How your phone knows where you are

Open a map indoors, in a basement, with no view of the sky, and the blue dot still finds you in about a second. Whatever is happening there, it is not satellites.

Your phone has four separate ways of working out where it is. GPS is the one everybody knows about, and it is frequently not the one being used.

Four methods, not one

The phone runs them in parallel and blends the results, weighting each by how much it trusts the answer at that moment. The trade-offs are what decide which one dominates.

The 4.9 metre figure is the official one, from the United States government’s own accuracy statement for GPS-enabled smartphones under open sky. The qualifier matters more than the number: accuracy degrades near buildings, bridges and trees, which describes most places people actually stand.

What the satellites are really sending

A GPS satellite does not know where you are and never learns. It broadcasts one thing, continuously, to nobody in particular: its own position and the exact time, kept by an atomic clock.

Your phone receives that message and compares the timestamp inside it to the time now. The difference is how long the signal took to arrive, and since radio travels at a known speed, that delay converts directly into a distance. One satellite puts you somewhere on a sphere. Two intersect into a circle. Three narrow it to a point. A fourth is needed because your phone has no atomic clock of its own, and solving for the clock error is what makes a cheap receiver work at all.

This is trilateration — measuring distances — and not triangulation, which measures angles. The distinction gets lost constantly and is the difference between what GPS does and what it does not.

It is also why a cold start used to take so long. The receiver needs to know where the satellites currently are, and that data arrives slowly over the satellite signal itself. Modern phones cheat: they download the same orbital data over the mobile network in an instant. That shortcut is called assisted GPS, and it is why your phone locks on in seconds where a 2003 car navigator sat blinking for a minute.

The Wi-Fi trick, which is the interesting one

Indoors, the method doing the work is almost always Wi-Fi, and it works in a way most people find slightly startling once it is spelled out.

Every wireless router broadcasts a permanent hardware identifier, whether or not you connect to it, whether or not you know its password. Your phone listens, hears a handful of nearby routers, and sends that list of identifiers to a server. The server holds a map of which router identifier sits at which location, finds the ones it recognises, and sends back a position.

Your phone did not join any of those networks. It did not need to. It simply overheard which routers exist nearby and asked a database what that combination means. The lookup is a single round trip, costs almost no battery, and works perfectly in a basement.

The obvious question is how the database was built. Partly by driving: mapping vehicles logged router identifiers alongside the street imagery they were collecting, an exercise that produced a considerable privacy row when the extent of it became public. Mostly, now, by the phones themselves. A device with a clear satellite fix and a list of routers it can hear has just produced a free, verified data point, and it reports it. Every phone with a sky view is quietly surveying the routers around it, including yours.

Which means your home router is, with near certainty, an entry in at least two commercial location databases — a detail worth holding alongside the rest of what your home network exposes.

What “precise location: off” actually does

Both major phone platforms now let you hand an app a deliberately blurred position instead of an exact one, and the setting does something more specific than it sounds.

The middle row is the one that gets underused. An app told your approximate location can still show local weather, local headlines and nearby shops perfectly well, because none of those need to know which building you are in. Almost every app that requests precise access would function identically without it.

“Always” deserves more suspicion than it usually gets. Background location is not one reading; it is a continuous record, and a continuous record of where a phone sleeps and where it spends weekdays identifies a person without ever being told their name.

Where the location goes after the phone hands it over

The operating system’s job ends the moment it answers the app. What happens next is not a location question at all.

Many apps embed third-party software kits for advertising, analytics or crash reporting, and those kits run inside the app with the app’s permissions. When you grant an app location access, you may be granting it to several companies at once, most of which are not named anywhere you were shown.

From there the data enters the same market described in how data brokers work, where movement histories are bought, merged and resold. That is the actual exposure, and it is worth being clear that no network-level tool addresses it — this travels inside an authorised app session, which is precisely the category a VPN does not hide.

What actually reduces this

Four things, in descending order of effect.

Audit the permission list rather than the settings screen. The question for each app is not whether it is trustworthy but whether it has any functional need to know where you are. Most do not, and the ones that do mostly need it only while open.

Downgrade to approximate wherever the app still works, which is most places. This is the single highest-value change available, because it breaks the link between you and a specific address without breaking anything you use.

Reset the advertising identifier occasionally. Location data is only valuable once it can be tied to a persistent identity across apps, and that identifier is the thread.

And treat “always” as a decision rather than a default. It is the only setting that turns a series of lookups into a biography.

Sources

• GPS.gov, GPS Accuracy — the 4.9 m open-sky figure for GPS-enabled smartphones and the conditions that degrade it
• Android developers, location permissions — the approximate versus precise permission model and what each returns
• GPS.gov, the GPS space segment — constellation size and what each satellite actually broadcasts
• Electronic Frontier Foundation, privacy — the civil-liberties analysis of location data: who can request it, and what the permission model does not cover