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What are Selcals

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
Code Frequency
A 312.6HZ
B 346.7HZ
C 384.6HZ
D 426.6HZ
E 473.2HZ
F 524.8HZ
G 582.1HZ
H 645.7HZ
J 716.1HZ
K 794.3HZ
L 881.0HZ
M 977.2HZ
P 1083.2HZ
Q 1202.3HZ
R 1333.5HZ
S 1479.1HZ