Precision Approach and Landing Systems

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Precision Approach and Landing Systems

• Approach generally the phase flight
  immediately preceding landing
• Non-precision Approach
• Only horizontal guidance provided
• Precision Approach
• Vertical and horizontal guidance provided

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Precision Approach and Landing Systems

• Precision Approach Categories
• Cat I 200 Ft. ceiling 2600 Ft. visibility
• Cat II 100 Ft. decision height, 1200 Ft. RVR
• Runway Visual Range
• 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

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Precision Approach and Landing Systems

• Note The higher the category, the more stringent
  the requirements for ground and air installations
  and pilot qualifications
• e.g.
• extra runway lighting (centreline), approach
  lighting
• redundant transmitters (hot spares)
• autoland

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Runway Visual Range
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Approach and Runway Lighting
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Precision Approach and Landing Systems

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

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Instrument Landing System (ILS)

• ICAO standard approach system
• Developed in late 1940s
• Over 110 installed in Canada

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Instrument Landing System (ILS)

• Components
• Localizer - horizontal guidance.
• Glide Path (or Glide Slope) - vertical guidance
• Marker Beacons - along - track position fixes
  (obsolete)
• DME (sometimes)
• Approach Lighting

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Instrument Landing System (ILS)

• Frequency (Localizer) 108MHz-112 MHz
• Even 10ths gives 40 frequencies
• As with VOR, interference from FM broadcast
  stations can be a problem
• Frequency (Glide Path) 329MHz 335 MHz (paired
  with localizer frequencies as specified by ICAO)
• Note Receiver automatically selects Glide Path
  frequency when Localizer frequency is tuned

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Localizer

• The localizer antenna array is located at some
  distance (usually about 1000 Ft.) off the stop
  end of the runway

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Localizer

• The antenna array radiates two signals each with
  its own antenna pattern. One pattern is modulated
  with a 150Hz AM tone, the other with a 90 Hz tone

When the aircraft is on the left of the extended
runway centreline, the 90Hz tone predominates and
when it is on the right, the 150 Hz tone
predominates. When the aircraft is on course, the
two tones are equal
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Glide Path

• The Glide Path antenna is usually located about
  1000 Ft down the runway from the threshold and
  400 Ft. off to the side

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Glide Path
The signal format is the same as for the
localizer but rotated 90?.
In order to minimize the height of the antenna,
the ground is used as a reflecting surface.
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Glide Path
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Glide Path
Note Because the ground is used in the
generation of the Glide Path signal, variations
in the ground conductivity and/or level will
change the Glide Path angle. One problem is snow,
which raises the level of the conducting surface.
During snow storms, the clearing of the area in
front of the Glide Path antenna has second
priority (after crash routes) NOTE The glide
path signal flares starting around 2600 Ft from
threshold. Also, the structure (deviations)
becomes quite large. Thus the Glide Path is not a
reliable source of vertical position information
for AUTOLAND systems
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Markers
Part of the ICAO specification for ILS includes a
facility called a marker. Markers are almost
extinct now, but their original function was to
provide an indication of the distance of the
aircraft from touchdown.
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Markers
The OUTER marker (modulated with Morse Code dash
dash dash etc.)is located about 4 NM from
threshold and indicates the start of the final
descent The MIDDLE marker (modulated with dash
dot dash dot etc.) is located about 2500 Ft from
threshold and indicates the decision point for
Cat I approaches The INNER maker (modulated with
dot dot dot etc.) is about 1200 Ft from threshold
and indicates the decision point for Cat II
approaches
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Markers
Markers all operate at a frequency of 75 MHz.
Their radiation patterns are narrow and pointed
upwards so that the aircraft receives the signal
only when it is directly overhead.
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ILS Aircraft Installations
The aircraft receiver is relatively simple since
all it has to do is measure the amplitudes of the
90Hz and 150Hz modulations and provide an error
signal The sum of the modulations is also
provided as an integrity check
Antenna
Flag
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ILS Aircraft Installations
Antennas Localizer Uses the same antenna as the
VOR. Either a half-wave dipole or a loop.
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ILS Aircraft Installations
Dash 8 LOC/VOR Antennas
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ILS Aircraft Installations
Antennas Glide Path Normally the Glide Path
antenna is a dual loop mounted inside the radome
Radar Antenna
Glide Path Antenna
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ILS Aircraft Installations
Antennas Glide Path On Long bodied aircraft the
radome is not a good location
3?
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ILS Aircraft Installations
Antennas Glide Path On 747s and other such
aircraft, the Glide Path antennas are mounted on
the nose gear doors. Another consideration is
that, in the radome, the radar antenna may
influence the reception from the Glide Path
antenna
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ILS
Accuracy Localizer Maximum error defined by
ICAO at the point where the average localizer
course crosses the runway threshold Cat I 35
Ft. (0.29? for a 6000Ft. Runway) Cat II 25 Ft.
(0.20? for a 6000Ft. Runway) Cat III 10 Ft.
(0.08? for a 6000Ft. Runway) Glide Path 0.056?
where ? is the Glide Path angle (0.168? for 3?
GP angle)
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ILS
Irregularities The course lines are generated by
antenna patterns which can be altered by the
presence of reflecting surfaces such as hangars
and other aircraft. Thus the localizer and glide
paths are never straight lines but the deviations
from nominal (called structure) are controlled
by the ICAO specification. The allowable
deviations decrease as the threshold is approached
2500 Ft
4 NM
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ILS
Integrity The ground station includes monitors
which will detect out of tolerance conditions and
either switch transmitters or turn off the
transmission. The receiver measures the
modulations and sets a flag if the sum of the
modulations goes below a given threshold
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ILS
Integrity The ground station includes monitors
which will detect out of tolerance conditions and
either switch transmitters or turn off the
transmission. Response time is 10 seconds for Cat
I and 2 seconds for Cat II and III. Thus Cat II
and III systems require that the standby
transmitter be on at all times (hot spare) The
receiver measures the modulations and sets a flag
if the sum of the modulations goes below a given
threshold
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ILS
Future of ILS Threats FM broadcast
stations Relatively few channels available
(40) Susceptibility to interference Limited Glide
Path angle (5??) Limited to straight in
approaches Strengths Large number of ground and
air installations Meets 99 of
requirements Guaranteed to be around until at
least 2015
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Microwave Landing System (MLS)
Background In the mid 1970s 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 issue a SARP
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.
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Microwave Landing System (MLS)
Background (continued) 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
ones 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.
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Microwave Landing System (MLS)
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?)
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Microwave Landing System (MLS)

• Canadas Role
• Several MLS facilities have been installed from
  time to time in Canada over the years.
• An experimental MLS was installed at Uplands
  Airport
• Two MLSs were installed at airfields in the
  coal-mining area of Alberta near Edson.
• Two MLSs were installed at Toronto Island
  Airport for a few years

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Microwave Landing System (MLS)
Frequency of Operation 5.031 to 5.0907 GHz
300kHz spacing (200 channels) Functions Provided
( options) Azimuth (horizontal) guidance
Vertical guidance Flare guidance (extra
accurate vertical guidance for the last 1000 Ft.
or so) Missed Approach guidance
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Microwave Landing System (MLS)
Principle of Operation 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
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Microwave Landing System (MLS)
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
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Microwave Landing System (MLS)
Principle of Operation 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?
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Microwave Landing System (MLS)
Principle of Operation 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
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Microwave Landing System (MLS)
Principle of Operation - Time Multiplexing 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.
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Microwave Landing System (MLS)
Principle of Operation - Beam Timing
?
Since the rate of scan and the dwell time are
known, the angle ? can be determined from the
measurement of t.
Where V is the scan rate 0.02?/s
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Microwave Landing System (MLS)
The other angle functions are provided in a
similar fashion
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Microwave Landing System (MLS)Azimuth Antenna
Installation

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Microwave Landing System (MLS)Elevation Antenna
Installation

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Microwave Landing System (MLS)
Airborne Installation For the frequency of 5 GHz
the wavelength is 6 cm Thus the antenna (a ¼ wave
monopole) is about 1.5 cm long Receivers are
very expensive due to the small market ( about
25,000)
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Microwave Landing System (MLS)

• Advantages over ILS
• Less susceptible to siting (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
• Disadvantages
• Expensive
• Not many ground stations to use it with

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Microwave Landing System (MLS)

• The Future
• 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)

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Augmented GPS (WAAS, LAAS)
As was mentioned in the section on GPS, the
position accuracy can be improved dramatically by
the use of differential techniques. WAAS is
capable of accuracies (95) of 3.2m
horizontally 6.0m vertically 18 Ft. Cat I Glide
Path gives 10 Ft. This is not quite adequate for
Cat I approaches. The vertical error being the
problem.
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WAAS Approaches

• About 525 LPV approaches (Lateral Precision with
  Vertical Guidance) have been approved. Limits
  300Ft/3/4 mile vis

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LAAS Approaches

• LAAS promises to provide Cat II and III
  capability but no approaches have been certified
  to date

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Possible Problem with GPS Approaches

• (See Notes)