In each plane, vertical and horizontal, a pair of antennas transmit two directional beams, directed close to either side of the ideal approach path. Each beam can be thought of as activating an electromagnet to either side of a centrally biased instrument needle. When the field strengths of the beams are equal, the bias will hold the needle in the centre of the instrument. When the aircraft is to one side of the ideal path, one field strength will be stronger than the other. The needle will be deflected to the appropriate side by an amount proportional to the difference in strengths, showing the angular displacement from the ideal path.
By keeping the indication central, the pilot or autopilot can maintain the ideal approach path in three dimensions down to the runway, or at least to a ‘decision height’ from which he can either land visually or divert if he cannot see the runway.
The actual transmissions use a different frequency for each plane. All Signals are horizontally polarised. Each antenna in the pair modulates the carrier wave at a different frequency (90Hz and 150 Hz), and it is the difference in depth of modulation (DDM) which moves the needle. The Signal strengths is arranged so that, when close to the intended path, the DDM varies linearly as the aircraft moves away from the path, allowing linear presentation on the instrument.
The transmission frequency for guidance in the horizontal plane, the “localiser’ frequency, is in the VHF band, between 108 and 111.975 MHz, using the odd first decimals, such as 110.50 or 109.35. (VOR beacons may use the even first decimals.)
The frequency used in the vertical plane, or ‘glidepath’ frequency, is in the UHF band (officially between 328.6 and 335.4 MHz, but actually between 329.30 and 335.00), at 150 kHz spacing. Every glidepath frequency is paired with a discrete localiser frequency, so that the equipment has only to be tuned to the localiser frequency for the glidepath frequency to be automatically selected.
The 90 Hz modulation is applied to the signal on the left side of the centreline, and the 150 Hz modulation to the right. The 90 Hz modulation is applied to the signal above the glide slope, and the 150 Hz below. The beams are arranged to give the desired glidepath angle. In most cases this would be the ‘nominal angle of 3 (a 4.9% slope) above the horizontal, but local conditions may require a different angle.
To confirm the aircraft’s range from the runway, marker beacons are provided under the ideal approach path which transmit an independent signal on a frequency of 75 MHz. When a marker signal is received, a light appears on the instrument panel, and an audio tone is heard.
The shape and direction of the localiser beams is controlled by the localiser aerial. In accordance with ICAO Annex 10, this must be placed beyond the end of the runway, at a safe distance to prevent it becoming an obstruction, in line with the runway. In some situations, the aerial may have to be placed to one side, which means it can only be used as a category I ILS (see below).
The localiser centreline is then at an angle from the runway centreline and the installation is called an ‘offset ILS. The localiser centreline of an offset ILS will cross the runway extended centreline at the same range from the threshold as an approaching aircraft would reach its decision height. If the offset is more than 2, the ILS cannot be used as a ‘precision approach aid’. It is regarded as an ‘airfield approach aid’ only.
There are may be three marker antennas, although two is the minimum, sometimes in conjunction with an NDB. The ‘outer marker is positioned at a range from touchdown sufficient to give height, distance and equipment functioning checks, usually just after the pilot descends on the final approach (Annex 10 recommends 3.9 nm from touchdown, but certainly between 3.5 and 6 nm). A ‘middle marker is placed (ideally 1050 m from touchdown), to indicate that visual references should be available, i.e, around category I decision height. The optional ‘inner marker, if fitted, is placed just short of the threshold itself, in the area where an aircraft would be at a category lI decision height.
Aircrafts have to be able to receive the marker signals on the glideslope for enough time to identify them and make calculations, so the beam is shaped to produce a signal over specific distances at the glideslope. Ideally, the outer marker should be 600 m wide, the middle marker 300 m, and the inner marker 150 m. The beam is broad enough to allow an aircraft within 2.5°of the centreline to receive it.
ILS reference point
The antenna for the glideslope is positioned alongside the runway, in such position that the glideslope passes over the threshold at a height of 50 ft. Th point in space is called the ILS reference point’.
Many lLS systems have a DME (Distance Measuring Equipment, frequency paired with them. In this case, the DME is intended to replace the markers, to give range indications from touchdown during the final approach. The output from such a DME is electronically adjusted so that the received ranges are only correct when the aircraft is on the ILS centreline in the direction of approach.
ICAO categorises ILS equipment and procedures. The operational objectives for these categories are as follows:
Cat I – to allow aircraft to approach a runway in a position to land with a decision height of 60 m (200 ft) above the runway, and a Visibility of not less than 800 m or runway visual range (RVR) of not less than 550 m.
Cat II– to allow aircraft to approach a runway in a position to land with a decision height of between 60 m and 30m (100 ft) and a RVR of not less than 350 m
Cat IIIa – to allow aircraft to approach a runway in a position to land with a decision height of 30 m or less (or no decision height) and RVR of not less than 200 m.
Cat IIIb– to allow aircraft to reach a position to land with a decision height of 15 m (50 ft) or less, and RVR between 200 m and 50 m.
Cat IIIc– to allow aircraft to land on the runway with no cloudbase or visibility restrictions.