How does your phone know where you are? GPS explained

Satellites that tell the time

GPS stands for Global Positioning System. It is a group of satellites run by the United States, circling Earth at around 20,000 kilometres up. There are enough of them that, from almost anywhere outdoors, several are above the horizon at any moment.

Each satellite carries extremely accurate clocks and constantly broadcasts a simple message: "this is who I am, this is exactly where I am, and this is exactly what time it is".

Other countries run similar systems: Galileo (Europe), GLONASS (Russia) and BeiDou (China). Most modern phones listen to several of them at once, which makes positions faster and more accurate. People often still call the whole thing "GPS".

Turning time into distance

Radio signals travel at the speed of light: very fast, but not instantly. When your phone hears a satellite's message, it compares the time stamped on the message with the time it arrived. The tiny delay tells it how far away that satellite is.

Here is an everyday comparison. If you see lightning and count the seconds until the thunder, you can estimate how far away the storm is. GPS does the same kind of sum, but with radio signals and timing measured in tiny fractions of a second.

Why it needs four satellites

Knowing your distance from one satellite only tells you that you are somewhere on a huge imaginary sphere around it. Add a second, and you narrow it to a circle. A third narrows it to a couple of points, and one of those is usually far out in space, so it can be ignored.

The fourth satellite fixes a practical problem: your phone's clock is nowhere near as precise as the satellites' clocks. The extra signal lets it correct its own clock error at the same time. With more satellites, the answer gets better still.

Importantly, your phone only listens. It does not send anything to the satellites, and the satellites do not know who is using them.

Why it struggles indoors and in cities

Satellite signals are weak by the time they reach the ground. They do not pass well through roofs and walls, and tall buildings can block them or bounce them around, which confuses the timing.

That is why your blue dot sometimes jumps across the road in a city centre, or spins in a circle indoors. To help, phones combine GPS with other clues: nearby Wi-Fi networks, the position of mobile phone masts, and motion sensors inside the phone. This blend gives a quick first guess, then GPS sharpens it.

Does GPS use your mobile data?

Receiving satellite signals is free and uses no data. What does use data is the map, live traffic and search results. If you travel abroad, or to places with poor coverage, try this:

1. Open your maps app while on Wi-Fi.
2. Look for an option to download an area for offline use, and save the region you will visit.
3. On the trip, you can switch off mobile data and still see your position moving on the saved map.

Keeping your location private

Your position is useful to you, and valuable to others. Many apps ask for it, and not all of them need it.

• Check which apps can see your location in your phone's privacy settings, and switch off any that do not need it.
• Prefer "only while using the app" over "always".
• Use "approximate location" where offered, for things like weather.
• Photos can store where they were taken. Check your camera settings before sharing pictures publicly, especially of your home or children.
• Be careful with live location sharing. Share it with people you trust, for a set time, and review the list now and then.

Tip: If someone you know has access to your phone or accounts, check for location sharing you did not set up, and for unknown tracking devices if you get an alert about one. Both major phone platforms can warn you about unfamiliar trackers moving with you.