When magma moves into a volcanic system, and closer to the surface of the earth, the area surrounding the volcano may move upward and outward. At Lassen Volcanic Center, this swelling is measured using the Global Positioning System (GPS).
Continuously recording GPS instruments are the most used type of volcanic deformation-monitoring equipment in the world. 24 satellites, orbiting the Earth twice each day, transmit their position in orbit to receivers on the surface of the Earth. The receivers record vertical and horizontal position, and by comparing these measurements through time, it is possible to determine the amount of movement for a specific location on the surface of the earth.
There are currently nine GPS receivers that make up the USGS and UNAVCO geodetic network at Mount Shasta. The first instrument was installed in 2005, and the latest instruments were added in 2007. These instruments provide real-time volcano monitoring data, though ground deformation has been negligible in the area for the last few decades.