How Accurate Is GPS?
Your phone finds itself by timing radio signals from at least four satellites, each carrying an atomic clock. Under open sky that puts you within 3–10 meters; buildings, trees, and the atmosphere stretch that to 20–50 meters — which is why the blue dot wanders even when you're standing still.
Satellites, clocks, and trilateration
GPS is a constellation of about 31 satellites (joined by Europe's Galileo, Russia's GLONASS, and China's BeiDou — modern phones use all four) orbiting roughly 20,200 km up. Each broadcasts an extremely precise timestamp from an onboard atomic clock. Your phone measures how long each signal took to arrive; multiply by the speed of light and you get a distance to each satellite. One distance puts you on a sphere, two on a circle, three on two points — and a fourth satellite resolves both the final point and your receiver's own clock error. That's why a 'fix' needs at least four satellites, and why the first fix after being indoors takes longest: the receiver is still downloading each satellite's orbital data.
Two details make the arithmetic harder than it sounds. The first is relativity: each satellite's atomic clock runs about 38 microseconds per day faster than an identical clock on the ground — roughly 45 µs gained because gravity is weaker at 20,200 km, minus about 7 µs lost to its orbital speed. Untreated, that is a positioning error of some 10 km a day, so the correction is built into the satellites' clock rate and the receiver's maths. The second is geometry: the constellation flies in six orbital planes inclined 55°, each satellite circling twice per sidereal day (about 11 h 58 min), which is arranged so that at least four are usually above the horizon wherever you stand. When they happen to be clustered in one patch of sky, the same signal quality yields a worse fix — that is what receivers report as poor DOP.
GPS is also no longer alone up there. Most phones now also hear Europe's Galileo, Russia's GLONASS and China's BeiDou, and — depending on where you are — regional systems like India's NavIC or Japan's QZSS. More constellations means more satellites in view, which is most of why a 2026 phone fixes faster in a city street than a 2015 one did.
Where the error comes from
Every meter of error is a nanosecond-scale timing problem. The signal slows unpredictably crossing the ionosphere and troposphere (2–5 m of error), satellite clocks and orbits drift slightly (±2 m), and — the big one in cities — signals bounce off glass and concrete before reaching you. That multipath effect means your phone times a reflection instead of the direct ray, which is why accuracy in a street canyon can degrade to 30–50 m while an open field gives 3 m. Tree canopy, your own body, and even holding the phone low add smaller versions of the same problem.
Altitude is measured the same way but suffers roughly 1.5–3× the horizontal error, because all the satellites are above you — there's nothing constraining the solution from below. That's why our elevation tool offers a terrain-model reading alongside the GPS one.
What the accuracy circle actually means
The circle around your dot is a statistical statement, not a boundary: typically it's drawn so there's a ~68% chance the true position is inside. A 20 m circle doesn't mean you're 20 m off — it means the receiver's own error estimate, derived from signal quality and satellite geometry, is 20 m. Geometry matters as much as signal strength: if the visible satellites cluster in one part of the sky (called high dilution of precision), small timing errors turn into large position errors, the way two nearly-parallel lines intersect sloppily.
Phones cheat — in a good way
A phone fix is rarely pure GPS. Assisted GPS (A-GPS) downloads satellite orbits over the network so the receiver skips the slow sky-search; Wi-Fi positioning matches nearby access points against crowd-sourced maps (good to ~15–40 m indoors); cell towers give a coarse 100 m–3 km estimate instantly. Your OS fuses all of it with the accelerometer, gyroscope and magnetometer, which is why the dot sometimes snaps to a new position: the fusion engine changed its mind about which source to trust. When you check your location on this site, the browser hands us whatever the OS fusion produced — we show its reported accuracy honestly rather than pretending to a precision it doesn't have.
The other modern cheat is a second civil frequency. Older receivers listened only to L1; since about 2018 many phones are dual-frequency, adding L5. Two frequencies let the receiver measure and subtract the ionosphere's delay directly instead of modelling it, and L5's design shrugs off reflected signals better — which is exactly the error that ruins fixes between tall buildings. Where the satellites in view are broadcasting it, that is the difference between a 5 m dot and a 1–2 m one, and it costs you nothing but a newer handset.
Getting a better fix
Practical steps, in order of impact: get open sky (even stepping away from a building face helps more than anything else); give the fix 30–60 seconds to settle instead of reading the first estimate; disable battery-saver location modes, which quietly switch to coarse sources; and on long recordings such as our GPS speedometer, keep the screen on and the phone unobstructed — many phones throttle GPS when locked. If two devices disagree, trust the one showing the smaller accuracy circle, and remember both can be right within their stated bounds. For sharing a spot precisely regardless of GPS noise, use the coordinate formats on the GPS coordinates page — a Plus Code or MGRS reference doesn't drift.