How GPS Actually Works: Satellites, Signals, and Trilateration

Your phone can tell you where you are within a few meters, anywhere on Earth, for free — using satellites orbiting 20,000 kilometers away. The system behind this is remarkable precisely because the core idea is so simple: measure how long radio signals take to arrive, and geometry does the rest. Here is how it actually works.

The Constellation

The Global Positioning System is operated by the US Space Force and consists of a baseline of 24 satellites, with around 31 operational at any given time. They orbit at roughly 20,200 km altitude in medium Earth orbit, arranged in six orbital planes so that at least four satellites are visible from virtually any point on Earth's surface at any moment.

Each satellite circles the planet about twice per sidereal day and continuously broadcasts two things:

• Ephemeris data — its own precise orbital position
• A timestamp — the exact moment the signal left the satellite, stamped by onboard atomic clocks

Step 1: Measuring Distance With Time

GPS signals are radio waves traveling at the speed of light (~300,000 km/s). Your receiver compares the timestamp in the signal with its own clock:

distance = (time received − time sent) × speed of light

A signal from 20,200 km away takes about 0.067 seconds to arrive. The precision required is staggering: a timing error of just one microsecond translates to a 300-meter position error. This is why each satellite carries multiple atomic clocks accurate to nanoseconds.

Step 2: Trilateration — Spheres That Intersect

Knowing your distance from one satellite places you somewhere on a sphere around it. Two satellites narrow you to the circle where two spheres intersect. Three satellites reduce that to two points — one of which is usually in space or deep inside the Earth and can be discarded.

So why does GPS need four satellites?

Because your phone does not have an atomic clock. Its cheap quartz clock is off by an unknown amount, which corrupts every distance measurement equally. The receiver therefore solves for four unknowns simultaneously: latitude, longitude, altitude, and its own clock error. Four satellites give four equations — enough to solve all four unknowns. Every additional visible satellite beyond four improves accuracy by over-determining the solution.

This is also why GPS receivers effectively get a free atomic-grade clock: after solving, your device knows the time to within nanoseconds. Many critical systems — power grids, stock exchanges, cellular networks — use GPS primarily for time, not position.

Einstein in Your Pocket

GPS is one of the few consumer technologies where both special and general relativity must be corrected for:

• Special relativity: satellites move at ~3.9 km/s relative to you, so their clocks run slower by about 7 microseconds per day.
• General relativity: satellites sit higher in Earth's gravity well, so their clocks run faster by about 45 microseconds per day.

Net effect: satellite clocks gain roughly 38 microseconds per day relative to ground clocks. Uncorrected, that would accumulate into about 10 km of position error per day. The satellite clocks are deliberately tuned to compensate before launch.

What Degrades the Signal

The theory gives centimeter math; the real world adds noise:

• Ionosphere and troposphere delay signals unpredictably (the largest error source; dual-frequency receivers largely cancel it)
• Multipath — signals bouncing off buildings arrive late, confusing the receiver in cities
• Satellite geometry — satellites clustered in one part of the sky give poor intersections (high "dilution of precision")
• Obstruction — the signal is astonishingly weak by the time it arrives (roughly the power of a 50-watt bulb seen from 20,000 km), so roofs, canyons, and dense canopy block it

We cover these in depth in our guide to GPS accuracy.

From Signal to Coordinates

The end product of all this physics is a pair of numbers — latitude and longitude — plus altitude. Those numbers can then be expressed in several coordinate formats (DD, DMS, DDM) or projected into meter-based systems like UTM.

See it in action: open our GPS coordinates finder to view your device's current fix, or click anywhere on the map to read coordinates the same way your receiver reports them.