How Does GPS Work? Four Satellites, a Cheap Clock and Einstein

How does GPS work? Your phone never transmits to the satellites. It listens to at least four of them and turns tiny time delays into distance. One analogy carries the whole explanation: clock towers ringing at noon in a foggy town square. Each GPS satellite carries atomic clocks, and its navigation message says when the signal left and where the satellite was. Light covers about 300 meters in a microsecond, so a clock that is off by a millionth of a second puts you about 300 meters from where you really are. Why four satellites and not three? With a perfect clock, three would be enough. Your phone has a cheap quartz clock, so it solves for four unknowns: three coordinates plus its own clock error. The same precise GPS time matters far beyond maps: mobile networks, power grids and financial networks rely on it. Then relativity: a clock in orbit gains about 38 microseconds a day compared with one on the ground. Left uncorrected, your position would drift about 11 kilometers every day. Engineers set each satellite clock slightly slow before launch, and in 1977 the NTS-2 satellite measured the effect to within about one percent of Einstein's prediction. Finally, why the blue dot sometimes jumps across the street: signals bouncing off buildings, indoor and underground signal loss, the ionosphere, and sometimes a map that is simply wrong. 00:00 Intro 00:13 It all starts with clocks 01:45 Distance comes from time 02:53 The fourth satellite 03:59 Up there, time runs differently 05:44 Why the blue dot sometimes jumps Sources: GPS.gov: Space Segment: https://archive.gps.gov/systems/gps/s... GPS.gov: GPS Accuracy: https://archive.gps.gov/systems/gps/p... GPS.gov: Timing Applications: https://archive.gps.gov/applications/... IS-GPS-200N: NAVSTAR GPS Space Segment / Navigation User Interfaces: https://www.navcen.uscg.gov/sites/def... Neil Ashby, Relativity and the Global Positioning System, Physics Today (2002): https://physicstoday.aip.org/features... Neil Ashby, Relativity in the Global Positioning System, Living Reviews in Relativity (2003): https://pmc.ncbi.nlm.nih.gov/articles... ESA Navipedia: An intuitive approach to the GNSS positioning: https://gssc.esa.int/navipedia/index.... NOAA: What is GPS?: https://oceanservice.noaa.gov/facts/g... NIST: Speed of light in vacuum: https://physics.nist.gov/cgi-bin/cuu/... #GPS #HowItWorks #Physics