How SELCAL Works

Almost every aircraft operating over the ocean or in remote areas has a SELCAL code registered with ASRI.  The SELCAL code is entered into field 18 of the aircraft’s flight plan that is filed with the relevant aviation authorities.   When a ground station wishes to communicate with an aircraft, the radio operator enters the required SELCAL code into the radio system which then transmits the SELCAL tones.  All aircraft monitoring the assigned radio channel will receive the SELCAL broadcast, but only those that have been programmed with the specific 4-alphabetic code will be alerted. The crew will then respond to the ground station operator using ICAO recommended voice radio procedures before confirming the message and the necessary actions.

SELCAL codes are made up of the various combinations of alphabetic characters represented as two pairs (for example AC-BD), with each character representing a unique audio frequency (see table at the bottom of this page). To transmit a SELCAL code, these audio frequencies are modulated onto the selected voice channel at the ground station transmitter.  This can be for either long range High Frequency (HF) communications, or more localized Very High Frequency (VHF) voice channels.  SELCAL codes are transmitted as two consecutive pulses, with the first pulse consisting of the first pair of audio tones (from our example, the AC part), and the second pulse the last pair of audio tones (the BD). The pulses are approximately 1 second in duration, separated by an interval of approximately 0.2 seconds.

The below figure gives shows how the example AC-BD code would be sent.