Which of the following principles explains how a VHF Omnidirectional Range (VOR) station determines an aircraft's magnetic bearing from the station?

1. By comparing the phase difference between a reference omnidirectional signal and a variable rotating signal.
2. By measuring the round-trip travel time of a radio pulse between the station and the aircraft.
3. By calculating the Doppler frequency shift of a signal transmitted from the station.
4. By measuring the signal strength ratio between the localizer and glide path signals.