ILS localizer and lateral sensitivity

Why does the localizer become more sensitive near the runway?

The signal compares angular position; physical displacement shrinks with range.

Difference in modulation depth

The localizer supplies lateral guidance by comparing the received depth of 90 Hz and 150 Hz modulation. Equal appropriate components define the intended centreline. The difference in depth of modulation produces deviation. The ground installation and protected sector determine where the signal is usable; reception elsewhere does not prove a valid front-course path.

A localizer is not a VOR radial selector. On conventional installations, rotating the course selector does not rotate the transmitted localizer centreline. Set the front course as required by the installed display and procedure so that the crew can interpret the geometry correctly.

An angular wedge

For an illustrative full-scale half-angle of 2.5°, the lateral distance represented by full scale is range × tan 2.5°. At 10 NM it is approximately 0.437 NM; at 2 NM it is 0.087 NM. Actual localizer course width is installation-dependent. The laboratory labels its angle as synthetic, not a universal ILS value.

Keep physical distance and dots separate

Use the laboratory’s 2.5° full-scale half-angle. At 8 NM, the full-scale lateral distance is 8 tan 2.5° = 0.349 NM. An aircraft displaced 0.07 NM is at roughly 20% of that angular scale. At 2 NM the same displacement is around 80% of scale. The aircraft has not become farther from the line; the angular comparison has become much more sensitive.

Move range while holding lateral displacement constant and predict the needle movement before reading it. Then hold angular displacement constant and watch the physical distance shrink. These are different experiments. Understanding the distinction explains why a correction acceptable farther out can become excessive close in, without inventing a universal heading correction or substituting this synthetic scale for a published facility specification.

When is a plausible localizer indication unusable?

Identity, sector, status and correct approach selection remain essential.

Front course and reverse sensing

The intended front-course sector provides the approach guidance described by the procedure. Back-course reception and conventional reverse sensing require specific equipment and procedure understanding; do not infer authority from a needle alone. False or distorted signals can also occur outside the protected region or when vehicles or aircraft interfere with critical signal areas.

Verify the localizer identifier, selected source, approach course and status. A receiver flag invalidates the deviation. In the laboratory, selecting invalid removes both the deviation marker and any steering interpretation while retaining the explanatory geometric picture.

Respond to trends

A small heading correction can be sufficient near the runway because a given lateral displacement creates a larger angular error. A large late correction can cause repeated crossings. Use the applicable stabilised-approach criteria and aircraft technique; the visual model teaches sensitivity and validity, not a control law or approach authorisation.

Distinguish a failure from a deviation

Imagine the aircraft is correctly established and the localizer suddenly flags. The last deviation may have been small, but the new flag changes the kind of information available: the receiver no longer supplies valid guidance. It is not evidence that the aircraft has moved to full-scale deflection, and it should not trigger a correction intended to chase the frozen or disappearing marker.

Contrast that with a valid needle moving steadily because the aircraft is drifting. In that case the trend describes geometry and can inform an appropriate correction within the approved technique. A signal disturbance near a critical area can create a third situation, where apparent motion may be misleading. Cross-check source, flags, geometry and approach conditions before deciding which explanation is supported by the indications.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.