As an example, wide alarm on a standard 3° glidepath,
(25% decrease in displacement sensitivity):
(nominal angle) ÷ [(100 - x)/100] = 0.72 ÷ (75/100) = 0.96°
Similarly, narrow alarm:
(nominal angle) ÷ [(100 + x)/100] = 0.72 ÷ (125/100) = 0.58°

As an example, wide alarm on a 5.8° localiser,
(17% decrease in displacement sensitivity):
(nominal angle) ÷ [(100 - x)/100] = 5.8 ÷ (83/100) = 6.99°
Similarly, narrow alarm:
(nominal angle) ÷ [(100 + x)/100] = 5.8 ÷ (117/100) = 4.96°

ILS 01 Appendix Guidance on Sideband Reference Phase

Testing
1  Introduction
1.1  The sideband reference glidepath characteristics are more susceptible to small phase changes in the transmitting aerial system than any other common glidepath.
1.2  A simplified way of seeing the problem is to consider the signal from the lower aerial. This signal alone would give twice full scale fly-down on a pilot’s indicator. It is only the presence of the upper aerial signal, when combined with the lower aerial signal, which produces the fly-up signal below the glidepath. Any error in the relative phase of the signals from the two aerials will result in the signal from the lower aerial having more effect. This results in weaker fly-up signals near the ground and a lower glidepath angle.
NOTE:  It is not the policy of the CAA to provide precise system adjustment procedures but in view of the high cost of flight inspection some guidelines are offered.
2  Guidelines
2.1  The guidelines in this annex are arranged as below:
2.2  Section 3 describes the faults which may exist if the phase alarm test has been attempted and failed.
2.3  Section 4 describes a procedure for adjusting and verifying that the transmitter aerial phasing is correct. If the phasing is verified as described using a repeatable method of de-phasing, the monitor phase alarms can be verified using only ground measurements. If there are any doubts about the accuracy of transmitter aerial phasing, it is advisable to apply the tests detailed in this section.
2.4  Section 5 describes the verification of monitor phase alarms based on results from tests in Section 4.
NOTE:  Sections 3, 4 and 5 are only suggested methods of avoiding excessive flying hours. The system operator/manufacturer may use any method of adjustment to ensure that the system will pass the required tests.
3  Non-compliance with Monitor Test
3.1  If the reduced fly-up and glidepath angle in both tests (phase advance and phase retard) are similar, but outside permitted tolerances:
This indicates that in the normal condition both the transmitter aerial phasing and the monitor signal phasing are correct. The only adjustment required would be to increase the sensitivity of the monitors. The CAA would not require further flight inspection. The changes in monitor sensitivity can be verified using ground test equipment.
3.2  If the fly-up and glidepath angle in both tests are not similar:
This indicates that either the transmitter aerial phasing or the monitor phasing (or both) are wrong. If, in either test, the fly-up was higher than in the normal condition, an error in the transmitter aerial phasing exists.
4  Verification of Transmitter Aerial Phasing
4.1  The transmitter aerial phasing should be adjusted to give the correct phase of signals in the far-field. This is the condition which gives maximum fly-up signals at low angles
- the usual measurement point is 0.32.
4.2  The correct phasing can be verified by altering the transmitter aerial phasing and making a level slice flight inspection. The system is then de-phased by the same amount in the opposite direction and the level slice repeated.
　
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本文鏈接地址：CAP 670 Air Traffic Services Safety Requirements 2(33)