It benefits us in the volcano world, too. GPS is a satellite-based system that provides accurate location and timing data to users worldwide. Its origin was for military defense purposes. The Department of Defense had a long-standing policy of intentional degradation of the satellite signals, a process called Selective Availability. GPS works by a system of precise clock-based radio signals.
Selective availability works by allowing the clocks in the satellites to drift.
At a stationary ground-based receiver, the tiny variations in the time signals make it appear as though the distance to the satellite is changing. Special codes were required to compensate for the clock drift. Anyone using the system without codes had to operate dual receivers in order to correct for the clock variations. We use GPS in several ways at the Hawaiian Volcano Observatory. Foremost is for our surface-deformation studies.