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

Right, let's pick this up with the ground monitoring of the ILS transmissions. This is the safety net that watches over the localizer and the glide path. Both the localizer and the glide path are automatically monitored by equipment located in an area of guaranteed reception. So there's a dedicated receiver site, positioned where it's certain to pick up the signals, and it constantly checks the health of what's being transmitted. This monitoring equipment will act when certain limits are exceeded. Let's take them one by one. First, the localizer at the reference datum shifts from the runway centre line by more than 35 feet for a Category I approach, 25 feet for Category II, or 20 feet for Category III. So the reference datum is the point where the localizer course is defined, and the tolerance gets tighter as the category gets more demanding. Second, the glide path angle changes by more than 0.075 times the basic glide path angle. So if the promulgated glide path angle is, say, 3 degrees, the monitor will act if it shifts by more than 0.075 × 3°, which is 0.225 degrees. Third, there is a power reduction in output of more than 50% from any transmitter. So if any transmitter's output power drops by more than half, the monitor reacts. When the monitoring unit detects any of these conditions, it will provide a warning to a control point and cause one of several things to occur before a standby transmitter is activated. The first is cessation of all radiation — the system simply stops transmitting. The second is removal of the identification and navigational components of the carrier — so the signal is stripped down. And the third is that a Category II or III ILS may permit operation to the lower categories I or II — so the system downgrades itself to a less demanding category rather than shutting down completely. Now let's move on to ILS coverage, starting with the localizer. The localizer coverage sector extends from the transmitter to specific distances. Within plus or minus 10° from the centre line, coverage extends to 25 NM, which is 46.3 km. Between 10° and 35° from the centre line, it extends to 17 NM, which is 31.5 km. And outside ±35° from the centre line, coverage extends to 10 NM, which is 18.5 km, if coverage is provided at all. These limits may be reduced — to 18 NM within the 10° sector and 10 NM within the remainder of the coverage — when alternative navigational facilities provide satisfactory coverage within the intermediate approach area. So if there's another navaid covering that area, the ILS localizer doesn't need to reach as far. Now the glide path coverage. It extends from the transmitter to a distance of at least 10 NM, which is 18.5 km, in sectors of 8° in azimuth on each side of the centre line. So horizontally, it covers 8° either side of the runway centre line. 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°, the vertical coverage runs from 0.45 × 3° = 1.35° up to 1.75 × 3° = 5.25° above the horizontal. The lower limit may be reduced to 0.3θ if required to safeguard the promulgated glide path intercept procedure. One important note: these are the sectors within which the ILS localizer and glide path emissions must provide correct indications. Radiated energy exists outside these vertical and horizontal sectors — the signal is still there, it just isn't guaranteed to be correct outside these defined sectors. Let me show you the coverage diagrams. So to summarise: the monitoring system watches the localizer position, the glide path angle, and transmitter power, and it can shut down, strip the signal, or downgrade the category. The coverage defines where the signals must be correct — 25 NM within 10° for the localizer, 10 NM minimum for the glide path, with vertical limits tied to the glide path angle θ.