SELCAL
Encyclopedia
In international aviation
, SELCAL or SelCal is a selective-calling radio
system that can alert an aircraft's crew
that a ground radio station wishes to communicate with the aircraft
. SELCAL uses a ground-based encoder and radio transmitter to broadcast an audio signal that is picked up by a decoder and radio receiver on an aircraft. The use of SELCAL allows an aircraft crew to be notified of incoming communications even when the aircraft's radio has been muted. Thus, crewmembers need not devote their attention to continuous radio listening.
(HF) or very high frequency
(VHF) radio frequency bands used for aircraft communications. HF radio often has extremely high levels of background noise and can be difficult or distracting to listen to for long periods of time. As a result, it is common practice for crews to keep the radio volume low unless the radio is immediately needed. A SELCAL notification activates a signal to the crew that they are about to receive a voice transmission, so that the crew has time to raise the volume.
An individual aircraft has its own assigned SELCAL code. To initiate a SELCAL transmission, a ground station radio operator enters an aircraft's SELCAL code into a SELCAL encoder. The encoder converts the four-letter code into four designated audio
tones. The radio operator's transmitter then broadcasts the audio tones on the aircraft's company radio frequency channel in sequence: the first pair of tones, each lasting about half a second; a silence of about 0.2 seconds; followed by the second pair of tones, each lasting about half a second.
The code is received by any aircraft receiver monitoring the radio frequency on which the SELCAL code is broadcast. A SELCAL decoder is connected to each aircraft's radio receiver. When a SELCAL decoder on an aircraft receives a signal containing its own assigned SELCAL code, it alerts the aircraft's crew by sounding a chime, activating a light, or both.
The crew next turns up the volume on the aircraft radio to hear the incoming voice transmission. Using ICAO radio protocol, they must verify with the transmitting operator that they are the intended message recipients. The crew then uses the received information.
The code is a sequence of four letters, written or transmitted as an ordered two sets of two letters each (e.g., AB-CD). The letters are chosen from a subset of the Roman alphabet comprising A through S, excluding I, N, O, T, U, V, W, X, Y, and Z. The letters within a given pair are written or transmitted in alphabetical order (e.g., AB-CD is an allowable distinct SELCAL code, as is CD-AB, but CD-BA is not). A given letter can be used only once in a SELCAL code; letters may not be repeated (e.g., AB-CD is allowable, but AA-BC and AB-BC are not).
Each letter designates a specific audio tone frequency.
To avoid confusion from two or more aircraft using the same SELCAL code, ASRI tries to assign code duplicates to aircraft that do not usually operate in the same region of the world or on the same HF radio frequencies. However, aircraft commonly move between different geographical regions and it is now routine for two aircraft with the same SELCAL code to be found flying in the same region. Therefore, air crew always verify both SELCAL and call sign
(i.e., aircraft tail registration
, or telephony designator and flight identification) to be sure their aircraft is the intended recipient.
Aviation
Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...
, SELCAL or SelCal is a selective-calling radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
system that can alert an aircraft's crew
Aircrew
Aircrew are the personnel who operate an aircraft while in flight. The composition of the crew depends on the type of aircraft as well as the purpose of the flight.-Civilian:*Aviator** Pilot-in-command** First officer** Second officer** Third officer...
that a ground radio station wishes to communicate with the aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
. SELCAL uses a ground-based encoder and radio transmitter to broadcast an audio signal that is picked up by a decoder and radio receiver on an aircraft. The use of SELCAL allows an aircraft crew to be notified of incoming communications even when the aircraft's radio has been muted. Thus, crewmembers need not devote their attention to continuous radio listening.
Use
SELCAL operates on the high frequencyHigh frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
(HF) or very high frequency
Very high frequency
Very high frequency is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency , and the next higher frequencies are known as Ultra high frequency...
(VHF) radio frequency bands used for aircraft communications. HF radio often has extremely high levels of background noise and can be difficult or distracting to listen to for long periods of time. As a result, it is common practice for crews to keep the radio volume low unless the radio is immediately needed. A SELCAL notification activates a signal to the crew that they are about to receive a voice transmission, so that the crew has time to raise the volume.
An individual aircraft has its own assigned SELCAL code. To initiate a SELCAL transmission, a ground station radio operator enters an aircraft's SELCAL code into a SELCAL encoder. The encoder converts the four-letter code into four designated audio
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
tones. The radio operator's transmitter then broadcasts the audio tones on the aircraft's company radio frequency channel in sequence: the first pair of tones, each lasting about half a second; a silence of about 0.2 seconds; followed by the second pair of tones, each lasting about half a second.
The code is received by any aircraft receiver monitoring the radio frequency on which the SELCAL code is broadcast. A SELCAL decoder is connected to each aircraft's radio receiver. When a SELCAL decoder on an aircraft receives a signal containing its own assigned SELCAL code, it alerts the aircraft's crew by sounding a chime, activating a light, or both.
The crew next turns up the volume on the aircraft radio to hear the incoming voice transmission. Using ICAO radio protocol, they must verify with the transmitting operator that they are the intended message recipients. The crew then uses the received information.
Code registration
An individual aircraft is given a SELCAL code upon application to the SELCAL code registrar, Aviation Spectrum Resources, Inc.: About Selective Calling (ASRI). The code is technically assigned to the owner-operator of the aircraft rather than the aircraft itself; if an aircraft is sold, the new owners-operators must apply for a new code.The code is a sequence of four letters, written or transmitted as an ordered two sets of two letters each (e.g., AB-CD). The letters are chosen from a subset of the Roman alphabet comprising A through S, excluding I, N, O, T, U, V, W, X, Y, and Z. The letters within a given pair are written or transmitted in alphabetical order (e.g., AB-CD is an allowable distinct SELCAL code, as is CD-AB, but CD-BA is not). A given letter can be used only once in a SELCAL code; letters may not be repeated (e.g., AB-CD is allowable, but AA-BC and AB-BC are not).
Each letter designates a specific audio tone frequency.
Alphabet-audio frequency equivalents
| A | 312.6 Hz |
|---|---|
| B | 346.7 Hz |
| C | 384.6 Hz |
| D | 426.6 Hz |
| E | 473.2 Hz |
|---|---|
| F | 524.8 Hz |
| G | 582.1 Hz |
| H | 645.7 Hz |
| J | 716.1 Hz |
|---|---|
| K | 794.3 Hz |
| L | 881.0 Hz |
| M | 977.2 Hz |
| P | 1083.9 Hz |
|---|---|
| Q | 1202.3 Hz |
| R | 1333.5 Hz |
| S | 1479.1 Hz |
Limitations
The current rules for SELCAL code assignment, with sixteen available letters/tones, limit the number of possible allowable codes to 10,920. Additionally, SELCAL codes assigned previously use a subset of only twelve letters/tones. Therefore, more than one aircraft may be designated by the same code.To avoid confusion from two or more aircraft using the same SELCAL code, ASRI tries to assign code duplicates to aircraft that do not usually operate in the same region of the world or on the same HF radio frequencies. However, aircraft commonly move between different geographical regions and it is now routine for two aircraft with the same SELCAL code to be found flying in the same region. Therefore, air crew always verify both SELCAL and call sign
Call sign
In broadcasting and radio communications, a call sign is a unique designation for a transmitting station. In North America they are used as names for broadcasting stations...
(i.e., aircraft tail registration
Aircraft registration
An aircraft registration is a unique alphanumeric string that identifies a civil aircraft, in similar fashion to a licence plate on an automobile...
, or telephony designator and flight identification) to be sure their aircraft is the intended recipient.
See also
- Aircraft Communication Addressing and Reporting SystemAircraft Communication Addressing and Reporting SystemAircraft Communications Addressing and Reporting System is a digital datalink system for transmission of short, relatively simple messages between aircraft and ground stations via radio or satellite. The protocol, which was designed by ARINC to replace their VHF voice service and deployed in 1978,...
- Airline teletype system
- SATCOMSATCOMSATCOM or Satcom may refer to:* Short for Satellite Communications and used frequently in the context of VSAT * Communications satellites or comsats...