ILS: Coverage, Errors and Categories

Coverage volumes

The localiser signal is usable within 10 degrees either side of the front course line out to about 25 NM, and within a wider 35 degrees either side out to about 17 NM; outside those angle and distance combinations the signal may still be present but is not guaranteed to meet accuracy standards. The glide path, by contrast, is only guaranteed to about 10 NM from the threshold, which is why aircraft are vectored or fly a published procedure to intercept the localiser well before they can expect reliable glide path guidance.

This asymmetry between lateral and vertical coverage is deliberate. Lateral guidance needs to work across a wide capture geometry so that radar vectoring and procedural turns can join the approach from varied directions, while vertical guidance only has to be trustworthy for the relatively short, straight final segment flown once the aircraft is already established on the localiser.

False glide paths

The glide path aerial array radiates the true path together with additional interference lobes above it. The first false glide path commonly appears at about twice the published glide path angle, so a standard 3 degree glide path has a false lobe near 6 degrees. Capturing that false lobe from below feels identical to capturing the real one: the needle centres and the approach looks normal, except it is roughly twice as steep and will arrive low and fast, or simply will not fit a stabilised profile.

The safeguard is procedural rather than something the needle itself can reveal: intercept the glide path from below and check the altitude at that point against the published crossing altitude at the final approach fix or outer marker. If the glide path centres well above that checked altitude, it is a false path, not the true one, and the correct response is to continue level, or go around, rather than follow the needle down.

Back beam and category minima

The localiser transmits behind the antenna as well as ahead of it, producing a back beam that can be flown as a back-course approach where published, though without a matching glide path and with reversed needle sensing unless the equipment corrects for it. Categories I, II and III describe progressively lower decision heights and required visual references, commonly quoted as a CAT I decision height not below 200 ft with a runway visual range not below 550 m, CAT II down to 100 ft with an RVR not below 300 m, and CAT III operations below 100 ft, or with no decision height at all, supported by progressively lower RVR. Exact minima depend on aircraft and operator approval, so treat these as commonly examined reference figures rather than universal constants.