INTRODUCTION

The MLS is a system of precission approach for landing by instruments and constitutes a kind of an alternative to the ILS system. It provides information about the azimuth, optimal angle of descent and the distance, as well as data about the reverse course in case of an unsuccessful approach. It has several advantages compared to the ILS, for example a greater number of possible executed approaches, a more compact ground equipment, and a potential to use more complicated approach trajectories. However for certain reasons, in particular the advancement of the GPS satelite navigation, was the installation of new devices halted and finally in 1994 completely canceled by the FAA organization. On rather seldom come across european airports we can an MLS.

The MLS provides an accurate landing approach for an aircraft in the area of the final approach, where the path of the final approach isn’t identical with the enlonged runway’s axis. The system works with a microwave beam that is transmitted towards the sector of approach and scans the sector both in the horizontal as well as the verical plane. An aircraft in the approach sector receives the signal and with the help of this beam evaluates it’s location in space. The aircraft’s position is therefore determined both in the horizontal direction of approach and the vertical plane, in whatever point of reach of the scanning beam. Because the microwave technology is radiated into the space of approach in a given time and it’s not spread out over different directions, no signal interruption results from various obstacles or terrain protrusions as it was with the ILS system. The MLS system can thus be situated also in developed areas, where an ILS system couldn’t be set up. An onboard computer enables to solve the approach manoeuvre from a random direction, for variously oriented runways, even along a curved of bend landing trajectory. The MLS system is approved by the ICAO for every three categories of an accurate landing approach.

BASIC ELEMENT OF THE MLS

The MLS system is comprised of ground pieces of equipment that are divided into the protractor components, rangefinder components, and the onboard hardware. The information about the angles of the approach course, descent, flare and the course of an unsuccessful approach are aquired through an onboard antenna or the aircraft itself by measuring the time between two passages of an oscillating lobe of a high frequency signal . The distance is determined with the help of an ancillary device, the DME rangefinder. The MLS system further sends with the help of phase modulation and time-division multiplexing additional data, as identification, system status and so on. The ground equipment consists in the basic configuration of an Azimuth Transmitter (AZ) with an added DME rangefinder, perhaps even a more precise DME/P, in close distance of a course transmitter and near an elevation transmitter, see Fig. 1. A scaled up configuration is supplemented with a course transmitter for an unsuccessful approach and a flare transmitter.

Figure 1 – A display of the MLS components and their approximate placement beside the runway.
(figure source: http://oea.larc.nasa.gov/trailblazer/SP-4216/photos/p42a.JPG)

Ground Distance Measuring Equipment (DME)

The rangefinder unit presents a DME which is positioned together with the course transmitter. In connection with requirements of accuracy of the MLS system arose a demand to refine the DME system, which was accomplished with the accurate DME/P rangefinder (along with the DME/W and DME/N). Hence the function of the DME is to provide a pilot information about the distance from a specific point which is essential for pinpoint calculation of the plane’s position in the three-dimensional space.

Ground protractor components

The ground principle of both protractor parts of the MLS system for horizontal and vertical homing of an aircraft is to create levelled emiting diagrams, oscillating at a constant speed in directions „TO‘‘ and „FROM“, and to measure the elapsed time between two passages of an oscillating plane lobe through an onboard MLS antenna.

Figure 2 – Scheme of a ground protractor set-up of the MLS system.

A runway fully equipped with the MLS system contains four transmitters. Two relays supply information about the angle of the azimuth (horizontal) plane and are located face to the runway, along it‘s axis. They are appended with a DME or DME/P rangefinder device, while one of the transmitters is dessignated for the course of approach and the other for the course of an unsuccessful approach. They are positioned 400-600 m from the runway’s threshold. Another two relays transmit angular information for the descent and flare (taking over the function of a descent beacon in the ILS). These are located at a distance of 120-150 m from the runway’s axis, while the transmitter of descent signals is situated 200-300 m from the runway’s threshold and the flare relay 700-1000 m from the beggining of the runway in the direction of approach. If the runway’s equipped with both azimuthal relays, then the relay whose antenna is turned in the direction of an approaching aircraft (the transmitter on the faraway side of the runway) represents an approach course transmitter and the relay close to the approaching aircraft takes over the function of an unsuccessful course transmitter. It’s similar also for the descent and flare relays.

PRINCIPLE OF OPERATION

The MLS system operates at a frequency band of 5031,0 – 5090,7 MHz on two separate channels at a mutual interval of 300 kHz. The protractor part of the MLS system provides continually information about an aircraft’s position relative to the runway both in the vertical and horizontal plane. The rangefinder part enables to measure the distance between an aircraft and the reference points in the approach process. The angular information for the approach course, descent, flare and go-around is determined by measuring the interval between two passages of an oscillating plane lobe through an onboard MLS antenna.

The MLS system is capable to provide coverage of maximum ± 60.0° in the azimuthal (horizontal) plane, whereby a typical device makes use of only ± 40.0° from the runway’s axis in the azimuthal plane for the final approach and ± 20.0° for a missed approach course, see Fig. 3. Of which the minimal ordained proportional homing sector is ± 10.0° from the runway’s axis. Thereafter is the space covered in the vertical plane from 0.9° to 15° with a coverage up to an altitude of 6000 m, for an approach distance of 37 km (see Fig. 4) and to a height of 1500 m and distance of 9,4 km for a missed approach.

Figure 3 – An illustration of the horizontal signal’s coverage and it’s oscillation.
(figure source: http://accessscience.com/loadBinary.aspx?filename=424150FG0020.gif)

A. SYSTEM DESCRIPTION

The time-referenced scanning beam Microwave Landing System (MLS) has been adopted by ICAO as the standard precision approach system to replace ILS. MLS provides precision navigation guidance for alignment and descent of aircraft on approach to a landing by providing azimuth, elevation and distance. The system may be divided into five functions:

1.     Approach azimuth;

2.     Back azimuth;

3.     Approach elevation;

4.     Range; and

5.     Data communications.

With the exception of DME, all MLS signals are transmitted on a single frequency through time sharing. Two hundred channels are available between 5031 and 5090.6 Megahertz (MHz). By transmitting a narrow beam which sweeps across the coverage area at a fixed scan rate, both azimuth and elevation may be calculated by an airborne receiver which measures the time interval between sweeps. For the pilot, the MLS presentation will be similar to ILS with the use of a standard CDI or multi-function display,

B. ILS LIMITATIONS

The Instrument Landing System (ILS) has served as the standard precision approach and landing aid for the last 40 years. During this time it has served well and has undergone a number of improvements to increase its performance and reliability. However, in relation to future aviation requirements, the ILS has a number of basic limitations:

1.     site sensitivity and high installation costs;

2.     single approach path;

3.     multi path interference; and

4.     channel limitations - 40 channels only.

C. MLS ADVANTAGES

As previously mentioned, ILS has limitations which prohibit or restrict its use in many circumstances. MLS not only eliminates these problems; but also offers many advantages over ILS including:

1.     elimination of ILS/FM broadcast interference problems;

2.     provision of ail-weather coverage up to ±60 degrees from runway centerline, from 0.9 degree to 15 degrees in elevation, and out of 20 nautical miles (NM);

3.     capability to provide precision guidance to small landing areas such as roof-top heliports;

4.     continuous availability of a wide range of glide paths to accommodate STOL and VTOL aircraft and helicopters;

5.     accommodation of both segments and curved approaches;

6.     availability of 200 channels - five times more than ILS;

7.     potential reduction of Category I (CAT l) minimums;

8.     improved guidance quality with fewer flight path corrections required;

9.     provision of back-azimuth for missed approaches and departure guidance;

10. elimination of service interruptions caused by snow accumulation; and

11. lower site preparation, repair, and maintenance costs.

D. APPROACH AZIMUTH GUIDANCE

The approach azimuth antenna normally provides a lateral coverage of 40º either side of the center of scan (see MLS Azimuth and Elevation Coverage figure, below). Coverage is reliable to a minimum of 20 NM from the runway threshold and to a height of 20,000 feet (ft). The antenna is normally located about 1000 feet beyond the end of the runway.

E. BACK AZIMUTH GUIDANCE

The back azimuth antenna provides lateral guidance for missed approach and departure navigation. The back azimuth transmitter is essentially the same as the approach azimuth transmitter. However, the equipment transmits at a somewhat lower data rate because the guidance accuracy requirements are not as stringent as for the landing approach. The equipment operates on the same frequency as the approach azimuth bur at a different time in the transmission sequence. On runways that have MLS approaches on both ends, the azimuth equipment can be switched in their operation from the approach azimuth to the back azimuth and vice versa. The MLS Azimuth and Elevation Coverage figure, below, shows MLS azimuth coverage volumes.

F. ELEVATION GUIDANCE

The elevation station transmits signals on the same frequency as the azimuth station. The elevation transmitter is normally located about 400 ft from the side of the runway between the threshold and the touchdown zone. The MLS Azimuth and Elevation Coverage figure, above, shows coverage volumes for the MLS elevation signal. It allows for a wide range of glide path angles selectable by the pilot.

G. RANGE GUIDANCE

Range guidance, consistent with the accuracy provided by the azimuth and elevation stations, is provided by the MLS precision DME (DME/P). DME/P has an accuracy of +100 ft as compared, with + 1200 ft accuracy of the standard. DME system. In the future it may be necessary to deploy DME/P with modes which could be incompatible with standard airborne DME receivers.

H. DATA COMMUNICATIONS

The azimuth ground station includes data transmission in its signal format which includes both basic and auxiliary data. Basic data may include approach azimuth track and minimum glide path angle. Auxiliary data may include additional approach information such as runway condition, wind-shear or weather.

An all-weather aircraft landing-guidance system that operates at microwave frequencies and provides deviations from the landing runway centerline using a time-referenced scanning beam (TRSB) technique. The MLS was standardized in 1988 and approved for use in international civil aviation until at least the year 2020. MLS is used to support low-visibility instrument precision approach and landing operations in North America and Europe. In addition to the fixed-base MLS equipment design, a compact mobile microwave landing system (MMLS) equipment design exists. The instrument landing system (ILS) is also standardized internationally and approved for use indefinitely as countries implement their transition to new technologies. Standards for a third landing system, the Global Navigation Satellite System (GNSS), based primarily on Global Positioning System (GPS) technology, exist. Multimode receivers enable an aircraft to conduct an instrument approach using ILS, MLS, or GNSS.