What it is and the problem it solves
GPS is a U.S. Department of Defense satellite-based hyperbolic navigation system. It solves the problem of determining absolute position and time anywhere on Earth with line-of-sight to satellites. It was built to replace earlier ground-based systems and support precision military operations.
How it works
GPS computes position in four-dimensional spacetime by solving navigation equations using pseudorange measurements from at least four visible satellites. It measures signal time delay, converts it to distance using the constant speed of light, and relies on atomic clock synchronisation across satellites and ground stations. It applies relativistic corrections—38 microseconds per day—to satellite clocks. It uses CDMA spread-spectrum modulation on L1 and L2 frequencies, assigning unique pseudo-random sequences to each satellite.
What works
The core mechanism works: pseudorange-based trilateration in four dimensions delivers metre-level accuracy for civilian users and centimetre-level for dual-frequency, corrected use. Atomic clock synchronisation, relativistic correction, and CDMA separation of signals function as designed. The 24-satellite constellation achieved full operational capability in 1993 and remains functional.
What does not
GPS does not provide guaranteed availability, resilience against jamming or spoofing, or real-time integrity monitoring. It does not work indoors, underground, or in dense urban canyons without augmentation. It does not deliver position without assuming receiver clock error is solvable—and fails if fewer than four satellites are visible.
What it changes
GPS replaced ground-based hyperbolic radio-navigation systems like LORAN and Decca with a single, space-based alternative. It enabled autonomous, real-time, global coordinate referencing for civil and military users alike—shifting navigation from infrastructure-bound to satellite-dependent.
Is it worth your time
Yes—if you need precise, all-weather, global positioning without infrastructure dependency. But it requires line-of-sight to four satellites, offers no native integrity or authentication, and depends entirely on U.S. military control and maintenance.
