There are 3 categories of precision approach: CAT I, CAT II and CAT III (spelled CAT1, CAT2 and CAT3). The localizer course is very narrow, normally 5. These categories have been defined in order to allow suitably qualified pilots flying suitably equipped aircraft to suitably equipped runways using appropriately qualified ILS systems. The localizer provides course guidance, transmitted at 108.1 to 111.95 MHz (odd tenths only), throughout the descent path to the runway threshold from a distance of 18 NM from the antenna to an altitude of 4,500 feet above the elevation of the antenna site. Wherever theres an ILS, the back of the localizer beam extends in the other direction. Just as a conventional VOR radial extends in both directions from the station, so also does a localizer. ILS approaches are defined by precision approach categories. Recall that the localizer portion of an ILS is essentially a VOR with a single radial. At the DH, the approach may only be continued if the specified visual reference is available otherwise, a go-around must be flown. DH shall be published on instrument approach charts (IAC) of the concerned airfield. The pilot should follow the ILS guidance until the decision height (DH) is reached. It should not be expected that the glide path will provide guidance to the touchdown point on the runway.Īn approach may not normally be continued unless the runway visual range (RVR) is above the specified minimum. The glide path provided by the glide slope transmitter is arranged so that it flares from 5 to 8m (18 to 27ft) above the runway. The glide slope is normally usable to a distance of 10NM (it can be extended when requested). The glide path projection angle is normally adjusted to 3° above the horizontal plane so that it passes through the middle marker at about 60m/200ft and the outer marker at about 426m/1400ft. It transmits a glide path with a beam width of 1.4°. The glide scope transmitter is located between 230m/750ft and 380m/1250ft from the approach end of the runway and offset between 75m/250ft and 198m/650ft from the runway centre line. The glide slope frequency is usually paired with the localizer frequency as the pilot enters only the localizer frequency in the aircraft instruments. The glide path radiates its signal only in the direction of the localizer front course.
The glide slope transmitter operates on one of 40 ILS channels within the ultra-high frequency (UHF) band from 329.15 MHz to 335 MHz.