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mge between 87.5 and 107.9 MHz and should be modulated
with a repmentative broadcast type signal.
Note 1.- The signal level of minus 87 d3m assumes o
combined anew and fieder gain qf 0 dB.
Note 2.- The mductim in the signal-to-noise ratio quoted
above is for the purpose of s&mhrdimhn when chechng that
receiving station e q u i m t m bench measurements met the
required immunity. In the planning ofjkquencies uorod in the
assessment qf protection fTom FM b m h s t inte@rence, a
vaiw not less than this, and in mny cases higher, &pending
on tk upemtiowl c i ~ u ~ u mien sin dividual caws, should
be chosen as th bosir ofdk intetfemnce assessma,
-7flm No. 71
Annex 10 - Aeromwliwl T e l e c o ~ u n ~ o n s
2. SELCAL
SYSSEM
2.1 This material is intended to w i d e information and
guidance relating to the o d o n of the SELCAL system. It
is associated with the Recommended Practices contained in
Part 11, Chapter 3.
1) Function. The purpose of the SELCAL system is to
permit the selective calling of individual sircraft over
radiotelephone channels linking the ground station with
rhe aircraft, and is intended to qperate on en-mute
frequencies with existing HF and VHF ground-to-air
communications transmitters and receivers with a
minimum of electrical and mechdcal modification.
The normal functioning of the ground-[+air communications
link should be unaffected, except at such time
as the selective calling function is being formed.
2) Principles qf qemtim Selective calling is accomplished
by the coder of the ground transmitkr sending
a single group of caled tone pulses to the aimaA
receiver and decoder. The airborne receiver and
decoder equipment is capable of receiving and interpreting,
by means of an indicator, the correct code and
rejecting all &r codes in the presence of random
noise and i&erence. The ground portion of the
coding device (ground selective calling unit) supplies
coded i n f o d o n to the $round-wair transmitter. The
ahborne elective calling unit is the special airborne
equipment which o mwith existing communications
~ceiverosn the aircraft to pennit decoding of the
ground-to-air signals for display on the signal
indicator. 'Ihe type of siw indicator can be chosen
to suit operational req&ments of the user and may
consist of a I q , a bell. a chime or any combination
of such indicating &vices.
- END -
7A1196
No. 71
Q ICAO 1996
N96, E I p 1 m
Order No. A10-3A
Printed in ICAO

　
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