A device in your pocket can pinpoint where you are on the entire planet to within a few metres, instantly, for free. It does this by listening to clocks on satellites 20,000 kilometres overhead — and the whole thing only works because of Einstein. Here’s how GPS actually works.
Key takeaways
- GPS satellites don’t track you — your phone works out its own position by listening to them.
- It works by measuring how long signals take to arrive, which reveals distance.
- Your device needs signals from at least four satellites to fix its location.
- The satellites’ atomic clocks must be corrected for Einstein’s relativity, or GPS would drift kilometres off within a day.
The first surprise: GPS doesn’t track you
The most common misconception is that GPS satellites watch your device and report where it is. They do the opposite. The satellites just broadcast; your device does all the calculating.
Each satellite continuously transmits a simple message: “I am satellite number X, my position is here, and the time right now is exactly this.” Your phone receives these messages and figures out its own location from them. The satellites have no idea you exist — which is also why a basic GPS receiver is entirely private, and why GPS works for unlimited users at once.
The core trick: turning time into distance
Everything rests on one fact: radio signals travel at the speed of light, which is constant and known. If you know how long a signal took to arrive, you can calculate how far it travelled — distance is simply speed multiplied by time.
So when your phone receives a satellite’s message, it compares the time stamped in the message with the time it arrived. The tiny delay — often just a fraction of a second — reveals exactly how far away that satellite is.
One distance alone isn’t enough. Knowing you’re 20,000 km from one satellite only tells you you’re somewhere on a huge sphere around it. That’s where multiple satellites come in.
Trilateration: pinning down a point
The method GPS uses is called trilateration — narrowing down a location by combining distances from several known points.
- One satelliteKnowing your distance from one satellite places you somewhere on the surface of a giant sphere centred on it. Far too vague to be useful.
- Two satellitesAdd a second distance and the two spheres overlap in a circle. You’re somewhere on that ring — better, but still not a point.
- Three satellitesA third sphere narrows the possibilities to just two points, and one of them is usually absurd (out in space or deep underground), leaving your real position.
- Four satellitesThe fourth isn’t for geometry — it’s to fix your device’s clock. This is the clever part, and it’s what makes cheap GPS possible.
Why the fourth satellite matters so much
Measuring distance by timing means your device needs an incredibly accurate clock. The satellites carry atomic clocks accurate to billionths of a second — but your phone can’t. A tiny error in your phone’s clock would translate into a huge error in position, because light travels about 30 centimetres every billionth of a second.
The elegant solution: use a fourth satellite as an extra equation. With four distances and four unknowns — your latitude, longitude, altitude and the exact time — the maths solves for all of them at once. In effect, GPS gives your cheap phone the accuracy of an atomic clock for free, purely through geometry. This is why your phone’s clock is so precise when GPS is on.
Why it’s sometimes wrong or slow
GPS is remarkable but not flawless, and the reasons are worth knowing.
- Buildings and terrain block signals. The signal is faint, so thick walls, tunnels and dense city “canyons” degrade it. Signals bouncing off skyscrapers before reaching you also cause errors.
- The atmosphere slows signals slightly. Passing through the ionosphere introduces small timing delays that systems try to correct for.
- A “cold start” takes time. A device that hasn’t been used in a while must first download where the satellites currently are, which is why the first fix can take a minute.
- Phones cheat to speed things up. Your phone also uses Wi-Fi networks and mobile towers to estimate location quickly, then refines it with GPS — which is why it locates you fast indoors even where pure GPS struggles.
The terms, explained
- GPS
- Global Positioning System — the US satellite navigation network. “GPS” is often used loosely to mean satellite navigation in general.
- GNSS
- Global Navigation Satellite System — the umbrella term. Other systems include Europe’s Galileo, Russia’s GLONASS and China’s BeiDou; modern phones use several at once.
- Trilateration
- Determining position by combining known distances from multiple reference points. Often mislabelled “triangulation,” which measures angles instead.
- Atomic clock
- An extremely precise clock based on atomic vibrations. Each GPS satellite carries several.
- Cold start / warm start
- How much satellite data a device already has. A cold start has none and takes longer to get a first fix.
- Assisted GPS (A-GPS)
- Using mobile networks and Wi-Fi to speed up and support GPS, especially indoors or in cities.
Prefer to watch? We break down how everyday technology works on video too — browse our Science & Tech video explainers.
Frequently asked questions
Does GPS know where I am, or does my phone?
Your phone. The satellites only broadcast their position and time; your device calculates its own location from those signals. A basic GPS receiver sends nothing back, so the system itself doesn’t track you.
Why do I need signal from four satellites?
Three fix your position in three dimensions, and the fourth corrects your device’s clock. Since distance is measured by timing, an accurate clock is essential — the fourth satellite provides that without needing an atomic clock in your phone.
Does GPS use my mobile data?
Core GPS is free and needs no data — it just receives satellite signals. But phones use data-assisted features (downloading map tiles or satellite info) to get a faster, more reliable fix, especially indoors.
Why is GPS less accurate in cities or forests?
The signal is weak and easily blocked or reflected. Tall buildings, tunnels and dense tree cover obstruct it or bounce it around, adding error. Open sky gives the best accuracy.
