How GPS knows where you are and why it sometimes does not

The blue dot on your phone is the end product of one of the stranger bargains in engineering: a fleet of satellites 20,200 kilometres overhead does almost nothing except broadcast the time, extremely precisely, and your handset does all the clever work of turning those time stamps into a street address.

Each GPS satellite, there are around 31 operational ones run by the United States Space Force, carries atomic clocks stable to within a few billionths of a second per day. It continuously transmits a signal saying, in effect, "the time is exactly X, and here is where I am in my orbit". Your phone receives that message and compares the broadcast time with its own clock. The gap between them is the signal's travel time, and since radio waves move at the speed of light, about 300,000 kilometres per second, travel time converts directly into distance. Know your distance from one satellite and you sit somewhere on a vast sphere around it. Add a second and a third satellite and the spheres intersect at your position.

There is a catch, and it explains why a fourth satellite is always needed. The satellites carry caesium and rubidium atomic clocks; your phone carries a quartz oscillator that would cost pennies to replace. Its timekeeping is nowhere near good enough, because at light speed a clock error of a single nanosecond corresponds to about 30 centimetres of position error, and a millisecond would put you 300 kilometres out. So the receiver treats its own clock offset as a fourth unknown alongside latitude, longitude and altitude, and solves for all four simultaneously using the fourth satellite's signal. Your phone, in a quiet way, is constantly disciplining its cheap clock against atomic time from space.

The physics gets stranger still. The satellite clocks run measurably differently from clocks on the ground: special relativity slows them because they orbit at nearly four kilometres per second, while general relativity speeds them up because they sit in weaker gravity. The net effect is a gain of roughly 38 microseconds a day, which sounds trivial until you multiply it by the speed of light: uncorrected, positions would drift by around ten kilometres every day. The correction is baked in before launch, which makes every uneventful sat-nav journey a routine confirmation of Einstein.

Why the blue dot lies in cities and dies in tunnels

The whole scheme assumes the signal travelled to you in a straight line. In central London or Manchester's office canyons it frequently has not: the signal glances off glass and steel facades before reaching your phone, arriving late and inflating the measured distance. This is multipath error, and it is why the dot places you on the wrong side of the road or marches you through a building. Receivers fight back by weighting satellites high overhead, whose signals arrive more steeply, and modern phones blend in other evidence, Wi-Fi access point databases, mobile cell IDs, barometers for altitude, which is also how a fix appears within seconds rather than the minute or so a cold GPS start genuinely takes.

Tunnels are simpler: the signal, at around 20 watts transmitted from 20,000 kilometres away, is astonishingly faint by the time it arrives, far below the background radio noise, and it does not penetrate rock or reinforced concrete at all. A sat-nav that keeps tracking you through the Blackwall Tunnel is not receiving anything; it is dead reckoning, extrapolating from your last known speed and heading, with in-car units adding wheel-speed and gyroscope data. Emerge somewhere the software did not expect and the dot leaps guiltily to your true position.

Jamming, spoofing and why it matters beyond your commute

That faintness is also the system's soft underbelly. A transmitter of a few watts can drown GPS frequencies across a wide area, which is why jammers, sold illegally to defeat fleet trackers in vans, occasionally knock out reception around motorways and, in one well-documented pattern, near ports and airports. Using or supplying them in the UK breaches the Wireless Telegraphy Act 2006, and Ofcom investigates interference reports. Spoofing is the nastier cousin: broadcasting counterfeit satellite signals so a receiver calculates a confidently wrong position, a technique now routinely observed affecting shipping and airliners near conflict zones in the Baltic and eastern Mediterranean.

Since timing, not just position, underpins mobile networks, energy grids and the time stamps on financial trades, a government review put the potential cost of a five-day outage to the UK at over £5 billion. Multiple constellations, Europe's Galileo, Russia's GLONASS, China's BeiDou, provide some redundancy, and modern phones listen to several at once. The blue dot, in other words, is less a fact than a continuously argued estimate, and knowing what it argues from tells you exactly when to distrust it.