Abstract: In a phase comparison radio navigation system in which phase-locked signals of the same frequency are radiated in succession from spaced stations and are compared in phase at a receiver, instead of locking an oscillator or oscillators at the receiver to received signals to provide simultaneously available signals for phase comparison, each received signal is compared in turn in digital phase comparison means with a free-running oscillator, two successive comparisons of the oscillator with one received signal being used to determine the phase error rate of the oscillator and this error rate being used to correct the other phase determinations.