How Phones Locate You With Wi-Fi and Cell Towers

A phone can estimate where it is without any satellite signal by looking at the Wi-Fi networks and mobile towers around it. Positions of these transmitters have been recorded in large databases, so the phone compares what it hears with what the database knows, and works out an approximate location.

Wi-Fi positioning

Wi-Fi access points constantly broadcast an identifier. A phone can pick these up without connecting. The phone sends a list of the identifiers it sees, along with how strong each signal is, to a location service. The service matches them against known positions of those access points and returns an estimate. Stronger signals suggest you are closer to that router.

Because routers have a short range, usually tens of metres, and are numerous in towns, this method can be quite good in dense areas. It works indoors where GPS often does not. It is weaker in rural places with few networks, and it can be wrong if a router has been moved since it was last recorded.

Cell-tower positioning

Mobile networks are made up of towers (cells), each with a coverage area. A phone knows which towers it can connect to and how strong each is. With one tower, the position is only known to be somewhere in its coverage area, which can be hundreds of metres or several kilometres. With several towers, timing or signal-strength comparisons can narrow the area. Cell-based positioning is a useful fallback when there is no GPS or Wi-Fi data, but it is the coarsest of the three.

Assisted GPS

These network sources also help satellite positioning. Giving the phone an approximate location and current satellite data, over the network, lets it find satellites much faster than it would on its own. This is often called assisted GPS. The satellite step itself still works as described in how GPS works.

Why your location sometimes jumps

A phone usually blends all sources and picks the best one at the moment. When you move from outdoors to indoors, the source may change from satellites to Wi-Fi, and the dot can jump slightly. The trade-offs are discussed in why phone GPS is inaccurate.

Privacy considerations

Network positioning means a location service can see which networks a device hears. Operating systems ask for permission before an app can access this information, and users can often choose between precise and approximate location. See location privacy and consent for the wider picture.

Also see why GPS does not work well indoors.