Vor

VOR (Very High Frequency Omni Directional Radio) Band: VHF Frequency: 108-117.95 MHz (approx 118 MHz). 108-111.95 MHz (approx 112 MHz) is shared with ILS. All odd first decimal for ILS All even first decimal for VOR. 112-118 MHz Exclusive for VOR.

Principle of Operation: Bearing by Phase compression. Working:(i) (ii)

Ground transmitter has two aerials. Fixed and rotating aerials. A fixed aerial transmits a horizontal polarized continuous wave signal of 30 Hz frequency modulates and called reference signal. This reference signal is received in same phase in all direction. Polar diagram of fixed aerial is circle.

(iii)

A directional signal is transmitted by which rotates electronically at the rate is 30 cycle per sec (30 Hz) i.e. 1800 RPM. The signal is of 30 Hz and amplitude modulated. A polar diagram of variable antenna is rotating figure is 8.

(iv)

(v) (vi) (vii) (viii)

This signal is received in different phase all around a transmitter. Both variable and fixed signal are in ways at N (M) magnetic north. The phase difference between two transmissions at any point around is equal to magnetic bearing of the aircraft from ground station. Aircraft equipment finds the phase difference and gives bearing information.

(ix)

VOR transmits 360° magnetic bearing which are know as Radials (or) QDR.

Radial:Radials are magnetic track radiating outward from a station. This diagram shows radials or QDM.

Difference between Cardioids and Limacon: Cardioids

Limacon

1. Cardioid is the polar diagram is receiver on a/c antenna.

1. Rotating Limacon is the polar diagram of  transmitter on ground.

2. It has well defined null point.

2. It has a signal in the null point also.

Cone of Confusion:(i) (ii) (iii)

VOR ground beacon transmits horizontal polarized signal which are inclined at 60°-80° to the horizontal plane. Due to this there is no signal in vertically above beacon. This area is in the form of cone. Some spills may exist over cone of confusion.

Indication Over cone of confusion:(i) (ii) (iii)

CDI needle oscillates. To / From indicator flickers. Failure warning flag may come and go off.

Indicator System in Flight Deck (or) cockpit:(i) (ii) (iii)

CDI (Course Deviation Indicator). OBS (Omni Bearing Selector). To / From Indicator.

CDI (Course Deviation Indicator):It shows deviation of aircraft from selected course or track. Max deviation of CDI is +/- 10°. With tolerance of +/- 2°.

OBS (Omni Bearing Selector):-

It is a rotating knob (or) dial in which any of 360° radials can be selected.

To / From Indicator:It does not show whether a/c is going towards or away from station. To / From indication are based on (i) (ii)

OBS selection. Position of a/c with respect to station.

Factors Affecting Accuracy of VOR:(i) (ii) (iii) (iv)

Site Error. Airborne Equipment Error. Propagation Error. Pilot Age Error.

(i)

Site Error:- (Also called Course Displacement Error)

The error is always present and is caused due to location of VOR beacon and structures around it. It is maximum permissible up to +/- 1°. In case of the error is exceeded transmission is automatically switched off (or) code is removed.

(ii)

Airborne Equipment Error:-

It is inherent in the a/c equipment and is induced during manufacture of equipment.

(iii)

Propagation Error:-

During propagation of signal, phase shift may take place due to deflection, refraction and diffraction. a) Site error b) Airborne equipment error. c) Propagation error.

(iv)

This three are called Aggregate error.

Pilot Age Error:-

As coming closer to VOR beacon, the radials converge and become close to each other. Due to this maintain a particular radial is tough. This error is called pilot age error.

Factors Affecting Range of VOR:(i) (ii) (iii)

Transmission Power Line of Sight. DOC.

(i)

Transmission Power:-

Higher the transmission power more will be range. To double VOR range, power as to be increased by four times.

(ii)

Line of Sight:Range of VOR depends upon height of a/c and elevation of 

beacon.

(iii)

Line of sight (nm) = 1.25 √Ht of a/c + 1.25 √Ht of beacon If the Ground beacon is on sea level Line of Sight (nm) = 12 √a/c FL. (We have to use Flight level)

DOC:-

It causes effective to reduce VOR range. DOC of VOR is similar to NDB. But it is given both in range and altitude. Note: DOC of VOR is valid Day and Night.

Accuracy of VOR Equipment: (i) (ii)

The accuracy of transmitter signal most be with in +/- 3° for 95% of time. Overall accuracy of transmitter and receiver, when all error is combined should be with in +/- 5° for 95% of time.

Failure Warning Device: A warning flag appears on a/c instrument if any of the following is detected. The following for failure warning: (i) No power or low power to the a/c equipment. (ii) Failure of a/c equipment. (iii) Failure if ground station equipment. (iv) Failure of indicator. (v) Weak signal either from reference or variable signal. (vi) It also comes during tuning.

Beacon Passage: It is indicated be positive change in To / From indicator.

Station Identification Code: An AM audio frequency signal is transmitted once in every 10sec. This identification code consist of three Morse code letter transmitted at a rate if 7words/ min. Every VOR station has monitoring located in the radiation field near transmitter. This monitor receives signals and compares it with known parameters. It will switch off the transmitter or identification code is removed in following cases.

(i) (ii) (iii)

Bearing information is wrong if more than 1°. Reduced signal strength by 15% or more. When transmitter is under maintenance.

Checking VOR Equipment on Ground: VOR equipment can be tested at a fixed point on ground. A Board will be on holding point of runway

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VOT (VOR Testing): This is installed to test a/c equipment during preflight check. The transmitter is called VOT. Frequency is published in AIP. Testing a/c equipment can be done from any position on aerodrome. Following are indications. (i) CDI should be center ⇒ OBS 180° ± 4° (i.e. 176° - 184°) TO / FROM indicator ⇒ TO (ii) CDI should be center ⇒ OBS 360° ± 4° (i.e. 356° - 004°) TO / FROM indicator ⇒ FROM

Doppler VOR: Principle is same bearing by the phase difference. Doppler principle is used to remove site error. Frequency and Band are same as normal VOR. On DVOR Fixed antenna transmits 30 Hz Amplitude Modulated. And instead of  rotating antenna, 52 antennas are fixed around fixed antenna, and transmit in 30 Hz Frequency Modulated.

Classes of Bearing: (i) (ii) (iii) (iv)

A B C D

⇒ ⇒ ⇒ ⇒

± 2° ± 5° ± 10° More than 10°

QDM: - Magnetic Heading to GO TO station in nil wind condition. Magnetic Heading + Relative Bearing = QDM QDR: - Magnetic Heading to GO AWAY FROM station in nil wind condition. QDM ± 180° = QDR

QUJ: - True Heading to GO TO station in nil wind condition QDM ± Variation = QUJ True Heading + Relative Bearing = QUJ QTE: - True Heading to GO AWAY FROM station in nil wind condition. QDR ± Variation = QTE QUJ ± 180° = QTE

By For Aviators.