Learn what an ILS antenna is, where localiser and glideslope arrays sit, and how 90/150 Hz signals guide an aircraft safely to the runway.
An ILS antenna is part of a ground-based Instrument Landing System in real-world aviation. Separate localiser and glideslope antenna arrays transmit overlapping radio patterns. The aircraft’s receiver compares 90 Hz and 150 Hz modulation in those patterns, showing corrections that keep the aircraft on the runway centreline and published descent path.
The two ground antenna arrays
A standard ILS uses separate localiser and glideslope arrays because lateral and vertical guidance are generated independently. They are components of the wider system described in our guide to how a complete ILS approach works.
| ILS component | Typical antenna location | Radio band | Guidance provided |
|---|---|---|---|
| Localiser | Beyond the far end of the runway, aligned with its centreline | Selected VHF channels from 108.10 to 111.95 MHz | Left and right |
| Glideslope | Beside the runway near the approach-end touchdown area | A paired UHF channel from 329.15 to 335.00 MHz | Above and below the descent path |
The published localiser frequency selects both components because each localiser channel has a paired glideslope channel. Modern avionics may tune that frequency automatically through the flight-management system.
Aircraft also have receiving aerials connected to their navigation receivers. Their number and location vary by aircraft, so “ILS antenna” can mean either an airport transmitter array or an airborne receiving aerial. In approach discussions, it usually means the ground installation.
How does the ILS radio signal guide the aircraft?
ILS guidance comes from measuring the difference between two modulated signal patterns rather than tracking the aircraft or sending steering commands directly.
- Receive the correct channel. The pilot or avionics selects the published localiser frequency and verifies the facility identification. The associated glideslope channel is paired automatically.
- Compare the modulation. Each directional field contains 90 Hz and 150 Hz modulation at different strengths. The receiver calculates the difference in depth of modulation, usually abbreviated to DDM.
- Generate lateral and vertical indications. On the front-course localiser, 90 Hz predominates to the left and 150 Hz to the right. For the glideslope, 90 Hz predominates above the path and 150 Hz below it. Equal modulation centres the relevant indicator.
- Follow or couple the guidance. The pilot follows the deviation bars, or the flight director and autopilot convert those deviations into pitch and bank commands. The antenna itself does not control the aircraft.
Localiser and glideslope beams become more sensitive as the aircraft approaches the runway, so large control inputs close to the threshold can cause overcorrection. Our breakdown of localiser versus full ILS guidance explains why receiving a localiser does not necessarily mean vertical guidance is available.
Does an ILS antenna land the aircraft?
No; an ILS antenna supplies precise approach guidance, but it does not land, flare or brake the aircraft by itself. Most ILS approaches are flown to a published decision altitude or decision height, where the crew must have the required visual reference or execute a missed approach.
Autoland is possible only when the approach facility, aircraft systems, operating procedures and crew qualifications support it. The ILS supplies alignment and descent information, while onboard autopilot, radio-altimeter, flare and rollout functions perform the landing sequence.
Why can ILS guidance be wrong or unreliable?
ILS indications become misleading when the wrong facility is selected, the signal is intercepted incorrectly or the aircraft is outside the protected part of the radio pattern.
- Wrong frequency or runway: verify the published frequency, identifier and inbound course. A valid-looking needle is not proof that the intended ILS has been selected.
- Glideslope intercepted from above: false glideslope lobes can exist above the normal path. Intercept the published glideslope from below at the charted altitude.
- Back-course reception: a localiser signal may be receivable from the opposite direction, but there is normally no usable glideslope and lateral indications may reverse unless the avionics has a back-course mode.
- Signal reflections: terrain, buildings and vehicles can distort an ILS signal. Airports protect critical and sensitive areas around the arrays, particularly during low-visibility operations.
- Simulator data mismatch: the visible 3D antenna is usually scenery, while guidance comes from navigation-facility data. If scenery and navigation data disagree, the beam may not align perfectly with the painted runway. For practical interception technique, see our guide to tuning and capturing an ILS in Microsoft Flight Simulator 2024.
Are localiser and RNAV approaches the same as ILS?
A localiser supplies only the lateral part of an ILS, while RNAV is a different navigation method that does not depend on the runway’s localiser and glideslope arrays. RNAV approaches commonly use GNSS-derived aircraft position, although their available vertical guidance and minima depend on the published procedure and onboard equipment.
Our comparison of ground-based ILS and RNAV approach guidance covers when each system is used and why their cockpit indications can appear similar despite working differently.