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 european airports
we can rather seldom come across 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 elements 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.
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Figure 1 – A display of the MLS components and their approximate placement beside the runwayGround 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.
Onboard equipment
The onboard equipment has to be able to
decode and process functions of the landing approach azimuth including
one with a high frequency of regeneration, the reverse azimuth, the
angle of descent, and necessary data to accomplish projected flights.
Information about the distance is decoded independently. The homing
angle is determined by measuring the interval between the reception of
the scanning lobes „TO“ and „FROM“. If the equipment is qualified, the
receiver has the option of manual or automatic selection of a landing
approach trajectory, an angle of descent and a reverse azimuth.
Operating in the automatic mode, the selection is made with the aid of
information present in the code names of the primary data.
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 4 – An illustration of the vertical signal’s coverage for various glide slope angles.
All data stated below is gradually transmitted on the same frequency with a repetitive frequency:
In order to maintain a synchronized timing
of the transmission’s individual data blocks, are all parts of the MLS
synchronized. Data about the distance is received separately on an
interconnected DME channel. Utilizing the MLS data with onboard
computers and control systems, it’s possible to carry out a precision
approach and landing in similar fashion as with the ILS system, on top
with the option to execute curved of broken arched trajectories of
approach and automatic landings. All parts of the MLS system include
their own monitor circuits that in the case of an out of tolerance
deviation of some outer MLS parameters switch the devices on a back up
array. In case of a long-time deviation the pilot gives notice about the
change to the traffic control.
The exact information about an aircraft’s
position enables to perform more complicated procedures, as flying along
a curved glide slope or using multiple glide slopes. An appropriate
precision allows to improve the air traffic flow on busy airports
through curved fly paths. ICAO quantifies the required system’s accuracy
as stated in the ICAO regulations Annex 10.
The complete accuracy limits include all
errors caused by the onboard equipment and radio waves broadcast.
They’re specified for a part of the flight path containing the reference
approach altitude and reference missed approach height for a go-around.
The reference landing height is 15 m (50 ft).
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