ILS Localizer 3

28.10.2004

Submission:

Requirement for localiser coverage +/-35 is due to support of transition from initial to intermediate phase of approach. Assuming, that other means of navigation can be used to support this phase of flight and ILS signal will be used only in phase of final approach, localiser coverage requirements could be modified.

Doc 8168 protects aircraft on ILS final approach by Obstacle Assessment Surfaces (OAS). Within the space bounded by OAS, the localiser signal must be sufficient to identify aircraft position relative to centreline to make a right corrective action to bring the aircraft into nominal flight path.

OAS's are based on probabilistic model of approach. It is in accordance with this model, that maximum lateral deviation from centreline is expected at the plane containing nominal glide path.

Now, how to calculate required localiser coverage at final approach?

From intersection of plane containing nominal glide path and transition surface "X".

Example:

Nominal glide path is 3.00° ; distance between localiser and threshold is 3000m.

Plane of nominal glide path:
z = 0.524 x + 15.00

Transition surface X:
z = 0.027681 x + 0.1825 y - 16.72

Intersection of these planes:
y = 0.13549 x + 173.81

Being slightly conservative ignoring that a line above crosses centreline somewhere between localiser and threshold, we can promulgate angle of this line as a limit of localiser coverage, i.e.

tan-1 0.13549 = 7.7°


That means, for this configuration, localiser coverage of ± 7.7° is safety minimum. Tighter coverage will not provide aircraft within OAS bounded airspace with appropriate localiser signal. By the way, the current localiser systems meet this criterion with clearance transmitter switched off. No consideration was taken for initial/intermediate phase of flight (e.g. radar vectoring to intercept localiser...).

Ivan Ferencz.

Head of NAVAIDS Division Civil Aviation Authority Bratislava Slovak Republic