Flat Earth Removal in PolSARPro

I am following PolInSAR training course in PolSARPro (https://earth.esa.int/documents/653194/656796/POLinSAR_Training_Course.pdf) and I have a question about flat earth removal step.

As you see in the image, there is a “2D flat earth file” required as part of the function.
Please can someone explain what this file is and how I can generate it?

Many thanks in advance

I also have this doubt. Would someone know about it?
Many thanks
Best wishes

You have to do “Flat Earth Estimation” in PolSARPro first. go to “Process”–> “In-SAR Functionalities”–> “Flat Earth Estimation”