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Introduction to: Radio Navigational Aids
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Lecture Topics
Basic Principles
Radio Directional Finding (RDF)
Radio Beacons
Distance Measuring Equipment (DME)
Instrument Landing System (ILS)
Microwave Landing System (MLS)
Hyperbolic Navigational System
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Defining Navigation
Navigation is the determination of the Position and Velocity of a Moving Vehicle on land, at sea, in the air, or space.
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What are Radio Navigational Aids ?Specialised Electronic and Communication Systems used for navigational purposes.
Long-range systems employ LF and HF bands.
Short-range systems employ VHF and UHF bands.
Microwave frequencies are used for long-range and short-range systems.
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Basic Principles of Radio Navigational Aides
Measuring the distance to a known location by measuring the round trip time of the Electromagnetic Waves.
Utilizing the directional radiation pattern of antennas to find the direction to a known location.
Antenna radiation patterns are of a prime interest in the field of radio navigation aids.
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Triangulation Systems
A
B
X
A and B are landmarks of known coordinates
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Line of Position (LOP)
90o90o
180o
135o
225o
270o
315o
45o
315o
270o
135o 45o
180o
225o
A B
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Bearings Measurement
A
B
X
A
A B
B
North
Heading
base-line
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Radio Direction FindingRDF equipment are used on board of aircraft, ships, and other moving platforms.
RDF systems employ a radio receiver with a rotatable Loop antenna.
Loop antennas have a special radiation pattern which is suited to the RDF function.
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Loop Antenna
one or more loops of wire
connection terminals
baseThe loop can be circular or square.
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Loop Antenna Radiation Pattern
Loop
Top View of Loop Antenna Radiation Pattern
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Loop Antenna Orientation
Directionof
Propagation
TxTx
Top View of Loop Antenna
vertically polarisedradio field
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Maximum Induced Voltage
Directionof
Propagation
TxTx
Top View of Loop Antenna
vertically polarisedradio field
Magnetic component
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Directionof
Propagation
TxTx
Top View of Loop Antenna
vertically polarisedradio field
Magnetic component
Minimum Induced Voltage
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Simple Direction Finder
Rx
Field Strengthmeter
RotatingLoop
Antenna
Bearing is found by searching for the minimum signal. Why ?
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Ambiguity of Simple RDF
Top View of Loop Antenna Radiation Pattern
Tx ? Tx ?
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Addition of Sense Antenna
Omnidirectional Antenna(Sense Antenna)
Loop Antenna
Voltage sum of Loop and Sense antennas
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Cardioid Antenna Pattern
Loop Antenna
v is proportional to 1 +cos
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Automatic Direction Finder (ADF)
Amp /2 Rx
SenseAntenna
Loop
IF AmpDetector
IF AmpDetector
Indicator
Motor
SynchroGen.
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Radio BeaconsA Radio Beacon (RB) is a transmitter used with RDF (300 kHz - 3 MHz).
Technically, RDF can be used to take bearing of any transmitter operating at the appropriate radio frequency.
Non-directional Beacons (NDB) are used specifically with RDF.
NDB transmit CW interrupted at regular interval by identification Morse code.
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Simple Radio Range System
/2Tx
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Radio Range Antenna Pattern
North
South
EastWest
A
N N
A
NENW
SESW
Morse Code:A :N :
CWCW
CW CW
CW = A + N
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VHF Omnidirectional Range (VOR)VHF Omnidirectional Range (VOR) provides Azimuth reading at the aircraft.
No direction finding antenna required at the aircraft position.
VOR Beacon, transmits continuous signals on one of 20 assigned channels in the 108 - 118 MHz band.
Limited range to line-of-sight, about 100 km.
Several VORs are needed to cover long distance over land.
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Principle of OperationAntenna with Cardioid pattern, rotating electrically at the rate of 30 Hz (Goniometer).
Bearing information is transmitted in the phase of the signal.
A separate 30 Hz reference signal is transmitted by an Omnidirectional antenna.
Bearings are calculated from the phase information received at the aircraft.
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Cardioid Antenna Pattern0o
360o
90o270o
180o
45o
135o
315o
225o
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VOR Antenna Pattern
North
X
30 revolutionsper second
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North
X
30 revolutionsper second
The signal at x is proportional to 1 + cos The signal at X is effectively AM modulated by 30 Hz
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Bearing Calculation
The amplitude of the signal at x is proportional to 1 + cos where
is the bearing of x relative to North.
The Cardioid pattern is rotated anticlockwise at the rate of 30 revolution s per second.
The amplitude of the signal at x is then proportional to:
1 + cos (230 t +
The phase difference
= cos (230 t +
cos (230 t
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Distance Measuring Equipment
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IntroductionDistance Measuring Equipment (DME) is used to measure the distance between an aircraft and a fixed point.
Generally the fixed point is a VOR or a destination point as an airport.
The useful range of DME systems is about 180 km, corresponding to a roundtrip delay of 1200 m sec.
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System ArchitectureAirborne equipment (Interrogator):
Transmitter
ReceiverGround equipment (Transponder) located at the fixed point.
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Basic Principle
TxRx
Transponder
1025 1025 -- 1150 MHz1150 MHz(126 channels)(126 channels)
962 962 -- 1024 MHz or1024 MHz or1151 1151 -- 1213 MHz1213 MHz
ReplyReply
InterrogationInterrogation
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OperationThe transmitter sends pseudo-random coded “interrogation pulses”
The transponder responds by sending “reply pulses” using the same pseudo-random pattern.
The onboard receiver intercepts the reply pulses that conforms to its pseudo-random pattern.
The time difference between sending the pulses, transponder processing delay, and receiving the pulses is used in calculating the distance.
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Transponder Access MethodThe system employs a random access method.
Up to 50 aircrafts may use the same transponder.
The interrogation pulses are pseudo-random coded.
The receiver only accepts replies that conform to the same pseudo-random pattern.
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Instrument Landing System
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IntroductionInstrument Landing Systems (ILS) are designed to guide an aircraft in its final approach and landing.
Three distinct subsystems are used:
Localiser,
Glide Slope, and
Markers.
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Localiser Plan
LocaliserTx90 Hz
150 Hz
5oRunway
(top view)
DIRECTION OF APPROACH
(108.1 - 111.9 MHz)
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LocaliserAids the pilot in lining up his/her aircraft in the proper azimuth approach to the runway.
Consists of a group of transmitters and antennas positioned at the far end of the runway.
The antenna radiation pattern has a 5o beawidth, centred along the runway.
The VHF frequencies used for the localise are in the range 108.1 to 111.9 MHz.
The useful range of the system is about 40 km.
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Glide Slope Plan
LocaliserTx90 Hz
150 Hz
5oRunway
(top view)
Tx
Tx
1o90 Hz
150 Hz
2.5o - 2.75o
DIRECTION OF APPROACH (side view)
(329.3 - 335.0 MHz)
(108.1 - 111.9 MHz)
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Glide SlopeAids the pilot in making his/her approach at the proper elevation angle to the runway.
Consists of a group of transmitters and antennas positioned beside the runway.
The antenna radiation pattern has a 1o beawidth, and elevated about 2.5o to 2.75o in the direction of approach.
The VHF frequencies used for the glide slope are in the range 329.3 to 335.0 MHz.
The useful range of the system is about 40 km.
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Frequency Arrangement
90 Hz
150 Hz
90 Hz 150 Hz
1o
5o
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Markers Plan
Runway
(top view)
MarkerTx
MarkerTx
400 Hz 1300 Hz
6 - 10 km
1 km
DIRECTION OF APPROACH
Carrier 75 MHz
75 MHz narrowvertical radio beam
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MarkersMarkers are transmitters that radiate continuous narrow vertical radio beams.
The carrier frequency is 75 MHz modulated by special tones.
The first transmitter is modulated by a 400 Hz tone, located at 6 - 10 km from the end of the runway.
The second transmitter is modulated by a 1300 Hz tone, located at 1 km from the end of the runway.
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Microwave Landing System
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IntroductionMicrowave Landing System (MLS) was designed to handle the increase in air traffic volume and to satisfy the demand for all-weather landing facilities.
Employs microwave frequencies (5 - 5.25 GHz band) rather than VHF.
MLS provides better accuracy, ease of application, and automation. enables landing down to zero ground visibility.
ICAO-approved replacement for the ILS system.
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ArchitectureMLS consists of three subsystems:
Localiser,
Glide Slope, and
Flare.The Localiser and Glide Slope subsystems serve the same purpose as in ILS.
The Flare provides information on the actual height of the aircraft above the plane of the runway.
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Aircraft Height Measurement
FlareTx
d
Runway(end view)
h
h = d tan
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Height CalculationThe angle q made by the aircraft and the runway at the point where the flare transmitter is situated is measured.
The distance d between the flare transmitter and the runway is known.
The height is calculated using the equation:
h = d tan
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Time Reference Scanning Beam
LocaliserTx
Runway
(top view)
DIRECTION OF APPROACH
A
B
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OperationThe system is based on Time-Reference Scanning Beams (TRSB), referenced to the runway, that enables an aircraft to determine precise azimuth and elevation angles.
The angular position is made by measurement of the time intervals between the TO and FRO azimuth antenna beam scan (typically +/- 60o) and UP and DOWN scan (typically 0o - 30o) of the elevation antenna pattern.
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Calculation
Tt1
The angles (or
are calculated using the equation:
t is the time difference between pulses and T is the scanning period from A to B and back to A
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Hyperbolic Navigation Systems
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IntroductionHyperbolic systems are based on measuring the difference in distance between a craft and two known sites.
Fixed distance difference between the craft and the two sites lie on a hyperbolic locus that passes between the sites.
Definition of terms:
base-line
base-line extension
line of position (LOP)
centre-line (CL)
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Basic Concept
A B
X
CL
LOP
base-linebase-lineextension
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Hyperbolic Navigation
A BD
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360o
d =
Decca ConceptDecca was the first of several hyperbolic electronic navigation systems developed during and after World War II.
It uses radio waves as a means of measuring the difference in distance between sites.
Master/slave radio transmitters arrangement.
Calculating the difference in distance based on phase difference measurement.
The range difference is given by:
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Decca Lines
A B
0o360o
or0o
720o
or0o
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A Master and two Slaves
A B1
B2
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Decca SystemTo overcome ambiguities and increase accuracy.
Three bas-lines system.
A set of four transmitters, a master and three slaves, referred to as a “chain”.
The base lines identified as Red, Green, and Purple.
The system reading is displayed on Red, Green, and Purple “Decometers” respectively.
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Decca Station Topology
purple
green
redA
B3
B2
B1
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Basic System
Tx340 kHz
340 kHz340 kHz
Tx340 kHz
Master Slave
Red340 340
340
Q340 340
Onboard receivingequipment
In phase
Red Decometer
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Possible Working System
Tx340 kHz
X 3X 4
Tx340 kHz
Master Slave
Red85 113.33
Q85 113.33
Onboard receivingequipment
In phase
Red Decometer
X 1/3X 1/4
340 340
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Practical 85 kHz Scheme
X 3X 4
Tx85 kHz
Master Slave
85113.33
Q85 113.33
Onboard receivingequipment
In phase
Red Decometer
X 1/6
340 340
113.33
X 8
14.166
85
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Decca Communication Receiver
5f
6f
9f
8f
5f 30f
30f
24f
18f
Purple
Green
RedRed Slave
Purple Slave
Master
Green Slave
Decometers
MultipliersIn Out
8f 24f
9f 18f
24f
18f
30f
6f
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Thank you for your attention