ILS Localizer and Glide Path

ILS Localizer: Lateral Guidance

The ILS localizer is a ground-based transmitter located beyond the stop end of the runway, aligned with its centerline. It operates in the VHF band (108.00–111.975 MHz) and emits two overlapping beams modulated at 90 Hz (left) and 150 Hz (right). The aircraft's receiver measures the difference in depth of modulation (DDM) to determine its position relative to the centerline. When the DDM is zero, the aircraft is precisely on the localizer.

Glide Path: Vertical Guidance

The glide path transmitter is positioned about 300 meters from the runway threshold and offset laterally (typically 120 meters from centerline). It operates in the UHF band (329.3–335 MHz) and provides a vertical signal, usually set at a 3° descent angle. The glide path uses similar modulation (90 Hz for 'fly down', 150 Hz for 'fly up'), and the aircraft's instruments show deviations above or below the ideal path.

ILS Approach Sequence

Pilots must first establish on the localizer (within half-scale deflection) before descending on the glide path. Descent is only initiated after confirming stable lateral alignment. If the aircraft deviates beyond half-scale on either signal, obstacle clearance is no longer assured, and a missed approach is mandatory.

Range Information and Verification

Distance from the runway is provided by marker beacons or DME, allowing pilots to cross-check altitude versus distance and confirm the glide path's validity. The outer marker or DME fix is commonly used to verify the aircraft's position relative to the glide path.

False Beams and Beam Bends

Both localizer and glide path antennas can produce side lobes or false beams, which may give misleading indications if the approach is not flown from below the glide path. Beam bends—localized deviations in the signal—can also occur and are assessed during flight testing to ensure safety.