Your phone does not know where you are. It works it out, moment by moment, from radio signals sent by satellites in medium Earth orbit. Those satellites carry atomic clocks and broadcast the time along with data that lets a receiver compute where each satellite was when it transmitted. From a handful of those signals, a receiver can pin down latitude, longitude, altitude and time.
Photo: Wikimedia Commons (CC BY-SA 4.0)
The basic idea: timing becomes distance
Each satellite puts the time its signal was broadcast into the codes it transmits. The receiver compares that with the moment the signal arrived and works out how long the trip took. Multiply by the speed of light and you have the range to that satellite. Because the satellite’s own position is known from the broadcast data, each range places the receiver somewhere on an invisible sphere around that satellite. Several spheres, and where they intersect, is you.
Why four satellites, not three
Three ranges would be enough to fix a position if the receiver had a perfect clock. No phone does. So GPS solves the clock problem in a clever way: treat the receiver’s own clock error as one more unknown, and take a fourth measurement. Four satellites, four unknowns, and the receiver returns latitude, longitude, altitude and time. Satellites carry atomic clocks, which measure the passage of time by counting oscillations of an electronic signal kept resonant with a state transition in an atom, according to the National Institute of Standards and Technology.
The atmosphere interferes
Signals do not travel through the atmosphere quite as they would through empty space. The ionosphere and the troposphere slow them down, and a receiver has to correct for those propagation delays or its ranges will be wrong. With the basic civil service and those corrections applied, the U.S. Federal Aviation Administration puts typical accuracy at about 7.0 metres, 95 per cent of the time, anywhere on or near the surface of the Earth.
GPS is one constellation of several
GPS is operated by the United States, but it is not the only satellite navigation system in orbit. The FAA notes three others: GLONASS, developed and operated by the Russian Federation; Galileo, developed and operated by the European Union; and BeiDou, developed by China. Together with their augmentations, these constellations make up what is called Global Navigation Satellite Systems, or GNSS, and all of the providers offer free use of their systems to the international community.
GPS is a U.S.-owned utility that provides users with positioning, navigation, and timing (PNT) services.
What it means in practice
Positioning is only part of what satellites give you. The same signals carry precise time, and that timing is what lets banks stamp transactions, power grids stay in step and mobile networks hand calls between towers. Your map app is the most visible user of a system that quietly underpins a great deal of modern infrastructure. See gps.gov for the official programme description, faa.gov for the technical walkthrough, and nist.gov for how atomic clocks keep the time that the whole system depends on.
Bottom line: GPS is a timing system that happens to produce positions. Measure how long a signal took to arrive from four satellites, and the geometry does the rest.
Photo: Eric Friedebach / wikimedia (by 2.0)
Photo: Eric Friedebach / wikimedia (by 2.0)