What are Selcals 

SELCAL and SELCAL32 description

It is an automatic recognition system of planes, for those crossing oceans (according to the specification: Aircraft Selective Call - ICAO Annex10). The SELCAL code is a four characters code identifying the plane. It is used in HF.

The new SELCAL mode, called "SELCAL32", is an extension of the standard SELCAL mode, backwards compatible with the former SELCAL. It increases the number of possible codes by about 200,000, to add to the 35,000 codes of the standard SELCAL.
The SELCAL32 is expected to be in service the 1st of April 2023.

About the 4 characters SELCAL(32) codes:
* a character must not be repeated ("ABAC" is not possible)
* the two characters of each pair must be ordered with the lowest alphanumeric character first, the SELCAL characters being ordered from the lowest to the highest in the following way "ABCDEFGHJKLMPQRSTUVWXYZ123456789", so "AB", "D3" and "58" are valid but "BA", "3D" and "85" are invalid.

Description :

Speed : 1 baud, the carriers duration being equal to 1 second,

Modulation : MFSK. Transmission of 2 different carriers during 1 second (+/-0.25 s) followed by a silence of 0.2 second (+/-0.1 s) followed by the transmission of 2 different carriers during 1 second (+/-0.25 s). The 4 carriers are different.

Receive mode: USB

Character set for SELCAL : 16 letters: A, B, C, D, E, F, G, H, J, K, L, M, P, Q, R, S. Each tone corresponds to a character (for example 312.6 Hz for "A" and 1479.1 Hz for "S")

Character set for SELCAL32 : 32 letters and figures: A, B, C, D, E, F, G, H, J, K, L, M, P, Q, R, S, T, U, V, W, X, Y, Z, 1, 2, 3, 4, 5, 6, 7, 8, 9. Each tone corresponds to a character (for example 312.6 Hz for "A" and 1557.8 Hz for "9")

Bandwidth : from 312.6 to 1479.1 Hz in SELCAL and from 312.6 to 1557.8 Hz in SELCAL32

Demodulation : non coherent. The tuning accuracy must be equal or better to 10 Hz in SELCAL but equal or better to 5 Hz in SELCAL32.

Lowest S/N : about - 6 dB

Selcal, or Selective Calling as it is more correctly  known, is an automatic recognition system that is operated by a two tone signal. The equipment is connected to the HF radios on aircraft and monitors for a call even when the squelch is turned up, and the pilots can hear nothing. This enables the pilots to have some aural peace when crossing the Atlantic or other oceans as HF radio can be very noisy. Selcals are made up of a four letter code and when heard  have a distinctive bing-bong sound.  As a flight enters the Oceanic FIR, a Selcal check is made the signal activates the on board Selcal receiver which alerts the pilots with a flashing warning light and an audible alarm.

Selcals are issued to airlines by the ASRI (Aviation Spectrum Resources Inc) in USA, and with a total of only 10920 available codes, duplications are possible. This problem is overcome by allocating duplicate codes to aircraft operating in different parts of the world, so in theory they should never be working on the same frequency. If however, duplicate Selcals appear on the same frequency the problem is generally resolved by moving one of the flights to another frequency.

The Selcal is made up of two pair of tones, the first pair being transmitted for approximately 1 second, with the second pair transmitted for the same duration following a 0.2 second pause. The individual tone frequencies are designated by letters A - S excluding the letters "I" and "O". A typical Selcal code is AB-CD, which indicates that the frequencies designated by letters "A" and "B" would sent followed by the frequencies designated by letters "C" and "D". Duplicate letters are not permitted in either pair, since simultaneous transmission of two tones of the same frequency would not be distinguishable by the aircrafts Selcal decoder. Also, the same tone is nor permitted to be used in both the first and second pair.

Most aircraft crossing oceans are fitted with Selcal equipment and normally the code allocated stays with that aircraft unless it sold and changes owner. 

Frequency Table

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