If you only process interferograms during half of the year, then you are only measuring the displacement during that time. You would need a strong geological reason to believe that there is no displacement in the other half of the year. You should try making one-year interferograms from the same season to look at the displacement during the rest of the year if you want to know the long-term rates. Time series analysis cannot predict how the displacement varies over a long time without a connected set of interferograms. In places where there are large variations in precipitation and temperature during the year, the groundwater extraction rate changes from wet to dry season, so the displacements can be very different at different times of the year.
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