Aircraft communications and navigation systems
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1 3.3 Aircraft equipment The aircraft is fitted with two antennas located on the nose and aft centrelines. An MLS receiver (often incorporated into a multi-mode receiver with ILS, marker beacon and VOR capability) is tuned into one of 200 channels and calculates azimuth and elevation guidance as described. The receiver operates in the frequency range 503 1 MHz to 509 1 MHz with 300 kHz spacing. Referring to Figure 1 3 .4 and Table 1 3 . 1 , the pulse timing is used to determine the three aircraft positions. An integral part of the MLS is a distance measuring equipment (DME) system to provide range; this can either be a conventional DME system as described in Chapter 1 1 , or a dedicated system operating in the 962 MHz to 1 1 05 MHz frequency range. DME frequencies are automatically tuned with the azimuth and elevation beams to provide range information. Typical MLS airborne equipment is illustrated by the CMA-2000 system, Figure 1 3 .5 (data and image courtesy of CMC Electronics). This system is installed on a number of military aircraft in the USA including the C-130 and Air Force One. Control of the MLS is via a control display unit (CDU), where the crew selects the desired MLS channel, together with azimuth and glide path approach angles. The system meets the requirements of ARINC 727 and provides three dimensional positional data within a large airspace volume. Azimuth and glide path guidance outputs are either displayed on a conventional course deviation indicator (CDI) or incorporated into multipurpose electronic displays. A summary of the CMA-2000 microwave landing system leading particulars is given in Table 1 3 .2.
Table 1 3.2 CMA-2000 m icrowave landing system leading particulars Feature
Specification
Range/channels
200 channels in C-band (5031 to 5090.7 MHz)
Control unit weight
6.7 kg
CDU weight
1 .4 kg
Power supply
1 1 5 V AC, 400 Hz, 60 VA nominal
Control unit microprocessor
8086, 1 28 kbyte EPROM, 64 kbyte RAM
Range
Up to 40 nm
Azimuth range
0' through 360'
Elevation range
2' to 29.5' (in increments of 0.1 ) '
Resolution
0.005'
Sensitivity
- 1 06 dBm
Dynamic range
95 dB
Digital interfaces
ARINC 429 and MIL-STD1 5538
Analogue interfaces
Synchro, DC voltages
Navigation aids
DME tuning, frequency tuning
Test your understanding 1 3.4 How many MLS channels are available?
Test your understanding 1 3.5 What frequency range does M LS use?
Table 1 3.1 Azimuth angle relationship Aircraft position
TO scan
FRO scan
Difference
A
6.6 ms
1 1 .5 ms
4.9 ms
B
5.7 ms
1 2.2 ms
6.3 ms
c
3.6 ms
1 4.2 ms
1 0.6 ms
Angle (+ is left)
