GPS works because its satellites carry atomic clocks so precise that Einstein’s relativity has to be built into the system: without correcting for the roughly 38 microseconds per day that satellite clocks gain in orbit, navigation errors would build up by about 10 kilometres a day.

GPS is usually described as a satellite navigation system, but the quieter truth is that it is also a clock system. The blue dot on a phone depends on satellites broadcasting time and position so precisely that small errors in time become large errors on the ground. The broad claim is right: without relativistic corrections, [...] The post GPS works because its satellites carry atomic clocks so precise that Einstein’s relativity has to be built into the system: without correcting for the roughly