The Global Positioning System is a network of satellites, ground control stations, and receivers that together can tell you where you are on the surface of the Earth to within a few meters, sometimes better. Your phone, your car's navigation, aircraft guidance systems, and precision munitions all depend on it. The core technology is a clever application of physics that dates back to the 1970s and was developed entirely by the U.S. military.
GPS is not the only such system in the world. Russia operates GLONASS, the European Union operates Galileo, and China operates BeiDou. Your phone likely uses signals from several of these simultaneously to improve accuracy. But GPS was first, and the term has become generic the way Kleenex has for facial tissue.
The Satellite Constellation
The GPS constellation consists of at least 24 operational satellites orbiting Earth at an altitude of roughly 20,200 kilometers, arranged in six orbital planes. The arrangement ensures that from anywhere on Earth, at least four satellites are above the horizon at any given time. In practice, many more are usually visible.
Each satellite continuously broadcasts two things: its precise location, and the exact time the signal was transmitted. The satellites carry atomic clocks accurate to within a few nanoseconds. That precision is not incidental — it is the foundation of the whole system.
Trilateration: Measuring Distance with Time
When your GPS receiver picks up a satellite signal, it compares the time the signal was transmitted to the time it arrived. Since radio signals travel at the speed of light, that time difference translates to a distance. If the satellite is 20,000 kilometers away and the signal travels at roughly 300,000 kilometers per second, you can calculate the travel time and thus the distance.
Knowing your distance from one satellite tells you that you're somewhere on a sphere of that radius centered on the satellite. A second satellite gives you a second sphere. The intersection of two spheres is a circle. A third satellite narrows that circle to two points, one of which is usually obviously wrong — deep in space, or underground. A fourth satellite is used to correct for the imprecision of the receiver's own clock, which is far less accurate than the atomic clocks on the satellites.
This process — determining position from multiple distance measurements — is called trilateration, not triangulation. Triangulation uses angles. Trilateration uses distances. The distinction is minor in practice but matters to anyone who works with the technology.
Why Four Satellites Instead of Three
Your receiver's clock is a chip on a circuit board, not an atomic clock. It drifts. The small timing errors it introduces create positional errors. The fourth satellite signal provides a mathematical check that allows the receiver to solve for its clock error as a fourth unknown, alongside latitude, longitude, and altitude. Four equations, four unknowns.
Accuracy and Its Limits
A standard GPS receiver in a modern smartphone is accurate to about three to five meters under good conditions. Military receivers using encrypted GPS signals can achieve sub-meter accuracy. The limiting factors are atmospheric interference — the ionosphere and troposphere slow signals slightly and unpredictably — and the geometry of the satellites visible at a given moment. A bad satellite geometry (all bunched up in one part of the sky) produces worse accuracy than a good one (evenly distributed).
Augmentation systems improve civilian accuracy further. The FAA's Wide Area Augmentation System provides corrections that bring aircraft GPS accuracy to within about one meter, which is sufficient for precision instrument approaches.
The Strategic Dimension
GPS was built by the military and is operated by the U.S. Space Force. The civilian signal, which anyone in the world can use for free, was originally intentionally degraded until 2000, when President Clinton ordered selective availability turned off. The reasoning was that commercial applications had made the degraded signal an economic liability.
The dependence of civilian infrastructure on GPS — financial systems use it for timestamping transactions, power grids use it for synchronization, telecommunications networks depend on it for timing — has made the system a high-value target. Jamming and spoofing attacks on GPS signals are a growing concern, particularly in conflict zones where adversaries have both the capability and the incentive to disrupt navigation.