PerThirtySix – How geometry, stopwatches, and Einstein’s theories work together to make GPS possible. “If you’re like me, you might be entirely dependent on GPS to navigate the world. At some point, you may have caught yourself wondering during those panicked moments when an exit is coming up and your phone is recalibrating: how does my phone even know where I am? The answer is in some ways simpler than you’d expect, and in other ways more complex. GPS is fundamentally a translation tool: it converts time into distance. A satellite sends a signal, your phone catches it, and the delay between those two events tells the phone exactly how far away the satellite is. Everything else is about making that measurement precise enough to be useful: accounting for bad clocks, satellite geometry, and eventually, Einstein’s theories. Every GPS measurement starts with a stopwatch. A satellite broadcasts a radio signal at the speed of light. Your phone receives it and checks how long the trip took. With this information, it’s straightforward to calculate the distance between a satellite and your phone…”