FINAL REPORT - Stakeholders - Ofcom

landing. He must carry either a radar altimeter or inner-marker receiver to measure the
decision height. He must also have an autopilot, and two pilots.
For Category I and II operations, two transmissometer measurements are made, one
made near the threshold and the other about midway down the length of the runway
(transmissometers measure the transmission of light through a medium – in this case,
providing information on the medium, since light’s characteristics are well-known). The
latter location provides useful information for aircraft taking off as well as for landing
aircraft during landing roll. The limitation of the transmissometer is that it measures
visibility on the ground, whereas pilots approaching a runway like to know the visibility on
the approach path.
Category III: This category is further subdivided in three classes:
• CAT IIIa: The decision height is lower than 100 ft and the RVR is not less than
700 ft.
• CAT IIIb: The decision height is lower than 50 ft and the RVR is not less than
150 ft.
• CAT IIIc: Zero visibility
These sub-categories can also be expressed as aircraft capabilities – for Category IIIa
operations, an automatic landing capability, i.e. the aircraft touching down on the runway,
is required. For Category IIIb operations, an automatic rollout capability in addition to an
automatic landing capability is required. For Category IIIc operations, the taxiing portion of
the landing must also be automatic.
3.4.3.7 C-Band MLS
Concept
The Microwave Landing System (MLS) originated in the early 1970’s. The MLS is a
precision approach and landing guidance system which provides two or three dimensional
position information and various ground to air data. International standardisation for the
MLS Time Reference Scanning Beam (TRSB) concept was reached by the International
Civil Aviation Organization (ICAO) in 1978.
The main components of MLS are:
• Elevation Subsystem;
• Azimuth Subsystem;
• Back Azimuth Subsystem;
• DME/P.
Technical Characteristics
The MLS duplicates and augments the capabilities of the ILS, which provides a ± 0.7º
proportional guidance region around the glide slope angle and a region of approximately
± 3.0° azimuth about the centreline approach of the instrumented runway. The MLS is
capable of providing a maximum ± 62.0° azimuth coverage region with a typical
installation using only ± 40.0° azimuth coverage region. The MLS elevation signal can
provide coverage up to +30.0° above ground level. The glidepath can be varied from 0.1
to 15°. This could allow helicopters to use an appropriate glidepath.
Two of the transmitters provide MLS azimuth functions; they are located at each end of
the runway, and are positioned facing the runway. With both runway directions equipped,
the azimuth antenna facing the approaching aircraft is configured as the approach
azimuth transmitter and the opposite antenna becomes the back azimuth transmitter. The
approach azimuth transmitter is used to guide the aircraft during an instrument (non-
Page 97