Field Guide · technology

Also known as: ILS

ILS (Instrument Landing System) is a precision-approach navaid that guides an aircraft down to a runway along a fixed lateral and vertical path, even in poor visibility. It uses two radiated beams — a localizer for left/right alignment and a glideslope for the descent angle — each formed from two overlapping tones. Where the tones are equally strong the aircraft is on course; an imbalance drives the cockpit needle toward the stronger side.1 Because the guidance is encoded in the depth of two amplitude-modulated tones, an ILS receiver only has to compare tone strengths, not measure phase.

runway 150 Hz lobe 90 Hz lobe on course: 90 Hz = 150 Hz
The ILS localizer overlaps a 90 Hz and a 150 Hz lobe along the centerline; equal tone depth means on-course, imbalance shows the deviation.

How it works

Both ILS beams work by difference in depth of modulation (DDM). The localizer, near 110 MHz in the VHF band, radiates a carrier modulated on one side of the runway centerline mostly by a 150 Hz tone and on the other side mostly by a 90 Hz tone; the two patterns overlap along the centerline where their modulation depths are equal. The receiver demodulates the AM envelope, measures how much 90 Hz versus 150 Hz is present, and deflects the course-deviation indicator toward the tone that dominates. The glideslope, near 330 MHz in the UHF band, does the same thing in the vertical plane so the aircraft can hold a typical 3° descent.

Along the approach, marker beacons (or, increasingly, DME distance) mark fixed points to the threshold. ILS installations are certified in categories (CAT I, II, III) according to how low a decision height and visibility they support, with CAT III enabling near-zero-visibility autoland. The localizer also carries a Morse identifier, much like VOR.

Relevance to SDR

ILS is a compact demonstration of AM tone-depth signalling: a software-defined radio parked on a localizer frequency can recover the composite audio, filter out the 90 Hz and 150 Hz tones, and compute their relative depth to reproduce the cockpit needle. It shares the aeronautical navigation bands with VOR and DME and rounds out the classic navaid set. GopherTrunk does not decode ILS; it is a land-mobile trunking scanner, and ILS appears here to give the AM-modulation family concrete real-world context.

Sources

  1. Instrument landing system — Wikipedia, for the localizer/glideslope structure, the 90 Hz and 150 Hz DDM principle, marker beacons, and CAT I/II/III performance categories. ↩

See also