Every GPS satellite is launched with a clock deliberately set to run slow, because Einstein’s relativity speeds it up by about 38 microseconds a day once in orbit — and without that built-in correction, your phone’s location would drift by roughly ten kilometres a day.

Every satellite in the GPS constellation carries atomic clocks, and before launch those clocks are deliberately set to tick at the wrong rate. They are adjusted to run slightly slow. The offset is small and precise, and it is built in on the ground, on purpose, by the engineers who designed the system. The reason [...] The post Every GPS satellite is launched with a clock deliberately set to run slow, because Einstein’s relativity speeds it up by about 38 microseconds a day once in orbit — and w