InSAR (Interferometric Synthetic Aperture Radar)
InSAR takes advantage of the phase of the microwave signal transmitted and received by a radar platform that most of the time is represented by a Synthetic Aperture Radar that is able to maintain range resolution regardless of the distance to the target.
The differences in the phase of the signal as measured from multiple satellite passes are translated to ground displacement values.
Since the phase of the signal varies rapidly within the signal wavelength (in the range of centimeters), detecting phase variations representing fractions of the signal wavelength leads to millimeter accuracy of the measured displacements.
The similar interferometric technique is applied for the gravitational wave detection at the Laser Interferometer Gravitational-Wave Observatory (LIGO). The gravitational waves arriving quasi-simultaneously in Hanford, Washington and Livingston Louisiana (some 3000 km separation) are detected through interferometry. This example proves the extreme accuracy of the InSAR technique.
Space-based InSAR reached a mature stage with the launch of ESA ERS 1/2 in 1991 and Canadian Radarsat-1 in 1995. The true operational stage was reached with the launch of Sentinel-1 constellation in 2014-2015 with data available worldwide for free.