Somewhere above your head right now, a constellation of satellites the size of a small car is broadcasting a signal so precisely timed that shaving even a millionth of a second off it would throw your phone’s blue dot blocks off course. GPS feels like magic, but it is really an elegant math problem: figuring out exactly where you are by measuring exactly how long light takes to reach you.

Signals Timed to the Nanosecond

The Global Positioning System is built around at least 24 operational satellites (the U.S. Space Force currently flies 31) orbiting about 20,200 kilometers above Earth, each circling the planet roughly twice a day. Every satellite carries multiple atomic clocks accurate to within nanoseconds, and continuously broadcasts its own precise time and orbital position. Your phone’s GPS chip is not sending anything back up — it is only listening, comparing the timestamp embedded in each incoming signal to the time on its own internal clock.

Trilateration: Turning Time Into Distance

Radio waves travel at the speed of light, about 299,792,458 meters per second, so the delay between when a signal left a satellite and when it arrived tells the receiver exactly how far away that satellite is. One distance measurement narrows your location to the surface of a giant sphere. A second satellite narrows it to a circle where two spheres intersect. A third narrows it to essentially a single point on Earth’s surface. A fourth satellite is what actually makes this practical, because it lets the receiver solve for its own clock error at the same time — cheap phone clocks are nowhere near as precise as the atomic clocks overhead. This process of computing a position from distances to several known points is called trilateration.

Why GPS Satellites Need Einstein

GPS would not work without accounting for relativity. Satellites orbiting at roughly 14,000 kilometers per hour experience special-relativistic time dilation that makes their clocks run slightly slower than clocks on the ground. But they also sit in weaker gravity, far from Earth’s mass, and general relativity says that makes their clocks run faster. The gravitational effect wins out, so GPS satellite clocks are built to tick at a slightly adjusted rate before launch — deliberately slower — so that once in orbit they match Earth time. Left uncorrected, these relativistic effects would introduce position errors that grow by about 10 kilometers per day, making the system useless within hours.

What Actually Limits Accuracy

Modern GPS chips in phones are typically accurate to within a few meters outdoors, though the raw signal is degraded by the atmosphere. Radio waves slow down slightly as they pass through the ionosphere and troposphere, and can bounce off buildings before reaching a receiver, a problem called multipath that is why GPS gets unreliable in dense cities or canyons. Augmentation systems, differential corrections, and modern dual-frequency receivers (which compare two GPS frequencies to estimate and cancel out atmospheric delay) have pushed consumer accuracy well beyond what the original 1970s-era system was designed to deliver.

One System Among Several

GPS is the oldest and most widely used satellite navigation system, but it is not the only one. Russia’s GLONASS, the European Union’s Galileo, and China’s BeiDou all provide similar global coverage, and most modern smartphones quietly combine signals from several of these constellations at once to lock onto a position faster and more precisely than any single system could alone.