Unauthorised use of ATC frequency is a situation where a person who is not an air traffic controller pretends to be one by transmitting radio messages to aircraft. VHF communication is not subject of any protection (e.g. encryption). Therefore anyone with a radio operating in the airband (frequency range 118-137 MHz) is technically capable of listening to and sending voice messages.
Following such an "instruction" may, depending on the specific circumstances, lead to e.g. Loss of Separation, Loss of Communication, Airspace Infringement, Controlled Flight Into Terrain (CFIT) and general disruption of flight operations. Therefore, in most countries impersonating an air traffic controller is a criminal offense.
Typical signs of unauthorised transmissions include:
- Transmissions are usually received only by the pilot or the controller and not both
- There are obvious mistakes in the phraseology used
- The "instructions" are usually out of the context and are not repeated if challenged
Additionally, pilots may have further reason to believe that an otherwise suspicious transmission if it was spoken by a different voice. While the voice change itself is no cause for alarm (the executive controller is often replaced after an hour or two at the position) the combination of illogical instruction and a voice change should raise suspicion and the pilot should consider requesting a confirmation. If no reply to such request is received the controller should be informed of the possible deceptive communication so that further action is initiated.
If a controller suspects (or a pilot report is received) that the frequency is subject to deceptive communications, they should:
- Inform all aircraft on the frequency about the situation and request all crews to ask for confirmation before complying with ATC clearances e.g. All stations, false ATC instructions have been received in the area of ... Exercise caution and verify instructions.
- Inform the supervisor (or other authority e.g. police in line with local instructions)
It is important that the best possible evidence is made available for any potential prosecution. Original tapes of communications received by the ground must be preserved where possible.
Locating the source of the messages is relatively easy using VDF (VHF direction finder) equipment. Whenever a transmission is made, the VDF determines its direction and the crossing point of two directions is the location of the antenna.