Understanding GPS
How GPS pinpoints your location
GPS positioning works by measuring your distance from several satellites at once. Each satellite broadcasts the exact instant its signal left; because radio waves travel at the speed of light, the receiver turns the tiny delay into a distance, or pseudorange. Three ranges would fix a point in space — but an ordinary receiver's clock is far cheaper and less stable than the satellites' atomic clocks, so a fourth satellite is needed to solve for the receiver's clock error too. That is why at least four satellites are required for a full 3D fix. Geometry matters as well: when the visible satellites are spread across the sky the fix is crisp, but when they bunch together the error grows — a factor called dilution of precision (DOP).
Signals, frequencies and accuracy
GPS satellites transmit on several radio frequencies in the L-band. The original civilian signal is L1 C/A (Coarse/Acquisition); modern receivers also use L5, a stronger signal designed for safety-of-life uses such as aviation. Comparing two frequencies lets a receiver cancel the ionospheric delay that is the single largest error source, which is why dual-frequency L1+L5 units are far more accurate. Military users have the encrypted P(Y) and M codes. In practice a smartphone under open sky is accurate to about 5 metres, dual-frequency receivers reach roughly 3 metres or better, and professional augmentation systems (SBAS, or real-time kinematic corrections) can achieve centimetre-level positioning.
| Signal | Frequency | Primary use |
|---|---|---|
| L1 | 1575.42 MHz | Civilian C/A + military P(Y) |
| L2 | 1227.60 MHz | Military P(Y); L2C civilian |
| L5 | 1176.45 MHz | Safety-of-life, high accuracy |
GPS vs GNSS: one of several systems
GPS was the first fully operational global navigation satellite system, but it is now one of four. The umbrella term GNSS (Global Navigation Satellite System) covers GPS alongside Europe's Galileo, Russia's GLONASS and China's BeiDou. Most modern receivers — including those in smartphones — track several constellations at once, which improves accuracy and reliability, especially in cities where tall buildings block part of the sky. You can watch the American fleet orbit in real time on our GPS satellite tracker.
| System | Operator | Baseline satellites | Altitude |
|---|---|---|---|
| GPS | United States | 24 | 20,200 km |
| GLONASS | Russia | 24 | 19,100 km |
| Galileo | European Union | 24 (+spares) | 23,222 km |
| BeiDou | China | 24 MEO (+GEO/IGSO) | ~21,500 km |
More than navigation: timing and vulnerabilities
Less visible but arguably more important is GPS's role as a global clock. Its atomic-clock timing signal keeps systems in step that few people associate with satellites: stock-exchange timestamps, mobile-network synchronisation and the phase alignment of electricity grids. That dependence is also a weakness. Because the signals arriving from 20,200 km are extremely faint, they are easily overpowered by jamming or spoofing — broadcasting counterfeit signals to feed a receiver a false position or time — a rising concern for aviation and shipping. Multi-constellation receivers and inertial back-ups are among the countermeasures.