A 0.001-Second Mistake That Puts You in the Wrong Country

GPS satellites mostly just broadcast the time, and your phone never answers them. So how does a phone with a cheap, drifting clock work out where you are to within a few metres? We work it out using real signals: data recorded by a GPS receiver at the US Naval Observatory in Washington, DC, run through a small GPS solver written for this video. Every number on screen comes from that real data or from our own runs: the flight times, the step-by-step fix, the 0.81 m answer, the effect of Earth's rotation and the atmosphere, and the whole-day accuracy. The signal-search part is a simulation using the real GPS C/A codes at the receiver's measured signal strength. Chapters 0:00 Satellites that only tell the time 0:35 Turning time into distance 1:28 The cheap-clock trick (why you need 4) 2:26 Hearing a signal weaker than the noise 3:40 Solving a real GPS fix, step by step 4:42 Earth's spin, the air, Einstein, and geometry 5:53 How close it got (and why phones do worse) Data and sources • Observation and navigation data: International GNSS Service (IGS), station USN8 (US Naval Observatory), 27 September 2026, via the BKG GNSS Data Center, https://igs.bkg.bund.de • IS-GPS-200, the GPS interface specification (C/A codes, navigation message, satellite clock frequency, ionosphere model, orbit equations) • N. Ashby, "Relativity in the Global Positioning System", Living Reviews in Relativity 6, 1 (2003) • GPS space segment: https://www.gps.gov/systems/gps/space/ • Assisted GPS: https://en.wikipedia.org/wiki/Assiste... • Coastlines: Natural Earth (public domain), https://www.naturalearthdata.com Music "Airport Lounge" by Kevin MacLeod (incompetech.com) Licensed under Creative Commons: By Attribution 4.0 License http://creativecommons.org/licenses/b... #gps #howitworks #computerscience