{"description":"Your phone finds you in seconds, anywhere on Earth, using satellites, atomic clocks and the speed of light. A short, clear explainer on the timing trick behind GPS, and why Einstein is part of it.","sections":[{"card_sub":"The timing trick inside every phone","card_text":"How GPS finds you","id":"s1","role":"hook","scene_type":"title","text":"Your phone finds you in seconds, anywhere on Earth. Yet it carries no map that knows where you are. So how does it know?"},{"card_sub":"About 31 satellites, 20,200 km up, circling Earth twice a day","card_text":"Satellites that shout the time","id":"s2","role":"setup","scene_type":"explainer","text":"High above the Earth, more than thirty satellites circle the planet twice a day. Each one carries an atomic clock, and calls out the same two things, again and again: where it is, and exactly what time it is."},{"big_number":"1 microsecond = 300 m","card_sub":"Signals travel at the speed of light, so arrival time becomes distance","card_text":"Timing is the measuring tape","id":"s3","role":"build","scene_type":"stat","text":"Those calls travel at the speed of light, and your phone times how long each one took to arrive. That timing is the measuring tape. A timing mistake of one microsecond, a millionth of a second, means roughly three hundred metres of error."},{"card_sub":"One sphere, then a circle, then two points, then you","card_text":"Spheres, then a point","id":"s4","role":"build","scene_type":"diagram","text":"One satellite only tells your phone it is somewhere on a giant sphere. A second satellite narrows that down to a circle. A third brings it down to two points, and one of those points is you."},{"card_sub":"It fixes your phone's own clock","card_text":"Why a fourth satellite?","id":"s5","role":"build","scene_type":"explainer","text":"But your phone has no atomic clock of its own, so a fourth satellite does one more job. It lets the phone correct its own clock, and solve for latitude, longitude, height, and time, all at once."},{"card_sub":"Position, measured in billionths of a second","card_text":"It is a clock, not a map","id":"s6","role":"landing","scene_type":"closing","text":"Even Einstein is on duty. Satellite clocks gain about thirty eight microseconds a day because of relativity. Left uncorrected, that would drift positions by about ten kilometres a day. Your phone knows where you are because it knows the time, to a billionth of a second."}],"title":"How GPS Finds You"}