The standard method for extracting ground movement using InSAR compares the phase wave of a radar dataset from one acquisition to the same location on a subsequent acquisition. The displacement of the phase signal enables reliable estimation of millimetric ground movement changes.
The radar wavelength of the sensor influences that magnitude of motion that can be reliably estimated between acquisitions. ESA’s Sentinel 1 sensor has a wavelength of 55mm, so it can typically observe changes of up to half of its wavelength, or 27mm, between Sentinel 1’s 6-12 day acquisition schedule. This range of ground movements is suited to detecting motion on mature dams and embankments. Where ground movement exceeds 27mm between acquisitions e.g. for newer dams or dams in critical situations, the traditional InSAR methods will underestimate ground movement.
To measure faster and larger movements i.e. greater than 27mm per acquisition, alternative methods or data are required. Speckle tracking is an InSAR method developed to track the “fast” movement of glaciers and would be appropriate for newly-constructed dams susceptible to rapid settlement following initial reservoir filling. Alternatively, in-situ ground movement (i.e. GPS peg data) data can be used to estimate the expected rate of change and therefore calibrate the model.