In the open sky, GPS is accurate to 2–5 metres. Walk into a city street lined
with glass towers and watch that number jump to 50 or 200 metres. The signal
bounces off buildings before reaching your phone — and the reflection looks
exactly like a direct signal. This is why your map shows you walking through
buildings.
GPS spoofing attacks on ships cause them to appear hundreds of miles off course.
The same multipath effect that blurs your city navigation can be weaponised.
In 2017, ships in the Black Sea reported GPS positions 32 km inland.
In 2019, dozens of vessels near Shanghai were falsely positioned in the middle of
Pudong airport. No hardware needed — just a signal transmitter and a laptop.
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▶ what causes urban canyon GPS error?
GPS works by measuring the time it takes signals from multiple satellites (you need at least 4)
to reach your receiver. The calculation assumes signals travel in a straight line at the speed of light.
In an urban canyon, signals bounce off glass and steel facades — called multipath —
and arrive slightly late, skewing the position fix by tens of metres.
Modern phones use several tricks to compensate: GLONASS (Russian), Galileo (European),
and BeiDou (Chinese) satellites in addition to US GPS give more fix candidates to average.
The accuracy field in the browser Geolocation API reflects the receiver's own
estimate of its uncertainty — though this estimate itself can be wrong in severe multipath.
Try this: compare accuracy indoors vs outdoors vs next to a tall building.
The jump is immediate and dramatic.