SELCAL (Selective Calling) is a non-voice mode used in the aeronautical HF bands, it sounds like two tones being transmitted, usually requested by the pilot of a plane, and sent by the air ground station. You hear "SELCAL Check" followed by the tones. Each tone (there are actually 4 of them) corresponds to a letter of the alphabet, and the four tones comprise a four letter identification for that airliner. In the pic below you can see a SelCal check for a particular aircraft, this is a unique identifier for it, AFKR. SELCAL is handy for the flight crew because when that tone is sent, it automatically notifies the crew that a message for them is coming. Almost like a pager in a sense. This transmission was received using a tabletop radio, SignaLink USB, and MultiPSK software.
