Operating instructions

AVIONICS SYSTEM
B-4
ILS frequency pairing
International agreements specify the carrier frequency pairings that can be used for localizer and
glideslope frequencies on ILS installations. A look-up table held within the Service Monitor
software contains spot frequencies from 108.10 MHz to 111.95 MHz, to provide the localizer
signal. A second table, cross-referenced to the first, contains the corresponding glideslope
frequencies. (See the ILS frequency pairings table at the end of this chapter.) A frequency
setting will only be accepted as valid if it corresponds to a frequency contained in one of the look-
up tables.
When a new localizer carrier frequency is set, the glideslope carrier frequency will re-set to the
corresponding localizer carrier frequency.
Marker beacons
The localizer and glideslope signals do not give any information as to distance from the runway
threshold. This information is provided by the ILS marker beacons.
The purpose of these markers is to inform the pilot/flight deck crew of the progress of the aircraft
along the landing glidepath. Each runway approach path is equipped with three beacons, all
transmitting on 75 MHz with amplitude modulation. The furthest from the touch down point, five
miles from the runway threshold, transmits a tone of 400 Hz and is known as the outer marker, the
next, at 3,500 ft from the threshold, transmits a 1,300 Hz tone and is known as the middle marker.
The third, at the runway threshold, transmits a 3,000 Hz tone and is known as the inner marker.
The airborne receiver demodulates the tones to give an audible and visual warning as the aircraft
passes each marker.
The simulated marker beacon signal
B2143
Fig. B-2 ILS MARKER screen and menu
The simulated Marker Beacon signal is a single RF carrier with accurately defined modulation
frequency and level. The modulation frequency is selectable from any of the three standard
marker frequencies or to any frequency within the range 20 Hz to 20.0 kHz. The parameters may
be altered by the user.
The Service Monitor ILS MARKER display screen and menu gives a digital readout of the
parameters relating to the simulated signal. The soft key menu allows the marker type to be
selected directly or the parameters to be adjusted by using the data input keys or the variable
control.