Before GPS, pilots navigated by radio. VOR stations broadcast signals on very high frequencies, and aircraft receivers interpret those signals to determine which radial the aircraft is flying along. A network of stations covered most of the developed world, and many remain in service today.

Each station transmits on a specific frequency and is identified by a three-letter code in Morse. Pilots tune the frequency, listen to confirm the station, then fly toward or away from it along a chosen radial.

VOR components and uses

• Ground station with a rotating signal
• Airborne receiver and display
• Radials defining airways and approaches
• Distance measuring equipment for range

VOR is line-of-sight, so reception degrades at low altitude or long distance. GPS has largely replaced it for en route navigation, but VOR still serves as a backup and supports many published instrument procedures.