Principle of Operation VOR
VOR is an abbreviation for "VHF Omnidirectional Radio Range", which implies that it operates in the VHF band. Adopted by ICAO as early as 1960, VOR has been the main short-range navigational aid for several years. Short range infers that ranges up to 200 NM can be expected. It is still the most commonly used short-range aid. As opposed to the NDB, which transmits a non-directional signal, the signal transmitted by the VOR contains directional information.
The principle of operation is bearing measurement by phase comparison. This means that the transmitter on the ground produces and transmits a signal, or actually two separate signals, which make it possible for the receiver to determine its position in relation to the ground station by comparing the phases of these two signals. In theory, the VOR produces a number of tracks all originating at the transmitter. These tracks are called «radials» and are numbered from 1 to 360, expressed in degrees, or ° . The 360° radial is the track leaving the VOR station towards the Magnetic North, and if you continue with the cardinal points, radial 090° points to the East, the 180° radial to the South and the 270° radial to the West, all in relation to the magnetic North. See Fig 1.1
Before we look in detail at how the system works the following example illustrates the principle and should make it easier to understand.
Think of a lighthouse at sea and imagine the white light rotating at a speed of one revolution per minute (60 seconds). Every time this white narrow beam passes through Magnetic North, a green omnidirectional light flashes. Omnidirectional means that it can be seen from any position around the lighthouse. If we are situated somewhere in the vicinity of the light sources and are able to see them, we can measure the time interval from the green light flash until we see the white light. The elapsed time is directly proportional to our position line in relation to the lighthouse.
The speed of 1 RPM corresponds to 6° per second, so if 30 seconds elapse between the time we see the green flash and the white rotating light, we are on the 180° radial, or directly south of the station (30 sec x 6°/sec = 180°). This calculation can be done from any position and the elapsed time is directly proportional to our angular position (radial). We could name these light signals, calling the green one the Reference (REF) signal and the white beam the Variable (VAR) signal.
The VOR system operates on frequencies between 108 MHz and 117.95 MHz. Channel separation is 50 kHz and the signals have a horizontal polarisation.
Frequencies between 108 MHz and 111.95 MHz are primarily used for the localiser part of the ILS but can be shared with short range VORs, or so-called terminal VORs. The VOR uses
frequencies having an even decimal as the first digit after the last MHz digit, while localizers use odd decimals. When assigning a VOR to this part of the frequency band, it is an essential requirement that it does not interfere with an adjacent ILS channel. The frequencies from 112.00 to 117.95 MHz are solely used by VOR, both on odd and even frequencies
108.10 Localizer 112.00 VOR
108.15 Localizer 112.15 VOR 108.20 T-VOR 112.10 VOR
108.30 Localizer 117.95 VOR
The ground equipment is set up on a fixed, surveyed site and consists of a transmitter driving a combined aerial system; one part producing the Reference (REF) signal, the other producing the Variable (VAR) signal. The REF signal is an omnidirectional continuous wave
transmission on the carrier frequency of that particular VOR station. It carries a 9960 Hz subcarrier that is frequency modulated at 30 Hz. Since this is an omnidirectional transmission, the polar diagram of the REF signal is a circle.
In the receiver, it is the 30Hz component of this signal that is used as a reference for measuring the phase difference. The variable signal (VAR) is transmitted from an aerial that is effectively a loop. This ‘loop’ produces a figure of 8 polar diagram, which is electronically rotated at 30 revolutions per second. When the two signals (VAR & REF) are mixed together, the resulting polar diagram will be a cardioid, but unlike the cardioid of the ADF, this does not have a «null» position. We call it a «limacon». It rotates at 30 revolutions per second, indicated with an arrow on fig.1.2
The rotation of the limacon creates an effective amplitude modulation of 30 Hz. The VOR receiver splits these two signals into the two original components. The two signals are processed through different channels and the phase of the 30 Hz modulations of the fixed REF signal and the VAR signal are compared in a phase comparator. The phase difference between these two signals is directly proportional to angular position with reference to the VOR station.
As explained, magnetic North is the normal reference for the radials, so when 0° phase difference is detected, the receiver is on the 360° radial from the station. The phase difference and variable signal at the cardinal points.
The description above is valid for the convential VOR, CVOR. The CVORs suffer from reflections from objects in the vicinity of the VOR site and it was found that errors due to this could have been reduced if the horizontal antenna dimensions were increased. This was not practical to do and a new system had to be developed: the DOPPLER VOR, DVOR. The CVORs are now gradually being replaced by DVORs, that will be described in the next section.
Doppler VOR (DVOR)
The Doppler VOR is the second generation VOR, providing improved signal quality and accuracy. The REF signal of the DVOR is amplitude modulated, while the VAR signal is frequency modulated. This means that the modulations are opposite as compared to the conventional VORs. The frequency modulated signal is less subject to interference than the amplitude modulated signal and therefore the received signals provide a more accurate bearing determination.
The Doppler effect is created by letting the VAR signal be «electronically rotated», on the circular placed aerials, at a speed of 30 revolutions per second. With a diameter of the circle of 13.4 meters, the radial velocity of the VAR signal will be 1264 m/s. This will create a Doppler shift, causing the frequency to increase as the signal is rotated towards the observer and reduce as it rotates away with 30 full cycles of frequency variation per second. This results in an effective FM of 30 Hz. A receiver situated at some distance in the radiation field continuously monitors the transmitter. When certain prescribed deviations are exceeded, either the IDENT is taken off, or the complete transmitter is taken off the air. We come back to this in the section Limitations and accuracy.
The VOR receiver does not know if it is receiving a signal from a CVOR or a DVOR and the pilot treats both types in the same way. The change of FM and AM for the REF and VAR signals, as compared to the CVOR, is compensated for by having the DVOR antenna pattern rotate the opposite way, compared to the CVOR.
About of DVOR/DME producers and their listed prices.
Today few company manufacturing DVOR and DME in world. I introducing some price of their system from my own experiency.
1.NORTHROPGRUMMAN http://www.parkairsystems.com/ USD 520,000-580,000*
2.Thales http://www.thalesatm-services.com/ USD 550000-650,000*
3. INDRA INTERSCAN http://www.interscan.com.au/ USD 380,000-460,000*
4. Telematics Co(Korea) www.telematics.co.kr/ USD 320,000-400,000
5. FERNAU AVIONICS http://www.fernau.com/ USD 430,000-500,000
6. ALCATEL Air Navigation Systems http://www.alcatel.com/ USD 480,000-580,000*
* Some above price's including installation and site surveying cost of DVOR and
counterpoise's cost. Counterpoise installation is very hard and too expensive.
DETERMINING THE INSTALLATION SITE
The area in which a DVOR is to be installed is determined by the responsible Civil Aviation Authority according to the international air traffic regulations. The area is generally sufficiently large to allow selection of a point with optimum topography and thus the optimum propagation conditions can be met. The installation is determined by means of a site survey at which a surveyor must always be present. ATM can provide an engineering consultant on site for this survey.
When the installation site has been determined, precise bearings must be taken, either with reference to trigonometrical points or, if a satellite receiver is available, via GPS receiver for increased accuracy. Please see fig 1.3
When the exact location of the DVOR facility is determined by the customer and/or the Navaid Supplier, preliminary topographical and geo-technical surveys shall be conducted. A site survey will be performed by a civil engineer.
These investigations shall provide information on existing ground conditions (soil, water, etc.) to determine:
— bearing capacity for the design of either high level foundations in the sand fill or deep piled foundations,
— conductibility for the design of the earthing network,
— existing services (pipes, cables, technical trenches, etc.).
The earth works shall allow the construction works.
If necessary the earth works shall include the demolishing of existing constructions and foundations.
Each site shall be connected to the road network of the airport. Light circulation only is expected during the normal operation. However, the access roads shall resist to the loads of 25 tons trucks and cranes.
The energy, telephone, multi-pair cables and the connections of the interface points to the external networks shall be assumed by others.
The overall dimensions of the DVOR counterpoise array and a general installation overview are given in figure.1.3 Details on the specific foundations are given in figure 1.3 The minimum deepness of each pier foundations must be at least 1.20 m under the ground These external dimensions are minimal, and the effective dimensions are to be computed by the Civil Engineer according to the nature of the soil and the local standard in force.
The installation of the earthing system shall start during the construction of foundations of antenna and shelter slabs.