12.10. Маркерный канал.
Маркерные радиомаяки работают на частоте 75 МГц и располагаются вдоль продолжения оси ВПП со стороны захода самолета на посадку. По международным стандартам устанавливаются два или три радиомаяка. При установке трех радиомаяков несущая частота ближнего к ВПП маяка модулируется частотой 3000 Гц, среднего - частотой 1300 Гц и дальнего - частотой 400 Гц. Глубина модуляции составляет (95±4) %.
Для опознавания маркерных радиомаяков, помимо разных частот модуляции, применяется различная манипуляция модулирующих частот:
- для ближнего маяка - шесть точек в секунду непрерывно;
- для среднего маяка - непрерывная последовательность чередующихся точек и тире, причем тире чередуются со скоростью два тире в секунду, точки - со скоростью шесть точек в секунду;
- для дальнего маяка - два тире в секунду непрерывно.
В системе СП-50 используются два маркерных радиомаяка - средний и дальний. Модулирующие частоты обоих маркерных маяков 3000 Гц.
Манипуляция несущей частоты производится таким же образом, как и в маяках ILS.
Рис.12.45. ДН антенны РММ.
Рис. 12.46. Функциональная схема маркерного приемника.
Рис. 12.47. Расположение МРМ.
12.11. Marker Beacon System.
The Marker Beacon System indicates to the flight crew that the aircraft is passing directly over a particular geographical location (such as a point along an air route) or points along an instrument landing path. The system provides both audio and visual indications of passage over any marker beacon transmitter.
All marker beacons radiate a 75 MHz RF signal modulated with either a 400, 1300 or 3000 Hz audio. A narrow pattern is radiated upwards from the ground, with little horizontal strength, so that interference between marker beacons is negligible.
There are two types of marker beacon transmitters, ILS Markers and Airways Markers.
ILS Markers
The ILS marker beacons are provided along the glide path as distance spot- checks. Each beacon radiates a fan-shaped vertical beam which is approximately ± 40° wide along the glide path by ± 85° wide perpendicular to the path.
Figure 12.48 shows the location of the ILS marker beacons from the end of the runway.
FAN MARKERS
Fig. 12.48 ILS MARKER BEACONS
The output power of each ILS marker beacon is 3 W and the carrier frequency is 75 MHz. Modulation of the carrier is either a 400, 1300 or 3000 Hz audio tone dependent on the marker and the depth of modulation is 95%. Each beacon radiates a horizontally polarised RF signal.
The outer marker is located approximately 4 miles (6.4km) from the runway threshold. The RF signal is modulated by a 400 Hz tone keyed two dashes per second. As the aircraft passes over this marker, a blue light illuminates and the 400 Hz tone keyed as continuous dashes is heard in the headphones.
The middle marker is located approximately 3500 feet from the runway threshold. The RF signal is modulated by a 1300 Hz tone keyed alternate dots and dashes. As the aircraft passes over this marker, an amber light illuminates and the 1300 Hz tone keyed as alternate dots and dashes is heard in the headphones.
The inner marker, when installed, is located 250 feet from the runway threshold. The RF signal is modulated by a 3000 Hz tone keyed six dots per second. As the aircraft passes over this marker, a white light illuminates and the 3000 Hz tone keyed as continuous dots is heard in the headphones.
The back course marker, if installed, is located at the opposite end of the runway from the inner marker also has a 3000 Hz tone and turns on the white light but the tone is keyed as continuous paired dots.
By monitoring the lights and headphones the flight crew are able to mark progress on final approach to the runway.
Fig. 12.49. AIRWAYS MARKERS
Airways Markers
When the maker beacons are used as airway identifiers, the 3000 Hz tone is used and the white light illuminates when the aircraft passes over the beacon. The tone is keyed with a Morse code identifier. The specific code is obtained from airways charts.
The transmitter power is typically 100 W to ensure lamp operation in an aircraft at any operating altitude.
Marker Beacon Receiver
The marker beacon receiver processes signals from a ground station and provides visual and aural signals to indicate passage over the ground beacon.
A typical receiver is shown in figure 12.50.
Fig. 12.50. MARKER BEACON RECEIVER
The marker beacon receiver is fixed tuned to receive 75 MHz amplitude modulated signals. The 75 MHz incoming signal from the aerial is applied through a selective LC filter to a crystal controlled converter where it is mixed with a local oscillator signal to generate an intermediate frequency (IF) signal.
This IF signal is filtered and amplified before being applied to the detector stage where the audio component (400, 1300 or 3000 Hz) is detected. The detected signal is amplified in the audio amplifier, the output of which divides and drives the audio power amplifier and the audio lights drive.
The output signal from the audio power amplifier is fed through an output transformer to the headphones for aural monitoring of the marker beacon system. Audio output from the audio light driver is fed through a three-stage frequency selective filter which separates the audio into its 400, 1300 or 3000 Hz components which turn-on the corresponding marker beacon lights.
Marker Beacon Sensitivity Switch
The sensitivity and automatic gain control of a marker receiver are important in determining the period of time during which a visual indication is obtained when flying over a marker beacon. A switch is provided to allow the pilot to adjust the sensitivity of the marker beacon receiver.
MARKER HIGH
LOW
Fig. 12.51. MARKER BEACON SENSITIVITY SWITCH
The HIGH sensitivity setting is used at high altitudes and the LOW sensitivity setting is used at low altitudes. The changeover is often made at 10,000 feet.
On LOW sensitivity, the aerial input required to light the marker lamps is typically 10 dB greater than on HIGH sensitivity.
Marker Beacon Lights
The marker beacon lights receive dc power from the marker beacon receiver.
The 400 Hz outer marker signal turns on the Blue lamp. The 1300 Hz middle marker signal turns on the Amber lamp. The 3000 Hz inner/airways marker signal turns on the White lamp. Each light assembly indicator cap is provided with an iris for brightness control.
Fig. 42 MARKER BEACON LIGHTS
Marker Beacon Aerial
The marker beacon aerial is designed to receive a horizontally polarised 75 MHz signal from the marker ground transmitter. The aerial is a blade type unit usually mounted on the bottom centreline of the aircraft.
Fig. 43 MARKER BEACON AERIAL