Download - Precision Approach and Landing Systems
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Precision Approach and Landing
SystemsApproachgenerally 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
SystemsPrecision 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 10thsgives 40 frequencies
As with VOR, interference from FM broadcast stations can be a
problem
Frequency (Glide Path): 329MHz335 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.) offthe 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 onthe 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 groundconductivity 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 ofthe 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 wasto 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
Outer Middle Inner
Markers all operate at a frequency of 75 MHz. Their radiation
patterns are narrow and pointed upwards so that the aircraftreceives 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 andprovide an error signal:
The sum of the modulations is also provided as an integrity check
Tuning Demodulation
90Hz
Filter
150Hz
Filter
+
+
+
-
Guidance Flag
Antenna
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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
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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 bealtered 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 bythe ICAO specification.
The allowable deviations decrease as the threshold is approached
4 NM2500 Ft
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ILS
Integrity:
The ground station includes monitors which will detect out oftolerance 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
ILS
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ILSFuture 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
Microwave Landing System (MLS)
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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.
Microwave Landing System (MLS)
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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 ofeconomic 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 instead.
Microwave Landing System (MLS)
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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)
Microwave Landing System (MLS)
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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
Microwave Landing System (MLS)
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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*
Microwave Landing System (MLS)
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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
ExampleAzimuth (horizontal):
The azimuth beam is shaped as follows:
Top View
Side View
Microwave Landing System (MLS)
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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
Microwave Landing System (MLS)
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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
Microwave Landing System (MLS)
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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
Microwave Landing System (MLS)
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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 ofmeasurements which takes 115 milliseconds (ms). This is called
time multiplexing.
Microwave Landing System (MLS)
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Microwave Landing System (MLS)
Principle of Operation - Beam Timing:
-40
+40
0
T0
t
Dwell Time
Since the rate of scan and the dwell time are known, the
angle can be determined from the measurement of t.
)( 02 tTV
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 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 approachesMuch less susceptibility to interference
Many more channels available
Increases runway usability in IFR conditions
DisadvantagesExpensive
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 powerthan 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 accuracycan 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.
WAAS A h
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WAAS Approaches
About 525 LPV approaches (Lateral Precision with Vertical
Guidance) have been approved. Limits: 300Ft/3/4 mile vis
LAAS A h
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LAAS Approaches
LAAS promises to provide Cat II and III capability but no
approaches have been certified to date
P ibl P bl i h GPS A h
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Possible Problem with GPS Approaches
(See Notes)