9.1. Selective calling (selcal) system. (ata 23)

9.2. Arinc communication and reporting.

Цель лекции: Изучение аппаратуры.

Вопросы лекции:

9.1.1. Назначение системы селективного вызова;

9.1.2. Selective calling (SELCAL) system;

9.1.3. SELCAL В737 и А320;

9.2. Назначение системы адресной связи;

9.2.1. Аircraft communications addressing and reporting system (ACARS);

9.2.2. ACARS В737 и А320.

9.1.1. Назначение.

Система селективного вызова позволяет осуществлять вызов конкретного самолета или вертолета с наземной станции по радиоканалу. Наземную часть системы составляют кодер (блок, осуществляющий селективную кодировку) и ВЧ- или СВЧ-передатчик. На борту устанавливается приемник, декодер и панель установки кода ЛА.

9.1.2. Selective calling (selcal) system.

The word Selcal is derived from the term Selective Calling.

The system is designed to relieve the flight crew from continuously monitoring the aircraft communication receivers and sorting out the aircraft's particular call-sign from the many which may be radiated from a number of ground stations in the course of a flight.

The Selcal decoder is in effect an automatic monitor which listens for a particular combination of tones which are assigned to an individual aircraft. Whenever a properly coded transmission is received from a ground station, the signal is decoded by the unit and the pilot is alerted by means of flashing lights and a chime that a radio transmission is being directed to the aircraft. The pilot can then listen to his receiver and hear the message.

Ground stations equipped with tone transmitting equipment call individual aircraft by transmitting two pairs of tones which will key only an airborne decoder which is set to respond to a particular combination of tones.

Each aircraft is allocated a Selcal code by "ARINC", which usually remains with it. The code consists of a four letter code, with no letter being repeated in any one code. Each letter is represented by an audio tone within the range 300 Hz and 1000 Hz. Letters from A through to M are used (excluding I) giving 12 tones in all.

LETTER	TONE FREQUENCY
A	312.6
B	346.7
C	384.6
D	426.6
E	473.2
p	524.8
G	582.1
H	645.7
J	716.1
K	794.3
L	881.0
M	977.2

TABLE - LETTER CODE AUDIO TONE FREQUENCIES

The codes are selected on the front of the Selcal decoder by rotating knobs, which in turn select four tuned reeds from those contained in the unit.

When a ground operator wishes to contact a particular aircraft by means of the Selcal unit, he/she selects the four-tone code which has been assigned to the aircraft. The tone code is transmitted by a radio frequency wave and the signal can be picked up by all the receivers tuned to the frequency used by the transmitter. The only receiver which can respond to the signal and produce the alert signal for the pilot is the receiver and decoder which has been set for the particular combination of tone frequencies.

Fig.9.1. SELCAL pulsis

The Selcal tones are detected by the HF and VHF receivers and the Selcal unit, which is connected to the audio output circuit of the receiver, it amplifies them and applies them to the four selected tuned reeds. The output of the unit is then used to alert the pilot.

The Selcal system consists of a decoder, control panel and communication receivers.

Fig. 9.2. SIMPLIFIED SELCAL SYSTEM

Selcal Decoder

Four rotary switches mounted on the front of the decoder permit selection of any 4 letter combination from the 12 letters available at each switch.

Each decoder requires four resonant reeds to establish the code for the unit. The reeds respond to audio frequencies and cause the generation of a voltage which is processed by the unit.

The resonant reed is essentially a frequency sensitive relay. When a resonant frequency is applied to the coil, the reed vibrates at that frequency. The amplitude of the vibration builds up rapidly and reaches a condition where a contact on the reed intermittently touches a fixed contact.

Fig. 9.3. SELCAL DECODER CIRCUIT

Assume that AMBL is selected as the four-tone code for the decoder. The proper code signal coming into the decoder consists of two sequential pulses, each composed of two tones (A and M, and B and L). The first pulse, A and M, causes the A and M reeds to vibrate in sympathy with the signals appearing on their respective coils. With these reeds vibrating, each at its own frequency, there are a number of times in each second when they touch their respective contacts simultaneously. At these times there is a completed circuit for the 28v dc to charge the capacitor Cj_.

An RC network across each pair of contacts smoothes the momentary reed contact closures so that current is relatively continuous, rather than in sharp pulses which flow only when both reeds are in contact at the same instant.

A resistor delays the charge into capacitor Cx so that random bursts of noise or unstable heterodyne tones will not instantaneously charge the capacitor.

Before the charge can drain from the capacitor, the second pulse tones enter the coil circuits. The B and L reeds now vibrate in sympathy with the signal on their respective coils and the discharge path to ground is completed a number of times each second, with an RC network smoothing the intermittent closures.

Relay K_x closes when the voltage it receives rises to about 10 volts. When it closes, it is electrically latched. It remains actuated until released manually by a reset circuit.

When relay K^ operates, contacts 1 and 2 provide current to actuate the relay coil and keep it latched. Contacts 5 and 6 provide a steady 28v dc signal to the control panel or chime speaker. Contacts 3 and 4 connect 28v dc to the relay oscillator. The relay oscillator provides one second ON/OFF pulses to the coil of relay K3. Relay K3 contacts provide the pulsed 28v dc signal to the control panel or chime speaker.

The decoder is reset for the next incoming signal by depressing the SELCAL pushbutton switch on the control panel. This connects a ground to the coil of relay K₁. Relay Kj is now shorted out and it shuts off the steady and pulsed 28v dc outputs to the control panel.

Selcal Control Panel

A typical Selcal control panel includes two rotary switches (SELCAL - 1 and SELCAL - 2) which are used to select an audio signal.

The selected audio is connected to the corresponding decoder input channel.

Fig. 9.4. TYPICAL SELCAL CONTROL PANEL

Two PRESS TO RESET Selcal indicators turn ON when supplied with a 28v dc signal from the decoder. When the indicator is depressed a reset signal is supplied to the decoder.