Microwave Landing System (TRSB) PDF

Chapter 4 MICROWAVE LANDING SYSTEM (TRSB) Learning Outcomes  Principle of Time Reference Scanning Beams (TRSB)  Sequence of Time Division Multiplexing  Computer and Airborne Interface AVIONIC SYSTEMS MICROWAVE LANDING SYSTEM (TRSB) 4.1 Introduction In 1972 ICAO published an ope...

Chapter 4 MICROWAVE LANDING SYSTEM (TRSB) Learning Outcomes  Principle of Time Reference Scanning Beams (TRSB)  Sequence of Time Division Multiplexing  Computer and Airborne Interface AVIONIC SYSTEMS MICROWAVE LANDING SYSTEM (TRSB) 4.1 Introduction 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 Reference Scanning Beam (TRSB) in the Super High Frequency (3-30 GHz) band. Signals at these frequencies are called microwaves, so the system became known as the Microwave Landing System or MLS. 4.2 Principle of Microwave Landing System (MLS) The justification for MLS system is based on the shortcomings of Instrumentation Landing System (ILS) which will become more significant with future growth of air traffic and need to maintain regularity and safety in all weather conditions. The principle drawbacks of ILS are: 1. Approaches are confined to a single narrow path 2. It suffers from multipath errors from its site position 3. The number of channels are limited 4. The quality of the guidance signals is depend on the nature of the terrain and can, for example, be seriously affected by snowfall. 5. Siting of ILS at some airports can be both difficult and expensive. At a few airports impossible. Practically microwave frequencies are independent of site conditions. Antenna radiation patterns can be tailored to lift the radiation from ground using arrays which, although large electrically, are relatively small in physical terms. ET0171/Chaganti Page 4-1 AVIONIC SYSTEMS 4.2.1 Time Reference Scanning Beams (TRSB) 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, and then back again after a specific time interval. The signal will be received twice at the airborne equipment, and the time between each signal relates to the angle from the reference line, which is the position of the beam when it starts its sweep. This can be seen in Figure 4-1. One fan beam sweeps horizontally to provide a position line azimuth, and at different time horizontally oriented fan sweeps up and down in a similar fashion to give a 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 Distance Measuring Equipment (precision 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. 4.2.2 Time-Division Multiplexing Technology allows every piece of information from each of the azimuth and elevation means 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), as shown in Figure 4-2. This is called multiplexing. ET0171/Chaganti Page 4-2 AVIONIC SYSTEMS -400 -400 ‘to’ scan ‘fro’ scan AZIMUTH AERIAL RUNWAY CENTRE LINE θ θ +400 +400 RECEIVED SIGNALS AMPLITUDE MEASURED THRESHOLD ‘to’ scan Time Interval is ‘fro’ scan ends Directly Related to ends Azimuth Angle θ Figure 4-1 TRSB Beam Principle ET0171/Chaganti Page 4-3 AVIONIC SYSTEMS missed flare approach flare elevation auxiliary elevation flare auxiliary auxiliary auxiliary elevation flare approach flare elevation approach azimuth azimuth data data data data azimuth 0 5.2 10.2 26 31 36.2 48 53.8 59 64 70.6 77.2 84 89.2 94.2 110 115 Time (ms) typical angle function ‘to’ guard ‘fro’ preamble scan time scan 10.2 26 ms Figure 4-2 TRSB Time-Division Multiplex Sequence ET0171/Chaganti Page 4-4 AVIONIC SYSTEMS 4.2.3 Auxiliary Data 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. Every piece of information includes a preamble to synchronise and prepare the relevant part of the airborne equipment for function transmission which contains the actual information signal, for example the beam sweep. 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. These are also 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. The time is not equally divided. Three elevation signals are received for every azimuth signals. 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. 4.2.4 Frequencies The time multiplexing technique allows all function to take place on a single channel. 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. 4.2.5 Azimuth and Elevation Coverage Azimuth and Elevation coverage for MLS system (with reference to runway) is shown in Figure 4-3 and Figure 4-4. ET0171/Chaganti Page 4-5 AVIONIC SYSTEMS Approach Missed Approach RUNWAY 0 20 400 10 nm 22.5 nm Figure 4-3 MLS Azimuth Beam 20 000 ft 10 000 ft 10 nm 200 200 22.5 nm Figure 4-4 MLS Elevation Beam ET0171/Chaganti Page 4-6 AVIONIC SYSTEMS 4.3 Airborne System Interface The block diagram of MLS system is shown in Figure 4-5. ANTENNA INDICATOR RECEIVER/ COMPUTER AUTOPILOT CONTROL PANEL Figure 4-5 MLS System Interface The control panel selects operational channel, the receiver provides reception and demodulation of the received pulses and then make elapsed time measurements on them. Digital signal processing is handled by a microcomputer inside the receiver, which also perform control functions required for the MLS receiver. The decoded preamble will be used to determine many of the parameters required for the elapsed time information. ET0171/Chaganti Page 4-7 AVIONIC SYSTEMS Computed angular information may be used in several ways. First, and perhaps most important, the angular information, both azimuth and elevation, will be detected from the receiver to be used with other equipment, such as an autopilot. The MLS receiver will be used primarily in precision approaches where an auto pilot is required. In this case of automatic landing, the information from the azimuth and elevation and the flare guidance scanning beam will be used to perform a completely automatic touch down without pilot intervention. For low cost system for use in a light aircraft, the pilot will be able to set in a glide path angle, and the receiver will drive a conventional dual pointer indicator. Example 4-1: How much Doppler shift would exist for a MLS signal at 5.03 GHz, received by an approaching aircraft moving at 135 knots (69 meters per second) ground speed? Doppler shift f   v  3  10 8 Wavelength ( )  5.03  10 9  0.06 meters 69  f  0.06 1150 Hz This amount of Doppler shift is rather low, but relative to the band-width of the MLS signal this amount of Doppler shift is significant. ET0171/Chaganti Page 4-8 AVIONIC SYSTEMS Notes ET0171/Chaganti Page 4-9