Tutorial ILS

Tutorial: ILS (localizer, glide slope and marker beacons)

What it is

The Instrument Landing System guides an aircraft onto a runway: a localizer says left or right of the centreline, a glide slope says above or below the descent path, and marker beacons give distances. OnAir reads the signals as needles and shows the identification. It is for learning and monitoring only; it is not an approach aid.

What you need

• A radio that tunes 108 to 112 MHz (localizers), 329 to 335 MHz (glide slopes) and 75 MHz (markers). The mode asks for 2 Msps, the lowest working rate is 100 ksps and the mode table spans 74 to 336 MHz. See Supported radios. An RTL-SDR covers all three bands.
• A VHF antenna for localizers, and a UHF-capable antenna for glide slopes; line of sight, close to the runway. Signals are weak off the runway axis.
• Frequencies: localizers on odd tenths between 108.10 and 111.95 MHz, for example 108.100, 108.150, 108.300, 108.350, 108.500 MHz (project database). Each localizer is paired with a glide slope frequency; OnAir computes it for you. The mode starts at 110.3 MHz.

Before you start

You need to be near the runway the localizer serves, ideally in the approach area. A localizer is directional: you hear it best on the extended centreline.

Step by step

1. Choose the group Aviation, entry Localizer and glide slope (mode ILS).
2. In the tuner, type the localizer frequency in localizer. The matching glide slope frequency in MHz is shown (or "(not an ILS channel)" if the frequency is not paired).
3. Press tune localizer, then Start. To listen to the glide slope press tune glide slope; tick hop to alternate between localizer and glide slope every 4 seconds so both needles are shown. The markers 75 MHz button tunes the marker beacon frequency.
4. Open Approach for the needles.

What you should see

A localizer shows a carrier with the 90 Hz and 150 Hz tones; the difference in depth of modulation (DDM) moves the needle. The Morse ident (listening for the 1020 Hz Morse) appears after a few seconds. The text labels "fly left", "fly right", "fly up" and "fly down" show which way the aircraft would correct.

Reading the screen

• Approach: localizer and glide-slope needles, marker beacon lights for outer (400 Hz), middle (1300 Hz) and inner (3000 Hz), and the ident.
• Stations: the same list as the VOR mode: import a CSV with Import CSV... to name the stations (see Tutorial: VOR).

Troubleshooting

• No carrier: wrong frequency or off the approach course.
• Needle jumps: multipath from buildings and aircraft.
• Glide slope shows nothing: its signal is only strong in the approach sector and needs the UHF band.
• Marker beacons silent: they only sound when an aircraft is overhead the marker.

Tips

Compare your reading with an aircraft actually flying the approach in ADS-B (Tutorial: ADS-B).

Legal and privacy notes

Public navigation aids; receiving is lawful in most places. Do not navigate by it. See Legal.

Under the hood

Localizer: carrier with 90 Hz and 150 Hz amplitude modulation; DDM = (M90 - M150) / depth; the ident is a keyed 1020 Hz tone. Glide slope uses the same tones in the 329 to 335 MHz band. Marker beacons: 75 MHz with 400, 1300 and 3000 Hz tones. ICAO Annex 10 vol. I.

Related

ILS mode page, Tutorial: VOR, Tutorial: Airband.