GPS (Global Positioning System)

The Global Positioning System (GPS), operated by the US Space Force, is a constellation of 31 active satellites in six orbital planes at 20,200 km altitude (MEO), each completing two orbits per day. A GPS receiver on Earth needs signals from at least four satellites to calculate its 3D position and time. Each satellite carries atomic clocks accurate to nanoseconds — the timing signal is the foundation of the system. Civilian accuracy (L1 C/A code) is typically 3–5 m; dual-frequency receivers (L1+L5) achieve <1 m. GPS also underpins critical infrastructure: financial transaction timestamps, power grid synchronisation, cellular networks, and air traffic control.