GPS
or: how we used Einstein to find the DoorDash guy
We built clocks so precise they count a cesium atom wiggling 9 billion times a second1, strapped them to rockets, flung about 30 of them into orbit at 8,700 mph2, then had to fix them with both of Einstein’s theories because time literally runs faster up there3. They scream the exact time at Earth nonstop, so a slab of glass in your pocket can hear four at once, do some algebra, and figure out where you’re standing on a spinning rock.
How it works, badly
Wait, is that real?
Yes, almost all of it.
How your phone finds you↩
About 31 working GPS A machine thrown sideways so fast that it keeps falling toward Earth and keeps missing. Thousands are up there relaying TV, maps, weather, and the internet.No overkill for this one yet. orbit roughly 12,550 miles up, circling Earth twice a day at about 8,700 mph; the exact count shifts as new ones launch and old ones retire. Each one broadcasts its exact time and position. Your phone measures how long each signal took to arrive, and since the signals travel at the speed of light, that delay becomes a distance. Four satellites are needed because there are four unknowns: your position in three dimensions, plus the error in your phone’s own cheap clock.
The Einstein part↩
The satellites move fast, which by Einstein’s discovery that time is stretchy: it runs slower the faster you move, and faster where gravity is weaker.No overkill for this one yet. makes their clocks run about 7 microseconds a day slow. They also sit farther from Earth’s gravity, which by general relativity makes them run about 45 microseconds a day fast. The net is roughly 38 microseconds a day fast. Left alone, that would drift your position by about 6 miles a day, so the clocks are tuned to tick slightly slow before launch.
The clocks↩
GPS satellites carry cesium and rubidium A clock that counts the wiggles of an atom instead of the swings of a pendulum; it is so steady that the second itself is defined by one.No overkill for this one yet.. A cesium atom flips between two energy states at exactly 9,192,631,770 cycles per second; that number is how the second itself is officially defined.
The fine print↩
A phone under open sky usually lands within about 16 feet, not inches, and it rarely listens to just four satellites; it grabs as many as it can, including other countries’ systems like Galileo, and leans on Your router yelling the internet across the room as radio waves, then listening very hard for your phone to yell back.Read the whole overkill → and A tall pole covered in radio antennas that hears every phone nearby and hands your calls and data to a wired network.No overkill for this one yet. too. And “mostly” is a joke: GPS timing quietly keeps cell networks, banks, and A continent-sized machine of power plants and wires that has to make as much electricity as everyone is using, every second.Read the whole overkill → in sync, which is less funny but more important than your burrito.
Sources: GPS.gov: space segment; ESA Navipedia: GPS space segment
Last fact-checked .