Operating instructions

AVIONICS SYSTEM
B-6
SELCAL
In the SELCAL mode the Test Set simulates the selective calling tones signals used on HF/VHF
channels to alert a selected aircraft.
The SELCAL system is typically used by commercial aircraft operators for their company
communications systems. The codes are regulated and issued by the government aviation
authority. The tones are designated in two pairs of two, in the form ‘BK-FB’ or ‘CP-AD’ or
‘BH-FB’. The first pair of tones from the call signal are transmitted simultaneously, i.e. BK, CP
and BH in the above example, followed after a specified time by the second pair, FB, AD and FB,
also sent simultaneously.
The receiver has a ‘listen’ condition which directs demodulated information to a tones decoder.
The reception of the designated tones triggers a warning device which alerts the aircrew that they
are required to communicate with the ground. Amplitude modulation is used.
The 16 available tones are designated by the letters A to S, corresponding to a table which has A
being 312.6 Hz, J, 716.1 Hz and S, 1.4971 kHz. The letters I, N and O are not used.
1 Sec. ± 0.25 Sec. 1 Sec. ± 0.25 Sec.
Tone K
749.3 Hz
Tone P
1083.9 Hz
Tone B
346.7 Hz
Tone C
384.6 Hz
Tone B
346.7 Hz
Tone D
426.6 Hz
Tone F
524.8 Hz
Tone A
312.6 Hz
0.2 Sec.
± 0.1 Sec.
0.2 Sec.
± 0.1 Sec.
‘BK-FB’ ‘CP-AD’
1 Sec. ± 0.25 Sec. 1 Sec. ± 0.25 Sec.
1 Sec. ± 0.25 Sec.
Tone H
645.7 Hz
Tone B
346.7 Hz
Tone B
346.7 Hz
Tone F
524.8 Hz
0.2 Sec.
± 0.1 Sec.
‘BH-FB’
1 Sec. ± 0.25 Sec.
C1413
Fig. B-4 Diagram of SELCAL tones timing
The simulated SELCAL signal
This is an RF carrier amplitude-modulated with dual tones as described above. The Service
Monitor
SELCAL display screen and menu gives a digital readout of the parameters relating to the
simulated signal, with the selected call sign centralized on the display in double size letters. The
soft key menu allows call signs and other parameters to be adjusted either directly or by using the
data input keys.