# Instrument Landing System (ILS): Localizer and Glide Slope

> Learn ILS localizer and glide slope operation for precision approach guidance.

- **Subject:** 062 — Radio Navigation
- **Section:** 062-02 — Radio Aids
- **Topic code:** 062-02-05
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/radio-navigation/radio-aids/instrument-landing-system-ils/

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The **ILS (Instrument Landing System)** is the standard precision approach system, providing lateral and vertical guidance for landing.

![ILS System Diagram](https://upload.wikimedia.org/wikipedia/commons/1/13/ILS_diagram.png)

## System Components

1.  **Localizer (LOC):** Provides lateral guidance. Operates in VHF (**108.10 - 111.95 MHz**, odd decimals only). Uses two lobes modulated at **90 Hz (left)** and **150 Hz (right)**.
2.  **Glide Path (GP):** Provides vertical guidance. Operates in UHF (**329.15 - 335.0 MHz**), automatically paired with the LOC frequency. Uses lobes of **90 Hz (above)** and **150 Hz (below)**. The standard angle is **3°**.
3.  **Marker Beacons:** All operate at **75 MHz**.
    - **Outer Marker (OM):** Blue, dashes. Indicates the FAF.
    - **Middle Marker (MM):** Amber, dot-dash. Near CAT I minimums (approx. 1 NM from threshold).
    - **Inner Marker (IM):** White, dots. Near the threshold (CAT II/III).

## Coverage and Accuracy

- **Localizer:** 25 NM within ±10° and 17 NM within ±35° of the centerline.
- **Glide Path:** 10 NM within ±8° of the centerline.
- **Sensitivity:** The ILS is much more sensitive than the VOR. A full-scale deflection on the LOC is **2.5°** (compared to 10° for VOR).

## Usage and Procedures

![ILS Indicator](https://upload.wikimedia.org/wikipedia/commons/d/d7/ILS_instrument.svg)

- **Established:** Considered established when deviation is less than **half scale** (1.25° on LOC, 0.35° on GP).
- **Interception:** Always intercept the glide path from **below** to avoid **false glide paths** (side lobes that appear at higher angles, such as 9°).
- **Back Course (BC):** Use of the localizer's back beam. On a normal CDI, indications are **reversed**, but on an **HSI**, correct indications can be obtained by setting the front course.

## Operation Categories (Installation)

- **Category I:** Guidance down to 200 feet (DH).
- **Category II:** Guidance down to 100 feet (DH).
- **Category III:** Guidance down to the runway surface.

## Critical and Sensitive Areas

- **Critical Area:** Zone where the presence of vehicles or aircraft causes unacceptable interference. Protected whenever the ILS is in use.
- **Sensitive Area:** Wider zone protected during low visibility operations (**CAT II/III**) to prevent beam fluctuations that the autopilot might erroneously follow.

## Approach Charts

![ILS Approach Chart](https://upload.wikimedia.org/wikipedia/commons/6/65/KTIW_approach_chart_ILS_RWY_17.svg)

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.