How GPS Works: How Your Phone Knows Where You Are

Open a map on your phone and a blue dot appears almost exactly where you’re standing. It feels like the phone is being tracked from space. In fact, it’s the other way round: your phone is quietly listening to satellites and doing the maths itself.

What GPS is

GPS stands for Global Positioning System. It’s a network of satellites run by the United States, operated today by the US Space Force, and free for anyone in the world to use.

The system is built around 24 orbital slots, arranged so that at least four satellites are visible from virtually any point on the planet. In practice more are flying: the US government counted 31 working GPS satellites in 2023. Each one orbits about 20,200 km above the Earth and goes round the planet twice a day.

How the satellites help you find yourself

Every GPS satellite carries extremely precise atomic clocks. It broadcasts a steady radio signal that says, in effect, “I am this satellite, this is exactly where I am, and this is exactly what time it is.”

Your phone picks up that signal and notes when it arrived. Radio waves travel at the speed of light, so the delay tells your phone how far away the satellite is. A satellite directly overhead is so far away that its signal takes around seven-hundredths of a second to reach you.

One distance on its own isn’t much use. It only tells you that you’re somewhere on a giant imaginary sphere around that satellite. But add more:

1. One satellite: you’re somewhere on a sphere around it.
2. Two satellites: you’re somewhere on the circle where the two spheres meet.
3. Three satellites: that circle crosses the third sphere at just two points, and one of them is usually far out in space, so it can be ruled out.

This method is called trilateration, and it’s the heart of GPS.

Why you need a fourth satellite

There’s a catch. The satellites have atomic clocks; your phone has a cheap one. Light travels about 300 metres in a millionth of a second, so even a tiny timing error in your phone would throw your position off by kilometres.

The solution is clever. Your phone treats its own clock error as one more unknown and uses a fourth satellite to solve for it. With four distances, there’s only one position and one clock correction that make all the numbers agree. That’s why GPS needs at least four satellites, and why a fix improves when more are in view.

GPS satellites don’t know where you are. Your phone works it out by listening, and it never has to send anything back.

Why GPS needs Einstein

The satellites’ clocks have to be adjusted for relativity, which sounds like science fiction but is very real.

• Because the satellites move fast, special relativity makes their clocks run slightly slow, by about 7 millionths of a second a day.
• Because they’re far from Earth, where gravity is weaker, general relativity makes them run faster, by about 45 millionths of a second a day.

Together, the satellite clocks gain about 38 millionths of a second a day compared with clocks on the ground. That sounds tiny, but at the speed of light it adds up to errors of around 10 km a day. So the satellite clocks are deliberately set to tick slightly slower before launch, and ground stations keep checking and correcting them.

How accurate is your phone?

According to the US government’s GPS website, a GPS-enabled smartphone is typically accurate to within about 4.9 metres (16 feet) under open sky. That’s good enough to tell which side of the road you’re on, but not always which lane.

Accuracy gets worse when the signals are blocked or bounced:

• Tall buildings in city centres reflect signals, so they arrive late and confuse the maths. That’s why your blue dot sometimes jumps across the street.
• Bridges, tunnels and dense trees block signals altogether.
• Indoors, the signals are often too weak to use at all.

Your phone doesn’t rely on GPS alone

Modern phones are smarter than a pure GPS receiver:

• Other satellite systems. GPS has company. Europe runs Galileo, Russia GLONASS and China BeiDou, and India and Japan have regional systems. Most phones listen to several at once, so more satellites are in view. The general name for all of them is GNSS (Global Navigation Satellite Systems).
• Assisted GPS. Your phone can download satellite position data over the mobile network or Wi-Fi, so it finds its first fix in seconds rather than minutes.
• Wi-Fi and phone masts. Nearby Wi-Fi networks and mobile masts give a rough position indoors, where satellite signals don’t reach.
• Motion sensors. The phone’s accelerometer and compass help it guess where you’re heading between fixes, for example in a tunnel.