Early GPS equipment. This equipment was among the first to use the Global Positioning System (GPS) that started in 1978. This equipment was built by Prakla-Seismos. GPS is a system of satellites in Earth orbit that broadcast signals. These allow ground-based equipment, like the one seen here, to calculate its position, elevation and the precise time. Modern GPS receivers are now small enough to be carried by hand. The GPS satellites must carry extremely accurate clocks, and need to take into account relativistic effects. The speeds of the orbiting satellites, relative to the Earth, cause a small but measurable slowing of time, as predicted by Einstein's Special Relativity theory.