Instrument Landing System (ILS) — Page 157, Lesson 141

Let’s start with the ground monitoring of the ILS. Both the localizer and the glide path transmitters are watched automatically by monitoring equipment that sits in an area of guaranteed reception — that is, a spot where the signal is always strong enough to be checked reliably. This equipment is set to act when certain limits are exceeded. First, the localizer at the reference datum — the point where the ILS is calibrated — must not shift from the runway centre line by more than 35 feet for a Category I ILS, 25 feet for Category II, or 20 feet for Category III. So the tighter the category, the smaller the allowed shift. Second, the glide path angle must not change by more than 0.075 times the basic glide path angle. So if the promulgated glide path angle is, say, 3 degrees, the allowed change is 0.075 × 3 = 0.225 degrees. Third, there must be no power reduction of more than 50% from any transmitter. When any of these limits is exceeded, the monitoring unit does two things: it sends a warning to a control point, and it triggers one of several actions before a standby transmitter is activated. Those actions are: cessation of all radiation — the transmitter simply stops radiating; or removal of the identification and navigational components of the carrier — so the signal is stripped down; or, for a Category II or III ILS, the system may be permitted to operate down to the lower categories, Category I or II, instead of its full capability. Now let’s move to ILS coverage. The localizer coverage sector extends from the transmitter to specific distances. Within plus or minus 10 degrees of the centre line, coverage extends 25 NM, which is 46.3 km. Between 10 and 35 degrees from the centre line, it extends 17 NM, or 31.5 km. And outside plus or minus 35 degrees, if coverage is provided at all, it extends 10 NM, or 18.5 km. There’s an important reduction allowed here. If alternative navigational facilities provide satisfactory coverage within the intermediate approach area, these limits may be reduced to 18 NM within the 10-degree sector, and 10 NM within the remainder of the coverage. So the localizer doesn’t always have to reach its full range if something else covers that area. Now the glide path. Its coverage extends from the transmitter to a distance of at least 10 NM, or 18.5 km, in sectors of 8 degrees in azimuth on each side of the centre line. So horizontally, you get 8 degrees of coverage on each side. Vertically, the coverage is provided from 0.45θ up to 1.75θ above the horizontal, where θ is the promulgated glide path angle. So if θ is 3 degrees, the vertical coverage runs from 0.45 × 3 = 1.35 degrees up to 1.75 × 3 = 5.25 degrees above the horizontal. The lower limit may be reduced to 0.3θ if required to safeguard the promulgated glide path intercept procedure. One crucial note: these are the sectors within which the ILS localizer and glide path emissions must provide correct indications. Radiated energy does exist outside these vertical and horizontal sectors — the signal is still there, it just isn’t guaranteed to be accurate enough for guidance. Let me show you the coverage patterns. shows the localizer coverage — you can see the 25 NM range within 10 degrees, the 17 NM between 10 and 35 degrees, and the 10 NM outside that. shows the localizer radiation pattern itself. And the glide path coverage is in Figures 9.6 and 9.7 — the horizontal coverage with the 8-degree azimuth sectors, and the vertical coverage from 0.45θ up to 1.75θ. So to tie it together: the monitoring system protects the integrity of the signal, and the coverage figures define where that signal is guaranteed to be correct. Both are essential for the ILS to be usable for a precision approach.