Introduction
46. As seen in the paragraph 36, an ILS suffers from major problems in avoiding multipath errors from its site position. An ILS installation on many aerodromes cannot be sited free enough of surrounding terrain or buildings to allow even Cat I approaches, let alone the Cat III approaches they desire. There are also other problems like the limited number of channels available and the restriction on approach paths which force aircraft with different performances to queue up to use it. There were also problems with ILS signals in precipitation, especially snow.
47. In 1972 ICAO published an operational requirement for a new type of non-visual approach and landing guidance system. This was to use a method called 'Time Referenced Scanning Beam’
(TRSB) in the SHF band. Signals at these frequencies are commonly called 'microwaves', so the system became known as the microwave landing system or MLS.
Time Referenced Scanning Beams
48. Basic Principle. The idea is for a ground station to sweep a narrow fan-shaped beam at a very accurate constant speed from one side of a sector to the other, then back again after a specific time interval (the 'guard time'). The signal will be received twice at the airborne equipment, and the
time between each signal
position line in elevation. The angle of approach is now known in both azimuth and elevation, and can be displayed in a similar fashion to that of ILS. The third part of the system consists of an accurate DME (precision DME or DME/P) signal to show the aircraft's position in range from the station. The aircraft's position can thus be determined in three dimensions. An aerial which transmits the beam scanning in the vertical plane is given in Fig 8-11.
Fig 8-10: TRSB Principle
Multiplexing
49. Technology allows every piece of information from each of the azimuth and elevation beams to be
obtained from signals on the same frequency. Each piece of information requires a very short time to obtain it. After one piece has been received it is used and stored until it is replaced. Meanwhile, another piece of information can be received, and again used and stored, then another. The total time taken to receive every piece of information required for the MLS system to function in this fashion is about 84 milliseconds. This is divided into specific periods or bands in which the individual pieces of information are transmitted (and received). This is called multiplexing.
Fig 8-11: MLS Aerial Transmitting Vertical Scanning Beam
50. In addition to guidance information, auxiliary information is also sent during the multiplex transmission. This includes the station identification, safety information such as the minimum safe glideslope angle, and more sophisticated information such as system condition, weather and runway conditions which can be displayed on modern cockpit displays if fitted. Every piece of information
includes a 'preamble' to synchronise and prepare the relevant part of the airborne equipment for the following 'function transmission' which contains the actual information signal, for example the beam sweep.
51. Beams can also scan in the opposite direction, away from the approach path, to provide guidance to aircraft in the missed approach segment. These are also useful on climb out after take-off. There are also test pulses, to check the serviceability of the system, and indicator pulses to give general guidance in the area between the approach and missed approach segments to guide the aircraft into the approach segment. Other signals may be transmitted to give guidance at the flare on touchdown for Category III approaches. The time is not equally divided. Three elevation signals are received for every azimuth signal. This indicates the greater danger of a rapid change in elevation angle compared with a change in azimuth angle. There are 40.5 elevation scans every second, and 13.5 azimuth scans.
52. Frequencies. There are 200 allocated channels, spaced 300 kHz apart in the band between 5031.00 and 5090.70 MHz. Each station uses one channel for all its transmissions except the DME/P, which uses similar frequencies to a normal DME. The DME/P frequencies are automatically selected.
53. Azimuth Coverage. In the approach segment, the horizontal area scanned by the guidance beams is +40° of the centreline, out to 22.5 nm from the station, although proportional guidance may be restricted to within 10° of
the centreline. Vertically, the elevation of 7.5°. There is a region over the runway in
either side of the centre line, and from 0.9° to 15°
vertically, out to 10 nm from the station and up to 10000’.
Fig 8-12: MLS Azimuth Guidance Horizontal Coverage
Fig 8-13: MLS Azimuth Guidance Vertical Coverage
A horizontal representation of the azimuth guidance coverage is at Fig 8-12, and a vertical representation at Fig 8-13.
54. Elevation Guidance Coverage. The specification for elevation coverage is slightly different. It only covers the approach sector, out to at least 20 nm, and within a horizontal angle which at least corresponds to that within which the azimuth proportional guidance is available.
In elevation, the minimum coverage is from 0.9°
to 7.5° above the horizontal, although it is recommended that it cover the whole of the azimuth approach sector. Representations of the coverage are at Fig 8-14 and Fig 8-15.
Fig 8-14: MLS Elevation Beam Horizontal Coverage
Fig 8-15: MLS Elevation Beam Vertical Coverage
55. DME/P. Like the DME stations used for ILS ranges, the DME/P is electronically adjusted to give ranges from touchdown. Correct indications are available within the coverage of the guidance beams in the approach sector.
Use of MLS
56. Approaches. The aim of the system is to allow aircraft to make approaches from whichever direction they may be coming towards the runway, with guidance down an accurate path to a decision height or to a position from where an automatic landing can be made. The circling approach requires a computer, or central processing unit (CPU) which can compute a safe three-dimensional approach path and compare the aircraft's actual path with the pre-computed one.
57. Aircraft without such a sophisticated CPU can still use the equipment, but only on the 'segmented' approaches. If the DME/P is not available, approaches can still be made, but only on a straight, ILS type, approach.
58. Accuracy. ICAO Annex 10 lays down the accuracy which must be achieved by a MLS system. Every requirement must be met at least 95% of the time. The most important requirement must be met at the MLS approach reference point, which like the ILS reference point is 50 ft above the point where the runway centreline intersects the threshold. Azimuth accuracy must be within 20 ft, elevation accuracy within 2 ft.
59. Monitoring. Each ground station monitors its output, and switches off the signal if any of
the following takes place:
(a) Transmitter power reduces below minimums.
(b) An error in the preamble transmissions.
(c) If the permissible accuracy at the reference point is exceeded for more than one second.
(d) The time synchronisation of the signals fails.
60. Errors. MLS is not immune from interference from vehicles or aircraft near the transmitter.
Multipath errors are still present. However, the interference from nearby fixed objects can be reduced by interrupting the scanning beam as it passes through such obstacles. At long ranges, the signal will have bent slightly so that the interruption is not noticed, and at short ranges the aircraft will not be in that part of the beam.
Airborne Equipment
61. Despite being mandated by ICAO, MLS has not been universally accepted as an ILS replacement. DGPS has been put forward as an alternative, and MLS development was discouraged.
Such uncertainty has led to manufacturers producing receivers which can accept signals from both new systems and display them. In fact, in order to minimise weight and space requirements during the transition from ILS to the new system, the receivers include the facility to receive and display ILS signals also. These are 'multi-mode' receivers.
62. Computer and microengineering technology allows inputs from each of the systems' aerials to be individually processed in modules within the same 'box' fitted to the aircraft. The various modules can then be selected for further processing within a main processor, and displayed on either a CDI, HSI or EFIS. Monitoring of the various signals can be carried out within the different modules, and failure indications displayed as necessary.
Advantages of MLS over ILS
63. The advantages of MLS over ILS are:
(a) The primary advantage is the freedom from siting errors. Large objects do not create multipath errors.
(b) MLS allows a variety of approaches, from many directions, including curved approaches, and a variety of possible glideslopes to cater for each aircraft's ideal performance.
(c) Aircraft can approach from different directions, rather than having to fly to a position
on the extended centreline of the runway in use. This expedites traffic flow in terminal areas.
(d) The different possible approach directions allow aircraft to effectively overtake as they descend, increasing the number of aircraft which can approach at one time and increasing the capacity.
(e) Guidance is more accurate with MLS than ILS.
CHAPTER 9