3.2 VOR System
3.2.1 Main Characteristics and the Destination of System
VOR system is created for determining the aircraft’s azimuth toward the ground- based radio beacon—an angle between the direction of North magnetic meridian passing through the radio beacon’s antenna system and the direction to the aircraft, counted clockwise. Azimuth’s determination makes it possible to use VOR for the following tasks:
marking the beginning, the end, and centerline of airways or sections of airways;
as an auxiliary aid at airfields using standard approach procedures;
as a reference point for aircraft;
as a source of navigation information (lines of position) for aircraft position fixing.
3.2 VOR System 93
Fig. 3.2 Types of indicating devices for displaying information from VOR
By measuring azimuths to two radio beacons, which coordinates are known, the aircraft’s position can be determined using the theta-theta method. It is important to note that the accuracy of such position determination will be relatively low.
VOR radio beacon forms and emits signals with horizontal polarization through non-directional antenna system, the class of emission is A9W.
The VOR radio beacon emits 360 radio beams (lines of position or radials) that are divided by 1° intervals which enables pilot to know on which radial he is flying TO or FROM the VOR radio beacon.
In order to identify, VOR radio beacon transmits an identification signal on the same VHF carrier which is used to provide the navigational function. In order to transmit the identification signals, AM carrier by two- or three-letter Morse code is used. This signal is transmitted with the speed of approximately seven words per minute and repeats every 30 s. The frequency of modulation’s tone signal is 1,020
± 50 Hz. Identification signal of radio beacon can also be transmitted via voice message.
Along with its main function execution, VOR radio beacon can provide the com- munication channel “earth-air” on the same VHF carrier which is used for navi- gational function execution. Radiotelephony communication does not interrupt the execution of a main VOR navigation function. Identification signals are not concealed at signals’ emission.
Onboard equipment of VOR is the receiver-indicator which, besides of the azimuth indication, allows to guide aircraft in FROM or TO the radio beacon modes following the set azimuth, as well as to determine the side and value of lateral deviation from the line of set path. On the different aircraft’s types, different types of indication devices can be used (Fig.3.2).
On practice, the following types of VOR radio beacons are used:
CVOR—conventional VOR is used to define airways and for en route navigation;
DVOR—a Doppler VOR (this overcomes re-reflected signals errors);
Fig. 3.3 VOR (DVOR) system operating principle
TVOR—terminal VOR which has only low output power (50 W) and is used in the terminal area of airports and covers a relatively small earth’s surface protected from interference by other stations on the same frequency band;
VOT—this is found at certain airfields and broadcasts a fixed omnidirectional signal for a 360° test radial. This is not for navigation use but is used to test an aircraft’s equipment accuracy before flight. More than±4° indicates that equipment needs servicing.
Allocated frequency diapason for VOR operation is divided into two parts: from 108 to 112 MHz (contains 40 channels), and from 112 to 117.95 MHz (contains 120 channels with step 50 kHz).
Frequency diapason 108–112 MHz is primarily an ILS band but ICAO has allowed it to be shared with short-range VORs and Terminal VORs: 108.0, 108.05, 108.20, 108.25, 108.40, 108.45, …,111.85 MHz (even decimals and even decimals plus 0.05 MHz).
En route radio beacons are used in the frequency band 112–117.95 MHz.
Phase method of azimuth measurement lies in the basis of VOR operating prin- ciple. Thereat, the information carrier can be either the envelope phase of AM fluc- tuations or the phase of high-frequency carrier.
In systems of the first type, dependence of AM fluctuation’s envelope phase from the azimuth is created by rotating the low-directivity polar diagram of a radio beacon’s antenna system. Cardioid-type polar diagram is normally used.
In systems of the second type, phase dependence of VHF carrier from azimuth is created by rotating non-directional antenna in a circumferential direction of specified radius. Thereat, Doppler effect is occurred and used for azimuth determination, which leads such systems to be called Doppler systems.
In both types of system, radio beacon emits the signal of reference phase as well as the signal, which phase of modulating fluctuation or frequency carrier depends on the azimuth (azimuth signal) (Fig.3.3). Reference phase and azimuth’s signals phases are the same and equal to zero when either the cardioid minimum or the position of rotating antenna are pointed to the magnetic north. Onboard equipment processes these signals and measures the difference between azimuth and reference signals’ phases, which is proportional to azimuth.
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Fig. 3.4 Radio beacons’ conventional signs
Fig. 3.5 VOR image on the navigation map
VOR radio beacons’ conventional signs in combination with DME and Tacan radio beacons, which are used on the aeronautical charts, are shown in Fig.3.4.
The “box” is put on the navigational map (Fig.3.5)—a rectangle containing data that identifies VOR radio beacon. This data include:
VOR’s (ST PETERSBURG) title that matches the town or any other ground point where it is located;
frequency (113.4 MHz);
call sign (SPB) that can be duplicated by Morse code or voice message;
Letter D before the frequency value indicates that VOR radio beacon is combined with DME.
On some maps, geodesic coordinates of radio beacon can also be specified (on Fig.3.5, this information is missing).
Fig. 3.6 Angular patterns of VOR radio beacons antennas