MLS – the original concept of MLS was as the ICAO standard approach aid to replace ILS.
In the late 1970s ILS was projected to suffer co-channel interference problems with a rapidly expanding industry. Most Cat 1 operations could tolerate some interference, but for Cat 2 and particularly Cat 3 this was seen as a major safety issue for autocoupled approaches.
History shows otherwise, with a less than anticipated runway expansion, and lower Cat 3 demand, also that modern ILS / auto flight systems and careful ILS location reduces the problem.
The US won the MLS competition for the system design (time ref scanning beam) which like other systems could offer area navigation type approaches, but the straight-in ILS replacement was the priority, and this, particularly for Cat 1 and the urgent safety upgrade of NPAs to ILS, could be met by the emerging GPS with FMS R Nav.
The FAA quietly dropped MLS in favour of GPS, leaving Europe to solve the higher integrity problems for Cat 3 ILS. Some US views will claim GPS capable Cat 3, but perhaps the integrity is not yet as that required in Europe.
What type of aircraft uses the 5.5 deg slope ?
Some time since I visited LCY:-
Originally Dash 7 at 7.5 deg, then BAe146 / Avro RJ (all variants) at 5.5 deg.
Fokker tried the F70 (not very successfully ?), Saab 340, and other turboprops … ???
Airbus 318, and several Biz Jet types … .
Flight Path Angle (FPA) is a useful instrument aid, but its use during approach and landing requires knowledge of where the intended point of touchdown is.
Hence some of the first applications were displays on HUD where the FPA could be overlaid on the runway – but this also required a marker which designated the required flight path angle, so that when all were coincident the aircraft was on the correct approach path.
The similar head-down application requires an electronic (ground referenced) glideslope to determine the point of landing, hence use of FPA as/with a FD.