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”How to form the interferogram?”
  Principle of InSAR / Advanced Course2

If you went through the principle of InSAR, let's see how the interferogram is formed.

InSAR image processing process 1

The first thing that we need in order to measure the surface deformation by InSAR is to get a pair of SAR images of a target area observed before and after the surface deformation. And the baseline related to the pair of SAR images must be short. So we need observations from approximately the same position in space.

In the case of ALOS, for example, the interferometry is possible in most area on the condition that the baseline is shorter than a few hundred meters. However, it is only possible in flatlands or urban areas on the condition that the baseline is more than one kilometer.

Figure 13. Eliminating a topographic effect, 2-pass method

Figure 13. Eliminating a topographic effect, 2-pass method

If the period between image acquisitions of the pair is long, the pair becomes less coherent (image degradation). However, the coherence loss due to the long period is small and the pair remains coherent on the condition that the baseline is short even if the period is longer than a few years.

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