MLS

الرحيم الرحمن الله بسم

MICROWAVE LANDING SYSTEM (MLS)

GROUP MEMBERS

Muhammad Waqar (05-CTE-19)

H.M Farhan GUL (05-CTE-32)

GROUP MEMBERS

Runway Visual Range (RVR):

The distance over which the pilot of an aircraft (on the centre-line of the runway) can see the runway surface markings of the light, is defines as RVR.

SOME RELATED TERMS

Decision Height:

It is the minimum height at which the pilot will make the crucial decision of either to land or to abort the attempt to land.

SOME RELATED TERMS

DME: Distance Measuring Equipment gives the air distance between a particular VOR (very high frequency Omni range) station and the aircraft. This facility of DME is invariably available at all VOR stations.

SOME RELATED TERMS

VOR: Very high frequency Omni range (VOR) stations give out radio signals in all directions at 1o intervals producing 360 radials. Each signal, usually of about 350km range, can be considered as a route or course that can be followed by an aircraft, by the use of cockpit instrument.

SOME RELATED TERMS

Flight Rules: Air Traffic is operated under two specific categories of flight rules.1) Visual Flight Rules(VFR)2) Instrument Flight Rule (IFR)

SOME RELATED TERMS

Operation under VFR are only possible when weather conditions are good enough for aircraft to visually manage safe distances and levels with respect to aircraft and the ground. In VFR conditions, responsibility lies with the pilot and there is a little air traffic control, as aircrafts are allowed to fly on the “see and be seen” principle.

VISUAL FLIGHT RULE (VFR)

When the visibility is poor or the cloud ceiling is below the requisite standards, IFR conditions prevail. In these conditions, the responsibility lies with air traffic control.

NOTE: The choice depends on weather conditions and prevailing traffic densities.

INSTRUMENT FLIGHT RULE (IFR)

PRECISION APPROACH AND LANDING SYSTEM

• Approach – generally the phase flight immediately preceding landing

• Non-precision Approach– Only horizontal guidance provided

• Precision Approach– Vertical and horizontal guidance provided

• Precision Approach Categories• Cat I: 200 Ft. ceiling 2600 Ft. visibility• Cat II: 100 Ft. decision height, 1200 Ft. RVR• Cat III:

– subcat a: 0 Ft. decision height, 700 Ft. RVR– subcat b: 0 Ft. decision height, 150 Ft. RVR– subcat c: 0 Ft. decision height, 0 Ft. RVR


• Note: The higher the category, the more stringent the requirements for ground and air installations and pilot qualifications

• e.g. – extra runway lighting (centerline), approach

lighting– redundant transmitters (hot spares)


• Precision Instrument Approach Systems– Instrument Landing System (ILS)–Microwave Landing System (MLS)–Differential GLS (Local and WAAS)


We switched from Instrument Landing System (ILS) to Microwave Landing System (MLS) due to limitations of ILS. The ILS has a number of basic limitations:

• Site sensitivity and high installation costs • Single approach path • Multi path interference• Channel limitations - 40 channels only. • Limited glide path angle (5 degree?)• Limited to “straight in” approaches

WHY WE SWITCHED FROM ILS TO MLS?

In the mid 1970’s the US was running into ILS frequency congestion problems in the North Eastern part of the country. (the 40 channel problem)

In an attempt to alleviate the situation, they proposed that ICAO has to specify a new type of landing aid that would use microwave frequencies (specifically about 15 GHz)

In response, two techniques were proposed. The US and Australia proposed a Time Referenced Scanning Beam (TRSB) system and the British proposed a Doppler system.


BACKGROUND

Because there was very little difference between the two systems and because there was perceived to be a great deal of economic benefit to the “winners”, the selection process became almost entirely political.

To no one’s surprise, the US/Australian system was adopted.

Unfortunately, the FAA, which was given the job of introducing the MLS into the civil aviation system, failed completely.

In 1994, the US government issued a statement that no further work would be done on MLS and that GPS would be used in stead.


BACKGROUND (CONTINUED)

This turned out to be premature. At the present time, only WAAS-based procedures have been certified for use and they do not quite meet Cat I requirements. LAAS for Cat II and III are still in the pre-certification stage.

Thus the door is still open for MLS to stage a comeback.

Of interest is that NASA uses a 15 GHz TSB MLS for landing the Space Shuttle. (Glide Path angle 19)


BACKGROUND (CONTINUED)

The Microwave Landing System (MLS) is an all-weather, precision landing system originally intended to replace or supplement the Instrument Landing System (ILS) MLS has a number of operational advantages, including a wide selection of channels to avoid interference with other nearby airports, excellent performance in all weather, and a small "footprint" at the airports.


http://www.powerset.com/explore/semhtml/Instrument_Landing_System

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.


Frequency of Operation:

5.031 to 5.0907 GHz

300kHz spacing (200 channels)


The system may be divided into five functions:

OPERATIONAL FUNCTIONS OF MLS

APPROACH AZIMUTH

BACK AZIMUTH

APPROACH ELEVATION RANGE

The azimuth station transmits MLS angle and data on one of 200 channels within the frequency range of 5031 to 5091 MHz and is normally located about 1,000 feet (300 m) beyond the stop end of the runway, but there is considerable flexibility in selecting sites. The azimuth coverage extends: Laterally, at least 40 degrees on either side of the runway centerline in a standard configuration. In elevation, up to an angle of 15 degrees and to at least 20,000 feet (6 km), and in range, to at least 20 nautical miles (37 km)

APPROACH AZIMUTH GUIDANCE

Coverage Volume of the Azimuth station


tp://www.powerset.com/explore/semhtml/Image:F0101010.gif


Coverage Volume of the Azimuth station

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.

BACK AZIMUTH GUIDANCE


The equipment operates on the same frequency as the approach azimuth but 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 elevation station transmits signals on the same frequency as the azimuth station. A single frequency is time-shared between angle and data functions and the elevation transmitter is normally located about 400 feet from the side of the runway between runway threshold and the touchdown zone. It allows for a wide range of glide path angles selectable by the pilot.

ELEVATION GUIDANCE

ELEVATION GUIDANCE

Elevation coverage is provided in the same airspace as the azimuth guidance signals: In elevation, to at least +15 degrees; Laterally, to fill the Azimuth lateral coverage and in range, to at least 20 nautical miles (37 km)

Coverage Volumes of the Elevation station

ELEVATION 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.

RANGE GUIDANCE

The MLS Precision Distance Measuring Equipment (DME/P) functions the same as the navigation DME, but there are some technical differences. The beacon transponder operates in the frequency band 962 to 1105 MHz and responds to an aircraft interrogator. The MLS DME/P accuracy is improved to be consistent with the accuracy provided by the MLS azimuth and elevation stations.A DME/P channel is paired with the azimuth and elevation channel.

RANGE GUIDANCE

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.

DATA COMMUNICATIONS

Angular position is determined by measuring the time of detection of a beam which is being scanned at a predetermined rate

Example – Azimuth (horizontal):

The azimuth beam is shaped as follows:

Top View

Side View


PRINCIPLE OF OPERATION

The beam is fan-shaped, that is, very narrow in the measurement (azimuth) direction and fairly wide in the other (vertical) direction meaning that it can be used at reasonably high angles

Top View

Side View



The beam is swept back and forth at a controlled rate

The amplitude of the sweep depends on the requirements of the system but is nominally 40



An aircraft thus detects the beam twice per period; once on the “to” sweep and once on the “fro” sweep. The receiver measures the time between the two detections or pulses.

t



To accommodate all of the required measurements (Az, El, Back AZ and Flare), each is assigned a time slot in a cycle of measurements which takes 115 milliseconds (ms). This is called time multiplexing.


PRINCIPLE OF OPERATION - TIME MULTIPLEXING

-40

+40

0

T0

t

Dwell Time

Since the rate of scan and the dwell time are known, the angle θ can be determined from the measurement of t.

θ

)( 02 tTV Where V is the scan rate = 0.02/s


PRINCIPLE OF OPERATION – BEAM TIMING

The antennas were much smaller, due to using a higher frequency signal. They also did not have to be placed at a specific point at the airport, and could "offset" their signals electronically. This made placement at the airports much simpler compared to the large ILS systems, which had to be placed at the ends of the runways and along the approach path.

MLS ANTENNAS

Azimuth Antenna Installation


Elevation Antenna Installation

An MLS azimuth guidance station with rectangular azimuth scanning antenna with DME antenna at left


MLS signals covered a very wide fan-shaped area off the end of the runway, allowing controllers to vector aircraft in from a variety of directions. In comparison, ILS required the aircraft to fly down a single straight line, requiring controllers to distribute planes along that line. MLS allowed aircraft to approach from whatever direction they were already flying in, as opposed to flying to a parking orbit before "capturing" the ILS signal. This was particularly interesting to larger airports, as it potentially allowed the aircraft to be separated horizontally until much closer to the airport.

MLS SIGNALS

• Less susceptible to sitting (reflection) problems

• Selectable glide path angles (up to 20) and azimuth approach paths

• Possibility of curved approaches• Much less susceptibility to interference• Many more channels available• Increases runway usability in IFR

conditions


ADVANTAGES OVER ILS


ADVANTAGES OVER ILS

• Smaller antennas due to use of high frequency signal

• Did not have to be placed at specific point of airport as in ILS

• Single frequency broadcasting• Greater accuracy than ILS (e.g. in case

of range with DME +/- 1200ft. in ILS and +/- 100ft. In MLS)

• Signal coverage is more

Many countries in Europe are interested in MLS for Cat II and Cat III operations because they are getting tired of waiting for LAAS. They are facing much more interference from their FM stations because they are permitted to use much higher power than in North America.

Four MLS facilities were installed at London Heathrow Airport in 2003 and British Airways has equipped 60 Airbus A320 aircraft with receivers. (2003)


THE FUTURE

1) www.wikipedia.com2) http://www.aviationpublishers.com3) Airport Design and Operation (second Edition) By Antnin Kazda & Robert E.Caves4) Airport Engineering By G .Venkatappa Rao 5) www.mimi.hu6)http://www.centennialofflight.gov/essay/Government_Role/landing_nav/POL14.htm

REFERENCES

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THANKS

Documents

MLS