SAR (Synthetic Aperture Radar) is a special radar installed on a satellite, etc. to measure the Earth's surface in detail.
Using SAR, we observe the same point on the ground more than two times. Then, by calculating the difference between these radar images we can detect surface movements in detail.
This technique is called the SAR interferometry and enables us to monitor the Earth deformation caused by earthquakes, volcanic activities, and subsidence from space.
This SAR Interferometry is outlined here.