Selective Calling System.
SELCAL is a signaling method which can alert an individual aircraft that a ground station wishes to communicate with it. SELCAL signals can be transmitted over either HF or VHF RTF. A SELCAL transmission consists of a combination of four preselected audio tones which takes approximately two seconds to transmit. The tones are generated by a SELCAL encoder at the ground stations and received by a decoder connected to the audio output of the aircraft receiver. SELCAL can relieve a flight crew from maintaining a listening watch on assigned frequencies, which can be especially helpful where ATC RTF still relies upon noisy HF channels.
Receipt of an assigned SELCAL code activates a flight deck call system which may be a light, an audible chime or both. On aircraft equipped with SELCAL, the flight crew can maintain a listening watch using either headsets or flight deck speaker.
HF SSB suppressed carrier mode is not used to transmit SELCAL signals so many aircraft HF SSB transceivers are designed to detect SELCAL signals transmitted in the full carrier mode even though the transceiver mode selector switch is selected to the suppressed carrier mode. Transceivers which do not have this feature must have the selector switch in the full carrier mode of operation to reliably detect a SELCAL signal but the mode selector switch must be returned to the suppressed carrier mode before aircraft voice transmissions are made.
SELCAL codes are assigned to aircraft operators and not to individual aircraft. Aviation Spectrum Resources (ASRI) is the registrar of SELCAL codes worldwide.
The SELCAL code is entered in field 18 of the flight plan, using the SEL/ indicator.
Older SELCAL units are based on 12 tones and are shared with other aircraft. ASRI, as the SELCAL registrar, has historically tried to minimise assignment of potentially conflicting duplicate SELCAL codes by relating code assignment to geographical area of aircraft operation. This has been increasingly less successful and it is now not unusual to have more than one aircraft with the same SELCAL identification operating in the same geographical area at the same time. Operators using these 12 tone codes are recommended to advise ASR of changes to the geographical areas of operation of their aircraft.
Newer SELCAL units are predicated on 16 tones and are also assigned on a shared basis since very few available SELCAL codes remain unassigned. There are currently no plans by ICAO to increase the number of available tones.
Flight crews of aircraft with SELCAL equipment are advised to be alert to the potential for duplicated SELCAL codes and to listen closely to the Flight Identification (ID), as well as SELCAL, to avoid taking a clearance meant for another flight.