The microwave landing system (MLS) was designed to succeed the ILS (instrument landing system): the same job, guiding an aircraft to a runway, done without the ILS's two fixed beams. Instead of one centre line and one glide path, the MLS fills a wide volume with scanned microwave signals, and lets each aircraft choose its own path through it.
The five services
The MLS's principle of operation covers five services:
horizontal course guidance during the approach;
vertical guidance during the approach;
horizontal guidance for departure and missed approach;
distance, from a DME (distance-measuring equipment) in its precision form, DME/P;
transmission of special information regarding the system and the approach conditions.
Where it lives on the dial
The name says microwave and the band delivers: MLS operates in the SHF band, on any one of 200 channels, on assigned frequencies, far above the ILS's VHF/UHF pair on the band ladder of the first lesson (SHF: super high frequency). The move up the ladder does more than add channels. The last lesson closed with multipath and beam bends, the reflections that bend an ILS where buildings and terrain crowd the site; the MLS's narrow scanned microwave beams are far less troubled by them, which is why MLS can be installed at aerodromes where, as a result of the effects of surrounding buildings or terrain, ILS siting is difficult.
In the cockpit
Because the signal fills a volume rather than defining one line, the receiver has to be told what to fly: the airborne equipment continuously shows the position of the aircraft in relation to a preselected course and glide path, along with distance information, during approach and departure. Preselected is the working word: the crew choose an approach azimuth and a glide-path angle within the coverage, and the display then reads exactly like an ILS flown on that chosen line. The system goes further: segmented approaches can be carried out, with a presentation of two cross bars directed by a computer which has been programmed with the approach to be flown: the aircraft follows leg after leg of a route through the coverage, the bars always commanding toward the programmed path.
Two practical notes close the cockpit picture. First, hardly any aircraft carries a dedicated MLS box: aircraft are equipped with a multimode receiver (MMR), in order to be able to receive ILS, MLS and GPS (global positioning system): one receiver for every approach aid of its generation. Second, the full flexibility is conditional, and an MLS without DME/P gives an ILS-lookalike straight-line approach: still precise, but a single straight final, exactly what the ILS already offered.
Curved approaches and DME/P
Why does the bending stop without the distance? Because the azimuth signal alone only tells the aircraft which angle from the runway it sits on; where it is along that angle is unknown without range. A curved or segmented path is a sequence of positions, not of angles, so segmented and curved approaches can only be executed with DME/P installed: the precision distance closes the fix, the DME lesson's ranging sharpened for the runway environment. With it, the approach can bend around terrain, noise-sensitive areas or traffic; without it, the MLS flies straight.