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Bearing Systems • General Principle: Need two signals: – 1. One whose phase varies with bearing from (or to) the transmitter – 2. A reference signal whose phase is constant regardless of bearing • Lighthouse example

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Page 1: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

Bearing Systems

• General Principle:

Need two signals:– 1. One whose phase varies with bearing from (or to) the

transmitter– 2. A reference signal whose phase is constant regardless of

bearing

• Lighthouse example

Page 2: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

VOR (VHF Omnirange)

• Frequency: 108-112MHz (even tenths)112-118MHz (every 0.1

MHz)

• RF Power Level Output (Ground Station):

100W

Page 3: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

VOR• In VOR, the variable signal is provided by a limacon

antenna pattern which rotates clockwise at 30Hz.

Note: a limacon has the equation

Thus an observer at any point measures an RF signal , amplitude modulated at 30 Hz

Page 4: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

VOR

VOR

ANTENNA

PATTERN

Page 5: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

VOR REFERENCE SIGNALThe Reference Signal is radiated in an omnidirectional pattern.

It is amplitude modulated by a 9960Hz subcarrier.

This, in turn, is frequency modulated at 30Hz

The FM (reference) modulation is in phase with the variable pattern when the observer is North of the station

(can be referenced to Magnetic or True)

Page 6: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

VOR SIGNAL GENERATION

Note the 1020Hz identifier

Page 7: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

ANTENNA ARRAYSIt is often impossible to generate a desired

antenna pattern with just one antenna

Using two or more antenna elements provides the designer with more design variables e.g.

1. Number of elements

2. Physical arrangement of elements

3. Amplitude and Phase of input signals

Page 8: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

ANTENNA ARRAYSTwo Element Example

Transmitter

Antenna

Pattern

Page 9: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

ANTENNA ARRAYSTwo Element Example

Transmitter

Antenna

Pattern90deg

Page 10: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

VOR ANTENNA

Page 11: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

VOR TRANSMITTER

Page 12: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

VOR RECEIVER

Page 13: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

VOR RECEIVER

HSI

(HORIZONTAL SITUATION INDICATOR)

Page 14: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

VOR ERRORS

• Since the system depends on the antenna pattern, any distortion of the pattern will cause errors

• Internal sources of error:– antenna or feed mismatch

• External sources of error:– Reflections from buildings, terrain, trees, etc

Page 15: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

DOPPLER VOR

• Sometimes a particular site has too many reflecting objects to permit the operation of a standard VOR

• In this case, a Doppler VOR is installed. • This permits a large aperture antenna array to be

used,i.e. an antenna array covering a large area.

• A large antenna array uses space diversity

Page 16: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

DOPPLER VOR

EFFECT OF APERTURE

Page 17: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

DOPPLER VOR

If an antenna, radiating a signal at a carrier frequency fC , is placed on the edge of a platform which rotates at an angular rate , a receiver at a distance will observe that the carrier is frequency modulated at the angular frequency .

This is due to the Doppler shift caused by the relative motion of the antenna and receiver .

Page 18: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

DOPPLER VOR

fc

r

v=r

fR=fCv/c

Page 19: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

DOPPLER VOR

fc

fc+9960

fc-9960

z

Note:

To maintain the

correct relationship

between reference

and variable signals,

the signal rotation

is counterclockwise

Page 20: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

DOPPLER VOR

Page 21: Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose

VOR as a NAVIGATION AID

• Accuracy: ± 3º• Availability:

(Two transmitters) 99.9%

• Integrity:

Ground - monitors

Air- receiver measures signal strength and modulation depth

• Availability:

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