Selective Calling (SELCAL) can be used on suitably equipped VHF and HF receivers, but in practice I've only seen it employed with HF.
— James Albright
While it seems self explanatory, there are some misconceptions that can be cleared up right here:
- All SELCAL codes have four letters and the only choices are A, B, C, D, E, F, G, H, J, K, L, M, P, Q, R, and S. That's 16 possible letters but you can only use each letter once. The letters are combined in pairs and within a pair they must be alphabetical. We end up with 10,920 possible combinations.
- The codes can be reused so you can find yourself in the same sky as another airplane with the same SELCAL code. (I've seen this once when crossing the Atlantic.)
- You can rely on SELCAL instead of monitoring the applicable radio in various parts of the world provided you file it in your flight plan and you test it with the controlling agency. This has to be done on each frequency you use, testing it on one doesn't count for others.
- You have to apply for a SELCAL code, more about that: Application.
SELCAL block diagram, from ASRI Users Guide, Figure 2-1.
- A state-of-the-art-technology when it was introduced for use in civil aviation in 1957, the Selective Calling (SELCAL) system permits selective calling of individual aircraft over the aeronautical mobile voice channels. It can be used on high frequency (HF) or very high frequency (VHF) channels and is designed to relieve flight crews from the need to continuously maintain a listening watch on their assigned radio channels.
- In order to uniquely identify individual aircraft using the SELCAL system, SELCAL codes were assigned to each aircraft. SELCAL code assignments were coordinated worldwide, on behalf of the International Civil Aviation Organization (ICAO), by Aeronautical Radio, Inc. (ARINC), which until recently had served as the registrar of the system since it was first introduced in 1958 for use by civil aviation.
- In January 2006 ASRI replaced ARINC as the registrar of the SELCAL system for the aviation community. ASRI, instead of ARINC, is now responsible for administering SELCAL codes, maintaining the database, and providing periodic reports on the status of the SELCAL system to ICAO.
Source: ASRI Users Guide, §1.1
Principles of operation
- With the selective calling system, the normal voice calling method is replaced with the transmission of coded tones to the aircraft over the voice communications channel. It is critical to remember that both the ground station and the aircraft need to be operating on the same HF or VHF frequency for the system to operate properly.
- When the ground operator desires to call a particular aircraft, the SELCAL code assigned to that aircraft is keyed into the tone generator. The tone pulses that are generated in the ground station tone generator are transmitted to the aircraft via the ground-to-air transmitter. A single selective call consists of a combination of four pre-selected audio tones whose transmission requires approximately two seconds. The tones are received by the aircraft receiver and presented to the decoder connected to the audio output of the receiver. The airborne receiver and decoder equipment is capable of receiving and interpreting the correct code and rejecting all other codes in the presence of random noise and interference. Receipt of the assigned tone code (SELCAL code) activates a cockpit call system for display on the signal indicator.
- The type of signal indicator can be chosen to suit operational requirements of the user and may consist of a lamp, a bell, a chime or any combination of such indicating devices. On aircraft equipped with SELCAL the flight crew has the capability to also maintain a conventional listening watch, using headsets or cockpit speaker.
Source: ASRI Users Guide, §2.3
- The SELCAL system was initially based on 12-tone codes which provided for a total of 2,970 individual code assignments. In the early 1980s, the registrar encountered a growth problem when the requests for code assignments from the aircraft operators exceeded the number of unassigned codes available. To address the problem, the inventory of available codes was increased by adding four more tones. This provided for another 7,950 code assignments, a solution that became applicable when provisions for the 16-tone code system were introduced in ICAO Annex 10 in 1985.
- Although the combined total number of SELCAL code assignments, based on the earlier 12-tone system and the subsequent introduction of the 16-tone system, which provided a total of 10,920 assignments, the inventory of unused codes eventually became exhausted. To continue to meet the requirements for new assignments, it was necessary for the registrar to assign duplicate codes. Flight crews, for example, can anticipate that they will receive a greater number of SELCAL activations for messages intended for other aircraft, a situation that raises the possibility of misdirected messages. The problems associated with this practice can be expected to increase over time.
- The registrar makes every effort to reduce the possibility of conflict caused by duplicate code assignments. The first and principal means is to attempt to maintain an adequate geographical separation between the aircraft assigned duplicate codes. However, this is not always possible, and aircraft with worldwide code assignments will occasionally operate simultaneously in the same airspace.
Source: ASRI Users Guide, §2.3
I once shared a hangar with a dissimilar aircraft that had the exact same SELCAL code. We didn't know that until during one crossing our neighbors answered our SELCAL.
- The four tones required for a single SELCAL code assignment are selected from a total of sixteen tones available in the ground station SELCAL installation. For convenient reference, the sixteen tones are designated by letters of the alphabet (“A” through “S”; with “I”, “N”, and “O” omitted). Tones “P”, “Q”, “R”, and “S” were the four tones that were added to the exiting 12 tones in 1985.
- Tone codes are made up of the various combinations of the tones and are designated by letters. An example of a SELCAL code would be AC- BD. Each transmitted code should be made up of two consecutive tone pulses, with each pulse containing two simultaneously transmitted tones. The pulses should be of 1.0 plus or minus 0.25 seconds duration, separated by an interval of 0.2 plus or minus 0.1 second.
Source: ASRI Users Guide, §2.3
Though the users guide doesn't say this, you can figure out from the total combinations number given that there are a few rules in the way the tones are transmitted:
- The possible letters are A, B, C, D, E, F, G, H, J, K, L, M, P, Q, R, and S.
- Mathematically speaking, 16 tones arranged in any possible combination 4 times results in 164 = 65,636 possible combinations.
- A given letter can only be used once, reducing the possibilities to 16 x 15 x 14 x 13 = 43,680 combinations.
- The first two tones are combined as are the second. Within each two-tone combinations the letters must be given in alphabetical order. So AB-CD is okay and so is CD-AB. But you cannot have CD-BA. So let's say the first letter is A. The second letter can be B through S giving you 15 possible combinations for the first tone. Now let's say the second letter is a B. The third letter cannot be an A or a B. Let's say it is a C. The fourth letter can only be a D through S for a total of 13 possible combinations. It gets complicated, eh? With these rules, we end up with 10,920 possible assignments.
I'm no mathematician but know when to ask for help. Nathan Kudlaty provides the answer below.
- With the selective calling system known as SELCAL, the voice calling is replaced by the trans- mission of coded tones to the aircraft over the radiotelephony channels. A single selective call consists of a combination of four pre-selected audio tones whose transmission requires approximately 2 seconds. The tones are generated in the aeronautical station coder and are received by a decoder connected to the audio output of the airborne receiver. Receipt of the assigned tone code (SELCAL code) activates a cockpit call system in the form of light and/or chime signals.
- The aircraft should:
- include the SELCAL code in the flight plan submitted to the appropriate air traffic services unit; and
- ensure that the HF aeronautical station has the correct SELCAL code information by establishing communications temporarily with the HF aeronautical station while still within VHF coverage.
- Pre-flight check
- The aircraft station should contact the appropriate aeronautical station and request a pre-flight SELCAL check and, if necessary, give its SELCAL code.
- When primary and secondary frequencies are assigned, a SELCAL check should normally be made first on the secondary frequency and then on the primary frequency. The aircraft station would then be ready for continued communication on the primary frequency.
Note.— Due to the limited number of SELCAL codes, similar code assignments to multiple aircraft may be expected. Therefore, the use of correct radiotelephony (RTF) procedures contained in this chapter is emphasized when establishing communications via SELCAL.
Source: ICAO Annex 10 Vol II, §5.2.4
I don't think anyone does this and doing so would add to the already congested frequencies. In practice, we do a SELCAL check on the primary frequency only. If that were to fail, we would maintain a listening watch rather than reattempt on the secondary frequency, but this is just our preference.
- Should the pre-flight check reveal that either the ground or airborne SELCAL installation is inoperative, the aircraft should maintain a continuous listening watch on its subsequent flight until SELCAL again becomes available.
- When an aeronautical station initiates a call by SELCAL, the aircraft replies with its radio call sign, followed by the phrase “GO AHEAD”.
- Once SELCAL watch has been established by a particular aircraft station, aeronautical stations should employ SELCAL whenever they require to call aircraft.
- In the event the SELCAL signal remains unanswered after two calls on the primary frequency and two calls on the secondary frequency, the aeronautical station should revert to voice calling.
- In principle, the SELCAL code in the aircraft should be associated with the radiotelephony call sign, i.e. where the flight number (service number) is employed in the radio call sign, the SELCAL code in the aircraft should be listed against the flight number. In all other cases, the SELCAL code in the aircraft should be listed against the aircraft registration.
Source: ICAO Annex 10 Vol II, §5.2.4
In practice the code is normally assigned to an aircraft, not its call sign.
All questions regarding SELCAL codes should be directed to:
2551 Riva Road MS 3-103 Annapolis, MD 21401-7435
Source: ASRI Users Guide, page 4-3
The application process is easy, it costs $100 to apply or transfer but there is no cost to maintain your SELCAL code.
A number of years ago I tried to solve this math problem: how can you determine the number of possible SELCAL codes give the rules of letter selection? I was unable to come up with an elegant solution and asked for help on this page. In September of 2016, Nathan Kudlaty came to the rescue with an idea to solve this puzzle by using a summation function for the first pair of letters. Thanks Nathan!
Before we continue, I did compile all 10,920 combinations on a spreadsheet, linked here: selcal_permutations.xlsx. You can follow along if you like.
The first thing to do is to come up with the total number of possible combinations for the first pair of letters. Consider all the pairings that start with the letter A: AB, AC, AD, AE, AF, AG, AH, AJ, AK, AL, AM, AP, AQ, AR, AS. That's 15. The first letter cannot be S because there would be nothing to follow. So the last starting letter is R and it only offers one combination: RS. So You have 15 possible combinations that start with the letters from A to R, and each of these have possible second letter combinations from 15 to 1. (Examine the linked spreadsheet, Tab L1L2 and see that there are 120 possible combinations of the first and second letters.) Mathematically, that can be written as the sum of possible combinations X added together from X = 1 to X = 15:
For the second pair we assume you already have a valid first pair. At first glance it would seem you would have 120 possible outcomes for the second pair. It seemed too complicated. But several readers, including Dave Peddicord, pointed out that the second pair would be like the first, but with two fewer options. Sure enough, as it turns out, every possible outcome of the second pair yields from 0 t 13 possible combinations. This is counterintuitive to be sure. But consider two extremes. Let's say the first two letters are AB, giving you 13 combinations for the second pair: CD, CE, CF, CG, CH, CJ, CK, CL, CM, CP, CQ, CR, and CS. The number of combinations decreases by one until you get to the last option when the third letter is R and you end up with only one possible combination: RS. (See Tab L1=A of the linked spreadsheet.)
All of that is expected. But you would think that by the time your first two letters are RS you will have ended up with no options at all. But that isn't true. Since the alphabetical rule only applies two individual pairs, the behavior is quite unexpected. If the first two letters are RS, the combinations of third and fourth letters still decrease from 13 (AB, AC, AD, AE, AF, AG, AH, AJ, AK, AL, AM, AN, AP, AQ, AR, and AS) to just one (PQ). So I would hypothesize that EVERY combination of third and fourth letters can be written as the sum of possible combinations Y added together from Y = 1 to Y = 13:
All of this is verified by the linked spreadsheet, which shows there are indeed 91 possible combinations of the second pair for all 120 combinations of the second pair. Statistically, we can find the total number of combinations by multiplying the two:
So that's the math. For me the first epiphany was Nathan Kudlaty's approach to using the "Sigma" summation for the first 120 combinations. The second epiphany was provided by Dave Peddicord and the idea that there would also be a standard behavior of the second pair. Simply doing the "brute force" examination for the second pair verified the 13 to 1 combinations applied to all 120 first pair combinations.
Aviation Spectrum Resources, Inc. Selective Calling (SELCAL) Users Guide, 61742 Rev C, December 30, 2013, available at https://www.asri.aero
ICAO Annex 10 - Vol II - Communication Procedures, International Standards and Recommended Practices, Annex 10 to the Convention on International Civil Aviation, Vol II, October 2001