Your phone can place you on a map within a few metres, anywhere on Earth, without sending a single signal to space. It only listens. The trick relies on extremely precise clocks, some clever geometry and, surprisingly, Einstein's theory of relativity.
Satellites that broadcast the time
The GPS system operated by the United States has around 30 active satellites orbiting about 20,200 km above Earth. Each carries atomic clocks and continuously broadcasts two things: the exact time the signal left, and where the satellite was at that moment.
Distance from time delay
Radio signals travel at the speed of light, about 300,000 km per second. Your phone notes when each signal arrives, subtracts the time it was sent and multiplies by the speed of light. A delay of about 0.07 seconds, for example, means the satellite is about 21,000 km away.
Finding the point where the spheres meet
Knowing your distance from one satellite puts you somewhere on a huge sphere around it. A second satellite narrows it to a circle, and a third to two points, one of which is usually far out in space. This is called trilateration.
So why does your phone need a fourth satellite? Because its own clock is cheap quartz, not atomic. An error of one millionth of a second equals a distance error of 300 metres. The fourth satellite lets the receiver solve for its own clock error as well as its position.
Einstein's correction
Here is the remarkable part. According to special relativity, the satellites' fast motion makes their clocks tick slightly slower, by about 7 microseconds a day. According to general relativity, weaker gravity high above Earth makes them tick faster, by about 45 microseconds a day. The net result is that satellite clocks run ahead by about 38 microseconds every day.
That sounds tiny, but uncorrected it would add position errors of about 10 km per day. Engineers deliberately set the satellite clocks slightly slow before launch so that they keep perfect time in orbit. Relativity is not an abstract theory here. Your navigation app depends on it.
Why phones locate you so quickly
- Assisted GPS. Your phone downloads satellite positions over mobile data instead of waiting to receive them from space.
- Multiple systems. Modern phones also use Russia's GLONASS, Europe's Galileo, China's BeiDou and, in many phones, India's NavIC. More satellites mean faster, more accurate fixes.
- Wi-Fi and cell towers. Indoors, phones estimate position from nearby Wi-Fi networks and mobile towers.
Why GPS goes wrong in cities
Tall buildings block satellites and reflect signals. A reflected signal travels a longer path, so the phone thinks the satellite is farther away. That is why your location dot jumps around between high-rises. Newer dual-frequency receivers reduce this error.
Engineering uses beyond maps
Surveyors use advanced GPS techniques to reach centimetre accuracy. Power grids, mobile networks and stock exchanges use GPS time signals to synchronise equipment. Engineers monitor bridges and dams for millimetre movements using fixed GPS stations.
Frequently asked questions
Does GPS use mobile data?
Receiving satellite signals needs no data. Downloading maps and assisted GPS information does.
What is NavIC?
NavIC is India's regional navigation satellite system, built by ISRO to cover India and the surrounding region.