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VHF FM broadcast interference
1.3.1 With reference to the Note of 2.3.3.2 of Part 11,
the immunity performance defined there must be measured
against an agreed measure of derogation of the receiving
system's normal performance, and in the presence of, and
under standard conditions for the input wanted signal. This is
necessary to ensure that the checking of receiving station
equipment on bench test can be performed to a repeatable set
of conditions, and results, and to facilitate their subsequent
approval. An adequate measure of immunity performance may
be obtained by the use of wanted signal of minus 87 dBm into
the receiving equipment and the signal modulated with a
1 kHz tone at 30 per cent modulation depth. The signal-tonoise
ratio should not fall below 6 dB when the interfering
signals specified at Part K2.3.3.1 and 2.3.3.2 are applied. The
broadcast signals should be selected from frequencies in the
range between 87.5 and 107.9 MHz and should be modulated
with a representative broadcast type signal.
Note I . - The signal level of minus 87 dBm assumes a
combined antenna and feeder gain of 0 dB.
Note 2.- The reduction in the signal-to-noise ratio quoted
above is for the purpose of standardization when checking that
receiving station equipment on bench measurements meet the
required immunity. ln the planning of frequencies and in the
assessment of prorection from FM broadcast inte@erence, a
value not less than this, and in many cases higher; depending
on the operational circumstances in individual cases, should
be chosen as fhe basis of the inte$erence assessment.
ANNEX I0 - VOLUME I11 -6/11191
No. 72
Annex 10 - Aeronautical TeLcommunicalions Volume III
2. SELCAL
SYSTEM
2.1 This material is intended to provide information and
guidance relating to the operation of the SELCAL-system. It
is associated with the Recommended Practices contained .in
Part II? Chapter 3.
1) Function. The purpose of the SJXCAL system is to
permit the selective calling of individual aircraft over
radiotelephone channels linking the ground station with
the aircraft, and is intended to operate on en-route
frequencies with existing HF and VHF ground-to-air
communications transmitters and receivers with a
minimum of electrical and mechanical modification.
The normal functioning of the ground-to-air communications
link should be unaffected, except at such time
as the selective calling function is being formed.
Principles of operation. Selective calling is accomplished
by the coder of the ground transmitter sending
a single group of coded tone pulses to the aircraft
receiver and decoder. The airborne receiver and
decoder equipment is capable of receiving and interpreting,
by means of an indicator, the comct code and
rejecting all other codes in the presence of random
noise and interference. The ground portion of the
coding device (ground selective calling unit) supplies
coded information to the ground-to-air transmitter. The
airborne selective calling unit is the special airborne
equipment which operates with existing communications
receivers on the aircraft to permit decoding of the
ground-to-air signals 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.
- END -
No. 72
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