Thales 421, Эксплуатация и обслуживание, Страница: 143 / 256

LLZ 421
Operation and Maintenance Maintenance
ILS 420
6−3
Ed. 01.10 SOAC
6.2.2 Check of Transmitter Parameters and Monitoring
There are various methods to validate periodically the transmitter− and monitor functions depending
on the procedures laid down by the national customer procedures. For example course alignment
may be checked by center line runs with a test vehicle weekly, but in case a farfield monitor is available
and by integrity and stability history, the maintenance intervals may be increased. The check of moni-
tor parameter alarms is proposed to be done with a change of corresponding transmitter parameter
(power level, DDM, SDM etc).
In addition, there are some special checks to be performed which are described separately. Item 16
’Polarization’ is a parameter that is normally validated during commissioning with flight check. Ac-
cording ICAO 8071 Vol. II it is recommended to perform it yearly as ground inspection. This parameter
can be measured on the course line at a suitable location for Cat. I. For Cat. II and III the preferred
location is on the runway centre line in the region of the threshold. One method of measurement in-
volves the use of two dipoles, one mounted horizontally and the other vertically. Provision is made
to combine equal proportions of the two signals in variable phase relationship and measure the DDM
with a suitable receiver. Half the maximum error introduced by adjusting the phasing can be taken
as the DDM error introduced by a roll attitude of 20 degrees.
For item 18, it is recommended to use a spectrum analyzer connected to a 50 dB directional coupler
which is inserted in the feeding cable at the output on top of the cabinet.
NOTE: The procedures to change transmitter parameters and check of monitoring alarms can
be performed locally via a connected Laptop/PC or remotely via the RMMC system.
6.2.2.1 Shut Down Timing Test (STT)
The purpose of the Shutdown Timing Test is to measure the time it takes to accomplish a complete
shutdown when the radiating signal source(s) is (are) invalid. The STT measures the time to complete
shutdown both on dual−equipment (DE) systems and on single equipment (SE) systems. The test
is performed with the PC User Program:
a) Use menu ’Commands’ − ’More commands’ − ’Transmitters’ − ’TX Miscellaneous’. Select ’Initi-
ate transfer/shutdown test’ to activate the STT. The result is shown in the window MON1(2) ’Trans-
fer Timing Data’: the item ’Test Procedure Result’ must show ’OK’.
6.2.2.2 Check of Synthesizer Difference Frequency
Test equipment: Test cable (Hirschmann RS 343−3239), Square Wave Generator; PC User Progr.
a) Remove jumper J8 at 125 Hz. Connect test cable to the testpoint and Generator (see Fig. 6−1).
Set output frequency of square wave generator to 125 Hz TTL level.
b) Read PC User Progr. indication ’Difference Frequency’. Vary 125 Hz to get warning and alarm indi-
cations. Note indicated frequency at transitions for Monitor 1 and 2. After test insert J8 as normal.
J5 J8
J8
8kHz
125Hz
front
SYN
Test point Test cable
Square Wave Generator
(MAL N 4−0,64)
1 2
3
4
Fig. 6−1 Test setup for for SYN difference frequency test