Why maps put you on the wrong road: GPS drift, urban canyons and dual-frequency reception

The map shows you on the flyover when you are on the service road beneath it, or two streets across in a market district, and it corrects itself a minute later. This is not a broken sensor or an out-of-date map. It is what satellite positioning does in a built-up area, and one piece of phone hardware makes a real difference to it.

How a position is worked out

A satellite transmits a signal stamped with the time it left. The receiver compares that with its own clock to get a travel time, and from several satellites it solves for a position. Because the receiver's clock is cheap and imprecise, the clock error is solved for as well, which is why four satellites are the practical minimum rather than three.

Everything that corrupts the travel time corrupts the position. In a city the dominant corruption is multipath: the signal bounces off a building and arrives later than the direct one, or the direct path is blocked and only the reflection arrives. The receiver measures the reflection's longer path in good faith. Errors of tens of metres are ordinary in a street lined with tall buildings, and the error is not random — it leans consistently away from the obstacles.

What the second frequency changes

Satellites broadcast on more than one frequency. Older phone receivers listened to one; newer ones can receive a second civil signal as well. Two benefits follow. The ionosphere delays signals by an amount that depends on frequency, so a two-frequency receiver can calculate that delay instead of modelling it. And the second signal has a structure that makes reflections easier to distinguish from the direct path, which is exactly the city problem.

Dual-frequency support is a hardware property, listed in the specification sheet when a maker considers it a selling point and omitted when it is absent. If your use of a phone involves navigating on foot in dense streets or driving where a parallel road matters, it is one of the few radio specifications with an obvious everyday effect.

Multiple constellations

Alongside the original system there are several others in service, and modern receivers use them together. More satellites means more of them visible above the rooftops, which helps directly. Support is listed as a string of system names in the specification; more is better, and the combination matters more than any single one.

What assisted positioning actually does

A cold receiver has to find satellites without knowing where they are, which takes time. Assisted positioning downloads the orbital data over the network so the receiver knows where to look, cutting the first fix from minutes to seconds. It also supplies a rough position from network information as a starting point.

That rough position is worth understanding, because it is the source of a common confusion: a phone with no satellite lock will still show a position derived from nearby wireless networks and cell towers. In a dense area that estimate can be good; in a sparse one it can be wildly wrong while looking confident. The blue circle on the map is the honest part — when it is large, the phone is telling you it is guessing.

Advice that works, and advice that does not

• Keep the phone's view of the sky clear. A dashboard mount low behind a metal-coated windscreen is a genuinely difficult environment.
• Leave the network location service on. It provides the starting estimate and the assistance data, and turning it off in the name of accuracy slows every fix.
• Recalibrate the compass, not the position. The figure-of-eight movement corrects the magnetometer, which controls the direction the arrow points. It does nothing for the position itself, which is why it feels like it works about half the time.
• Expect the first fix after a flight to be slow. The receiver's stored orbital data is stale and its assumed position is a continent away.

Magnetic interference is a real and underrated cause of "the arrow points the wrong way": magnetic mounts, cases with closures and speakers all disturb the compass. If the position is right and the heading is wrong, the problem is magnetic rather than satellite.

Battery, and why navigation is expensive

Continuous positioning keeps the receiver, the screen and often the mobile radio busy at once, and it is one of the heaviest things a phone does. The heat that follows drives the processor to slow itself, which is why navigation on a hot day feels sluggish as well as thirsty — the mechanism is in why phones slow down while navigating. Downloading the map region in advance removes the mobile radio from that list and is the single most effective thing you can do about it.