Ground-based Non-Directional Radio Beacons coupled with an airborne receiver (radio-compass or Automatic Direction Finder) comprise one of the primary radio navigation systems. Once tuned to the NDB frequency, the airborne receiver indicates the direction towards the airport with an accuracy of several degrees which allows an easier ground approach. Both civilian and military aircraft use the NDB receivers.
The Non-Directional Beacon is a ground-based omni-directional transmitter operating in the Medium Wave 190 – 535 kHz band. The transmitted signal carries a three-character airport ID in Morse code which is repeated every 30 seconds. That ID is transmitted as a 1020Hz tone with 600 ms spacing between characters. The range of NDB is depends on the transmitter output power and time of the day of the transmission. For a beacon with an output power of 100 Watts, that range would be 350 km during daytime and 50% more at night due to relatively better MW emission conditions.
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