Persistent scatterer interferometry measures ground movement to a few millimetres a year from a satellite several hundred kilometres away. The claim sounds implausible and it is worth understanding why it is not.
Radar measures phase as well as amplitude. Compare the phase of two acquisitions of the same place and the difference tells you how much closer or further the ground has moved — at a precision set by the wavelength, which for C-band is about 5.6 cm. Fractions of a wavelength are measurable, so millimetres are in reach.
Three things go wrong. First, most of the ground does not reflect radar consistently — vegetation moves, water changes, soil gets wet. PSI works only on persistent scatterers: rock, buildings, infrastructure, bare ground. On a Dutch dyke covered in grass, the scatterers are the concrete revetment and the access road, not the grass.
Second, the atmosphere delays the signal by an amount that varies day to day and looks exactly like ground movement. Separating the two requires many acquisitions and a model of how atmospheric delay is spatially correlated.
Third, phase wraps. A measurement of movement is only unambiguous within one wavelength, so a fast-moving point can be misread. Dense time series and spatial consistency resolve this — usually.
The 2025 dyke deformation paper and its reproduction code are on the Terravek Labs page.