Atomic Clock (Space)

Atomic clocks aboard navigation satellites are the foundation of satellite-based positioning. They measure time by counting the oscillations of atoms (typically caesium or rubidium) at known frequencies. GPS satellites carry rubidium and caesium atomic clocks accurate to 1 nanosecond per day (about 1 second in 100,000 years). Since a GPS receiver calculates its position by measuring the time signals take to travel from satellites, clock accuracy directly determines position accuracy: 1 nanosecond of timing error equals 30 cm of range error. Galileo satellites carry hydrogen maser clocks that are even more precise. Relativistic corrections (both special and general) are applied because the satellites' speed and altitude cause time to run differently than on Earth's surface.