SELCAL

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In international aviation, SelCal or SELCAL is a selective-calling system that allows a ground radio operator to alert an aircrew that the operator wishes to communicate with that aircraft.

Because of the background noise level experienced on HF radio frequencies, aircrews usually prefer to turn down the audio level of their HF receiver until alerted via SELCAL of a message specifically intended for their aircraft. When the ground station operator wishes to communicate with an aircraft, he enters into the SELCAL encoder the 4-letter code of that aircraft, which is usually included in its flight plan, and transmits that code over the assigned radio channel. All aircraft monitoring that channel receive the SELCAL broadcast, but only those (preferably only one) that have been programmed with that 4-letter code will respond by sounding a chime or otherwise alerting the crew. The crew will then set their volume control higher to listen to the voice traffic and, using ICAO recommended radio procedures, assure that the message is intended for them.

There is a critical shortage of possible 4-letter codes, which has required re-use of the same code by more than one aircraft. Duplicate codes are usually assigned to aircraft operated in widely separated areas of the world, and usually do not have the same HF radio frequency assignment. However, there are occasions when two or more aircraft having the same code may be operating in the same general area, and will respond to the same transmission. Therefore, SELCAL should not be used as a substitute for proper voice identification procedures.