Selective Calling or SELCAL is a technique by which an aircraft has a unique identifier and a ground station can contact the aircraft for communication. This eliminates the aircraft crew from monitoring the HF frequencies on a regular basis and the subsequent background noise as a result.
The High Frequency Communications system (HF) must have SELCAL capability to process the ground station contact and alert the crew through audio or visual means. One of the components in the SELCAL system is a decoder. The decoder allows a ground station equipped with tone encoding equipment to call the specific aircraft by transmitting 2 pairs of assigned audio tones. The decoder is set to respond to the specific code assignment set on the front of the decoder. Upon reception of the tone code, the decoder signals the HF system that a message is waiting through audio or visual means.
Decoders are available in remote and panel mount variations. Some decoders may have the ability to monitor as few as 2 communication radios or up to 5 simultaneously.