What is Localiser in aviation? The Localiser, often abbreviated as LOC, is a crucial component of the Instrument Landing System (ILS) that guides aircraft during the final approach phase to a runway. It provides lateral guidance by transmitting radio signals that pilots use to align the aircraft precisely with the runway centerline. Understanding the Localiser is essential for both aviation enthusiasts and professionals, as it plays a vital role in ensuring safe and accurate landings, especially during poor visibility conditions.
Understanding the Role of Localiser in Aviation Navigation
The Localiser in aviation serves as the primary source of horizontal guidance for aircraft on approach paths. It emits a radio frequency signal that pilots interpret via instruments in the cockpit to maintain alignment with the runway’s extended centerline. The Localiser transmitter is mounted on the ground beyond the departure end of the runway, typically halfway along its length, providing accurate directional information over a distance of up to 25 nautical miles (nm) from the runway threshold.
Technically, the Localiser operates in the frequency range between 108.10 MHz and 111.95 MHz within the Very High Frequency (VHF) band. The signal is modulated with two audio tones: 90 Hz and 150 Hz. The difference in depth of modulation between these two tones creates the directional indication. When the aircraft is precisely on course, the two tones are equal. If the aircraft drifts to the left or right, one tone becomes dominant, signaling the pilot to adjust the heading accordingly.
How the Localiser Works During an Instrument Landing System Approach
The Localiser works in conjunction with the glide slope component of the Instrument Landing System to provide both lateral and vertical guidance. The Localiser ensures the aircraft remains on the runway’s centerline, while the glide slope maintains the correct descent angle, usually set at approximately 3 degrees. Together, these signals enable pilots to execute precision approaches even when visibility is severely limited.
During an ILS approach, the pilot tunes the aircraft’s navigation receiver to the Localiser frequency associated with the runway in use. The navigation indicator displays deflections to the left or right of the centerline based on the Localiser signal. When combined with the glide slope indication for altitude control, the pilot can perform a stabilized approach and safely land the aircraft. The Localiser’s signal coverage is designed to ensure reliable signals up to 35 degrees either side of the runway centerline within the final 10 nm, narrowing progressively beyond that distance to avoid interference.
Technical Specifications and Limitations of the Localiser System
The Localiser transmitter uses an antenna array positioned across 3,000 feet from the runway centerline on the approach end, carefully designed to generate a signal pattern that ensures maximum accuracy. The course width of the Localiser beam is typically set at 3 to 6 degrees, with signal modulation set so that full-scale deflection of the cockpit needle corresponds to a lateral displacement of approximately 2.5 degrees from the runway centerline, which translates to roughly 400 feet at a 10 nm distance.
Despite its importance, the Localiser does have limitations. Terrain, buildings, and adverse weather conditions can sometimes cause signal reflections or attenuation, leading to deviations or “false courses.” To mitigate this, modern aircraft use built-in signal monitoring and cross-check Localiser data with GPS and other navigational aids. Additionally, the Localiser is only designed for straight-in approaches; it does not provide guidance for circling approaches or curved trajectories.
For further technical details on the Localiser and its operational principles, you can explore the Federal Aviation Administration’s Official documentation here: FAA Aeronautical Information.
For More: What is RNP in Aviation? (Required Navigational Performance)