GPS Needs Einstein

Your GPS receiver listens to signals from at least 4 satellites orbiting 20,200 km above Earth. Each satellite broadcasts a precise timestamp, and your phone calculates distance by measuring how long each signal took to arrive.

Those satellites move at 14,000 km/h, so special relativity slows their clocks. General relativity speeds them up in weaker gravity. Net effect: satellite clocks gain 38 microseconds daily. Without Einstein's corrections, GPS would drift 10 km per day.