ILS: localiser, glidepath and DME

What it is

• The ILS gives an approaching aircraft a radio beam to follow: the localiser (LOC) for left/right guidance along the runway centreline and the glidepath (GP) for the descent angle. A DME gives the range.
• Prestwick has ILS CAT I on both ends of the main runway (UK AIP AD 2.19): RWY 12 “IPP” and RWY 30 “IKK”, both localisers on 110.300 MHz with glidepaths on 335.000 MHz, and a shared DME on channel 40X that reads zero at each threshold. CAT II/III are listed as not applicable.

How controllers use it

• The two ends share one frequency, so they are interlocked: only one may radiate at a time, and the other cannot be switched on locally or remotely (CAP 670 ILS10.27–29). When the runway changes, the ILS direction is changed with it.
• The tower watches a navaid serviceability indicator (CAP 670 ATC01.6) and tells pilots if the ILS is degraded or off. The AIP tells pilots not to descend below 2,000 ft before intercepting the glidepath unless radar-directed.
• Example: with runway 30 in use, IKK radiates on a 3.5° glidepath while IPP stays off; if the wind swings round and runway 12 comes into use, the ILS is switched to IPP on a 3° path.

UK rules and practice

• CAP 670 recommends standby equipment with automatic changeover for CAT I systems and standby batteries (ILS10.32–34).

At Prestwick

• RWY 12 (IPP): glidepath 3°, reference datum height 53 ft. The AIP warns the guidance does not permit coupled approaches below 200 ft.
• RWY 30 (IKK): glidepath 3.5°, reference datum height 56 ft. A weak signal may be experienced below the glidepath at 8° right of the centreline.
• A 2026 tender covers replacing both ILS and the DME for runway 12/30 (contract 8 Jun to 7 Dec 2026).

// Through the eras