What is GPS?
The Global Positioning System (GPS) is a system of geolocation used all over the world. Every GPS receiver (like the ones we have in our cars and our phones, or the ones scientists use in the field) receives data from a group of satellites in orbit around the Earth. By cross-referencing the information from at least four of these satellites, the GPS receiver can determine very precisely its own location at the surface of the Earth in three dimensions (latitude, longitude and altitude).
By monitoring signals of GPS receivers fixed at the surface of the Earth, scientists can measure accurately the tectonic plate motion.