration process.
Radiometric calibration only affects the intensity is therefore not required for InSAR. If you want two images of Sigma0 in one stack, first calibrate them and then perform coregistration.
As you know TanDEM-X products are like below format (fig1):
Two products are not co-registered with some second difference
A xml file is coregistered (I used this for processing).
So based on your suggestion, then I should use two products are not coregistered. Convert them to sigma0 and then coregister them.
I’ve converted one of them in sigma0 but still it has so much noise.
Would you please let me know what is the best processing chain over TanDEM-X intensity image?
yes, you can open the single scenes with the XML files in the TDX1 and TSX1 folders separately and apply calibration.
The noise in the image is not affected by calibration. Radiometric calibration simply translates the measured intensities into comparable and standardized units and reduces the impact of the incidence angle. Accordingly, the processing chain depends on what you want to do with the data. InSAR doesn’t require calibration to Sigma0, but other approaches, might need it.
Actually, I made a DEM map by using H band of TanDEM-X (two images) and then I want to overlay back scatter image with DEM map and compare them.
Sometimes we have high DEM and I want to compare this high DEM with backscatter image and see if it is high or not.
I do not know these steps are enough or not?
Calibrate -> output: sigma0
Input:sigma0-> Speckle (lee5x5)
Rang-Doppler terrain Correction
yes, these steps are totally fine to generate a calibrated ground range terrain corrected image.