Instrument Landing System (ILS) components: which of the following statements is incorrect regarding typical siting of ILS elements relative to the runway threshold?

Introduction / Context:
The Instrument Landing System (ILS) comprises a localizer (lateral guidance), glide slope (vertical guidance), and marker/locator beacons marking distance to threshold. Understanding typical siting aids comprehension of approach geometry and equipment clearances.

Given Data / Assumptions:

• Standard straight-in ILS approach configuration.
• Approximate distances are used for teaching purposes.

Concept / Approach:
The localizer antenna array usually sits beyond the departure end, aligned on the extended centerline, while the glide slope transmitter is offset to the side of the runway near the touchdown zone—not on the centerline. Outer and middle markers (or their locator equivalents) lie along the approach path at characteristic distances (≈ 7.2 km and ≈ 1.0 km).

Step-by-Step Solution:
Evaluate each siting statement.LOM ≈ 7.2 km from threshold → acceptable.LMM ≈ 1.0 km from threshold → acceptable.Localizer ≈ 300 m beyond far end on centerline → typical depiction.Glide slope: it is installed to the side of the runway near the touchdown zone, not “at the centre of the runway”; the wording is incorrect.

Verification / Alternative check:
Airport approach plates and equipment layout diagrams show glide slope masts off the runway edge to avoid obstacles and to maintain correct signal geometry.

Why Other Options Are Wrong:

• The three other siting statements match common practice; only the “centre of the runway” phrasing is wrong.

Common Pitfalls:

• Confusing marker beacons with DME fixes; many modern ILS installations use DME instead of markers.

Final Answer:
The glide slope antenna is installed at the centre of the runway about 150 m on one side.