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Understanding the Localizer in Aviation: How It Works, Coverage, and Course Width Explained

When pilots fly an Instrument Landing System (ILS) approach, one of the most critical navigation elements guiding them to the runway is the localizer (LOC). While the glideslope provides vertical guidance, it's the localizer that ensures the aircraft remains correctly aligned laterally with the runway centerline. Without it, flying a precise approach in low visibility would be nearly impossible.


In this article, we’ll explore how the localizer works, where it sits in the full ILS system, its service volume, and what determines its course width.



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What Is a Localizer?

A localizer is a ground-based radio navigation component of the Instrument Landing System that provides lateral (left/right) guidance to align an aircraft with the runway centerline during an approach.


It transmits signals along the runway heading, which aircraft receivers interpret to display whether the aircraft is left or right of the intended approach path. Pilots follow the localizer to maintain centerline alignment all the way to the threshold.


How the Localizer Works

The localizer operates using VHF radio signals in the 108.10 MHz to 111.95 MHz frequency range, with odd tenths reserved exclusively for LOC (e.g., 108.1, 108.3, 108.5…).


Dual-Signal Transmission

The system works by broadcasting two overlapping lobes:

  • One side modulated at 90 Hz

  • The other modulated at 150 Hz


Where the signals overlap equally is the on-course line, representing the runway centerline. If the aircraft drifts left or right, the imbalance between the two signals grows, and cockpit instruments display an offset needle, instructing the pilot to correct.


How pilots interpret it:

  • Needle centered: aircraft is on course

  • Needle deflects left: aircraft is right of centerline (fly left to intercept)

  • Needle deflects right: aircraft is left of centerline (fly right)


Simple in concept—but extremely precise.


Localizer Service Area

Every localizer has a defined area in which signal accuracy is guaranteed. This service volume ensures reliable navigation from miles away during an approach.


Standard Localizer Coverage

  • 35° coverage either side of centerline out to 10 NM

  • 10° coverage either side of centerline out to 18 NM


This lets aircraft intercept the localizer from a distance, providing time to establish on course before descending on the glideslope.


Localizer Course Width

Localizer course width isn't fixed—it's designed to widen as distance from the runway increases, providing sensitivity close to touchdown while remaining manageable further away.


Typical course characteristics:

  • At the runway threshold, full-scale deflection is approximately 700 feet wide

  • Course width typically varies from 3 to 6 degrees total

  • The closer the aircraft is to the runway, the more sensitive the needle becomes


This increasing sensitivity is why small corrections near minimums are crucial—tiny deviations show up quickly.


Localizer Back Course (Bonus Concept)

Some localizers also radiate in the opposite direction from the runway, enabling a back course approach. This offers lateral guidance only and requires special awareness to avoid reverse sensing depending on equipment settings.


Not all ILS installations support back course usage, but where available, they provide valuable redundancy.


Why Localizers Are Still Important

Despite advancements in satellite navigation like RNAV (GPS) LPV procedures, localizers continue to be widely used because they are:

  • Extremely reliable

  • Resistant to GPS interference

  • Standard at major airports worldwide

  • Capable of precision alignment even in low visibility


The localizer remains a foundational navigation element for instrument pilots.


Final Thoughts

The localizer may be just one half of the ILS, but it plays a pivotal role in ensuring safe, precise landings. It guides aircraft laterally using simple yet powerful radio signals that pilots interpret intuitively through cockpit instruments.


From how it generates guidance to how its course width tightens closer to the runway, understanding the localizer provides deeper insight into why instrument flying is both art and science—a mix of precision technology and pilot skill.



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