Aviation GPS

The GPS unit receives digital satellite transmissions that they use to determine their location, based on the distance from the satellites.
The satellite transmission includes the position of the satellite and the exact time of transmission. The time it takes to get to your GPS shows how far away the plane is from the satellites. To ensure clock accuracy, each satellite GPS carries four atomic clocks. Knowing the location of time and the source of three satellite signals gives your unit enough information to triangulate the location of your plane, but only your GPS has a clock in the complex of comparable accuracy.
The key to GPS accuracy is the tremendous accuracy of atomic clocks. The technology of atomic clocks placed in an oscillating field, making the field to resonate at a frequency that matches the magnetic resonance of the atoms.