Every aid in this course so far told the pilot where the aircraft was. The instrument landing system (ILS) tells the pilot where to go: down one defined line in space that ends at the touchdown zone. The ILS has three main components:
the localiser (LOC) — guides in azimuth, holding the aircraft on the extended runway centre line;
the glide path (GP) — guides in the vertical plane, holding it on the descent;
range information — provided by marker beacons or DME (distance-measuring equipment), ticking off the distance still to go.
Where the antennas stand
Three components, then: the LOC for azimuth, the GP for the vertical plane, markers or DME for range. The two guidance antennas stand at opposite ends of the job:
the LOC antenna — located on the extension of the runway centre line at the stop-end: the far end of the runway, seen from the approach, so its signal is beamed back along the full length of the runway and out along the approach path;
the GP antenna — located beyond the runway threshold, laterally displaced to the side of the runway centre line: a few hundred metres in from the threshold, abeam the touchdown zone, and off to the side so that the antenna itself never stands beneath the aircraft while the descent path it defines meets the runway at the touchdown point.
Frequencies and pairing
The ILS LOC frequency band is 108.0-111.975 MHz, and within it only the frequencies with an odd number in the first decimal are ILS LOC frequencies. This is the same shared shelf the VOR lesson taught from the other side: 108.10 or 109.30 is an ILS, while 108.20 or 109.40 can be a VOR (VHF omnidirectional radio range). The vertical half lives elsewhere: the GP operates in the UHF band (ultra-high frequency), but the pilot never tunes it, because the UHF GP frequency is selected automatically by being paired with the LOC frequency. One number set on the receiver brings both beams in.
How the beams steer
Both guidance signals are built from the same trick: two overlapping lobes, one modulated at 90 Hz and one at 150 Hz. Seen from the approaching aircraft, the LOC radiates the 90-Hz lobe on the left of the centre line and the 150-Hz lobe on the right; the GP radiates the 90-Hz lobe above the descent path and the 150-Hz lobe below it. The receiver measures how deeply each tone modulates what it hears: on the centre line, and on the path, the two depths of modulation are equal; off them, the nearer lobe predominates, and the difference between the depths grows with the angular deviation. That growing difference is the raw material of the deviation needle: which tone predominates says which side of the beam the aircraft is on, and by how much says how far off it sits.
The approach in profile
In the vertical plane, the recommended GP is 3 degrees, a slope gentle enough for a stabilised descent and steep enough to clear what lies under the approach. Along that slope, marker beacons produce radiation patterns to indicate predetermined distances from the threshold along the ILS GP: narrow beams pointing straight up, which the aircraft crosses one after another on the way down. All marker beacons operate on the 75-MHz carrier frequency, one frequency for every marker everywhere, which works because each beam is so narrow that the receiver only ever hears the one directly below. What distinguishes them is the audio: the modulation frequency of the tone is low, medium or high, and the identification is a continuous keying of that tone:
outer marker (OM) — a low tone keyed as 2 dashes per second continuously; the cockpit indicator is coloured blue. The OM often shares its site with a locator, the low-powered NDB of the bearings module (non-directional radio beacon);
middle marker (MM) — a medium tone keyed as a continuous series of alternate dots and dashes; the indicator is amber;
inner marker (IM) — a high tone keyed as 6 dots per second continuously; the indicator is white.
Markers are not the only way to measure the approach: marker beacons are sometimes replaced by a DME paired with the LOC frequency, the pairing mechanism of the DME lesson putting continuous distance where the markers gave single ticks. Either way, the procedure keeps one checkpoint sacred: the final-approach area contains a fix or facility that permits verification of the ILS GP-altimeter relationship, and the outer marker or DME is usually used for this purpose. Crossing that fix, the altimeter reading is compared with the published crossing altitude: if the aircraft is truly on the GP, the two must agree.