# Subtracting flate-earth formula

Hi,
I need to know how SNAP subtract flat-earth phase during interferogram formation.

I mean the formula and data used to calculate the interferogram.

Many thanks!

Hi ,

I think this is the right formula, so can anyone help me to have a real example.

what role do the DEM in this step of calculation?

Nobody want help me with this?

I wish you take a little time to help me !

Maybe you could show us in which context you need this and what you already have found out. Any first findings, points where you exactly struggle.
I don’t think anyone is giving a maths lecture here

did you see these slides? http://www.its.caltech.edu/~ee157/lecture_note/Interferometry.pdf
Slide 3-10 could maybe show what you are looking for. But you will have to go back a little to understand the different contributing variables.

thank you ABraun
I am using Snappy to perform some calculation now I am stagnant in this step (flat-earth subtracting)
I will read the doc you’d sent and thanks

Dear @ABraun…I saw this formula by chance here and I have a question about it; I think 4pi should be (2pi*K) and ‘K’ depends on standard mode (K=1) or ping pong mode (K=2), is not it?

If so, I wonder; why in every place it is written (4pid/landa) and in my idea; correct formula should be (2piK*d/landa) .

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Your 2pi equation is correct. But “d” is computed using the round-trip time and lightspeed. When you use slant range for the calculation, the equation becomes 4pi

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So, you mean; if I want to write a formula single-pass interferometry with no (2pip/landa) because we do not have displacement term in single pass; then it should be this:

And if it is for repeat pass; as anyway in repeat pass; p=2; so; then it should be this:

Am I right?

Are we talking about the flat-earth phase calculation?

No, flat-earth is part of this big equation. This formula is InSAR formula.

Then please ignore my previous response. Check the following link to see if you can find the information you need: https://cpb-us-w2.wpmucdn.com/sites.gatech.edu/dist/5/462/files/2016/08/AESS-IFSAR-Tutorial.pdf

I could not understand how equation (3) page 3 was acquired. Could you please elaborate ?

Here are some slides that discuss flat-earth phase:

https://earth.esa.int/documents/507513/1077921/2009_NEST_Training_InSARTheory.pdf/68e6081f-1c7b-4eb9-8cbb-3cd9eded61b4

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I will study it to understand further, I am grateful for your kind sharing

this link seems to have been changed. I couldn’t find the slides, could you provide another link please?

Maybe you can check this instead:
InSAR Displacement Mapping with ERS data page 13-14.