GPS uses data transmitted by orbiting satellites to locate points on the ground. The USGS has made baseline GPS measurements at several volcanoes in the United States and in Latin America. In the event of an awakening of one of these volcanoes, GPS receivers would be set up at these points again to determine whether or not measurable deformation had occurred and to monitor for precursory deformation that might herald an eruption.
Satellites transmit signals which are observed by ground based receivers.
Observed signal travel times from satellite to receivers are used to compute ranges from satellite to receiver. Ranges from four satellites are required to determine a position on the earth relative to a reference ellipsoid. In it's simplest civilian application, single point positioning, with a handheld receiver, accuracy in position is approximately 75 meters. For geodetic applications, with many hours of observation utilizing two or more sophisticated GPS receivers and rigorous data processing, accuracies of a few centimeters over baselines of 100 kilometers are routinely obtained. The use of satellite technology eliminates the need for survey points to be intervisible.