Running Faster

The basic idea

Your GPS receiver doesn't transmit anything — it only listens. A constellation of ~30 satellites orbits Earth, each broadcasting a signal that says: "I am satellite X, and right now it is exactly time T."

Your phone receives signals from at least 4 satellites and figures out how far away each one is by measuring how long the signal took to arrive (distance = speed of light × time).

Trilateration (not triangulation)

With the distance to 3 satellites, you get a 3D position. The 4th satellite is needed to correct for clock errors in your cheap phone receiver — satellites carry atomic clocks worth $100,000+, your phone doesn't.

General relativity matters

This is the wild part: GPS satellites move fast (orbital velocity) and experience weaker gravity than Earth's surface. Both effects cause their clocks to drift:

• Special relativity (speed): satellite clocks run slower by ~7 μs/day
• General relativity (gravity): satellite clocks run faster by ~45 μs/day
• Net effect: +38 μs/day, which is corrected for in the satellite firmware

Without these corrections, GPS would accumulate ~10km of error per day.

Why this is cool

GPS is one of the few everyday technologies where ignoring Einstein would visibly break something. Relativity isn't just abstract physics — it's in your pocket.