Operating instructions

AVIONICS SYSTEM
B-2
Avionics system overview
General description
The Avionics system test mode produces test signals which simulate the ground transmissions of
VHF Omnidirectional Range (VOR) air navigation beacons, Instrument Landing System (ILS)
aircraft blind landing equipment and Selective Calling (SELCAL) aircraft communication
systems.
The presentation and control of the generated signals is tailored to the functional testing and
performance evaluation of airborne radio navigation and communication equipment. The
parameter values of the test signals can be varied by the user, thus allowing tests to be made to
determine tolerances and ‘out of specification’ figures.
The portability and versatile power supply requirements of the Service Monitor allow tests to be
carried out to equipment in the laboratory, the workshop or on the flightline.
The modulation applied to ILS and VOR test signals is available at the AF output connector on the
front panel.
The test signals provided within the Avionics System test mode are described starting on page B-
2. Instructions for using the Avionics System test mode are described starting on page B-7.
ILS
In the ILS mode the Test Set simulates the Instrument Landing System transmissions. The
purpose of these signals is to indicate to the pilot/flight deck crew, the position of the aircraft
relative to the landing glidepath, glideslope and runway threshold. The centerline of the runway
glidepath is marked by a pair of signals known as the localizer beams. The correct angle of
descent is marked by a similar pair of signals called the glideslope beams. The distance from the
runway threshold is indicated by three marker beacons.
Localizer signal
The two signals used to provide the localizer beams are transmitted using aerials with sharply
defined forward lobes. These are aligned to partially overlap along the glidepath center line,
giving an overlap area and thus an area of equal signal strength, of approximately 5°. Both signals
are derived from a common RF generator and operate within the frequency band 108 to 112 MHz.
The signal transmitted to the left of the runway, as seen by an approaching aircraft, is amplitude
modulated at 20% with a 90 Hz signal. The signal transmitted to the right of the runway is
similarly modulated but with a 150 Hz signal. From this it will be seen that an aircraft flying
along the center line will receive two signals of equal level. As the RF carriers of the two signals
will be of the same frequency and phase, the receiver will interpret them as a single signal,
modulated at 90 Hz and 150 Hz.
If the aircraft is flying to the left of the center line the signal with the 90 Hz modulation would
predominate and give a greater depth of modulation reading. Similarly, if flying to the right of the
center line the 150 Hz signal would be the stronger.
The receiver used to interrogate the signals demodulates the two tones and uses the difference in
modulation depth between the two to indicate the aircraft’s position relative to the glidepath center
line.
The localizer beam can also be modulated with a Voice/Identity signal which will be carried with
equal strength on each transmission.