If you measure enough places on the surface, you can plug all the data into computer programs that can estimate how deep the magma chamber is, how big it is, and where new magma is. These techniques have all been perfected at the Hawaiian Volcano Observatory, and are now used all over the world. This photo shows a GPS receiver on the south flank of Kilauea. Photo by Steve Mattox, March 1992. It uses satellites that broadcast a signal and receivers that pick up and record the signal.
Hikers can use a hand-held GPS to find their position.
Farmers here in North Dakota use GPS to locate specific areas in their fields that need special attention. Volcanologist use GPS as a powerful tool to detect movement at shallow depths beneath the surface of volcanoes. With GPS, the velocity is the speed of light. The signal from the satellite includes the time it was sent. The receiver notes the time the signal arrived.