Something wrong about Kompasat-5 sigma naught equation


I want to obtain sigma naught in dB of Kompsat-5 SCS image and I processed one image using radiometric calibration, multi-looking, terrain correction and speckle filtering. But I find the value of sigma naught in dB is positive. Just like the following

I calculated the sigma naught manually according to the equation of sigma naught in the SNAP HELP. I found the result was still positive.
So I told to KARI and they sent me a different equation to calculate the sigma naught in dB. I used this equation and found the result is negative and reasonable. I uploaded the equation file from KARI here. Please help me check it and I want to obtain the correct result in SNAP.

K5_Sigma_Naught_Equation.pdf (240.2 KB)

Is it positive after the calibration step already?

Thanks for your response.
Yes, it is positive from the radiometric calibration.

according to the help Kompsat is not supported. But in this case no Sigma0 would have been calculated at all.

Do you know any source for Kompsat-5 sample data so I could try it myself?

Did you select any input band in the second tab? For some products (e.g. TerraSAR-X) both Amplitude and Intensity are displayed and I once experienced that the output is different when the wrong one is selected. I suggest selecting nothing and let SNAP decide.

I can find the equation in the SNAP Help. Just in the below of your captured image.

I processed the K5 data again without selecting any input band. The result is still positive. I can send the Kompsat-5 SCS data to you through e-mail or other way.

sorry, I only had a look at the list.
In this case the calibration somehow doesn’t apply the formula from the help, I’m afraid.

Thank you. Could you update the SNAP to support the Kompsat-5 data?
If I calculated the sigma naught using the bandmath of ENVI based on the equation. The result is very strange because of the local incidence angle acquired from the GIM (Geographical incidence mask) layer data.

The SNAP Help says High Resolution and Standard Modes of Kompsat-5 are supported. So I am afraid the equation in the SNAP Help is exactly inaccurate. You can try the equation I uploaded to update the calculation of sigma naught. It will be also helpful for me. Thank you.

I am not a developer, sorry.

Oh, sorry. Thank you, anyway.

KompSAT-5 support is coming. My guess is with the release of SNAP 7 in end of June. Maybe @marpet has confirmation about this?

Great news. Thank you for the information.

According to this issue, it is implemented and will come with SNAP 7:

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Did you select the “Save in dB” option when you calibrate the K5 product? By default this option is not selected. Therefore, by default the output sigma naught is not in dB, meaning that it is positive.

Yes, thank you. We can select the “Save in dB” when we calibrate the K5 data. But the value is also positive.

I think it is same as we convert the K5 data to dB after the terrain correction.

Thank you. It shows currently only K5 in CEOS format is supported. K5 in GeoTiff/XML format will be added and supported by SNAP 7.
However, I only have K5 data in HDF format. I am not sure whether HDF (suffix .h5) format is same with CEOS or not.
If K5 in HDF format is also supported now, I think the equation of sigma naught for K5 data in SNAP Help might be inaccurate and is different from the equation provided by KARI. The problem is that I processed the K5 SCS data in SNAP and the sigma naught in dB is positive.

We have updated the calibration according to equations given in document “Radiometric Calibration of KompSat-5” dated 2017-02-18. The positive dB value problem should be fixed. The fix should be in the next release. Thank you for pointing out the problem.


That’s great. Thank you.