Kraków 2022-10-05
VOR / DVOR / DEM navigation system.
Let's start from the beginning. A beacon is a transmitting or transmitting-receiving radio that transmits a radio signal used in aerial radionavigation. The radio signal is broadcast at regular intervals and includes the beacon code, usually Morse character. This signal may also be transmitted only after receiving the query. In air transport, it is used to determine the direction of flight using a radio finder.
VOR beacon.
VOR in English, i.e. VHF Omni-directional Range). VOR is a type of beacon used in aviation, using the radio band in the range of 108 - 117.950 MHz. The signal of this beacon provides azimuthal information, i.e. azimuth - the angle between the north and a given horizontal direction. This enables the presentation of information such as: 1 The magnetic bearing of the aircraft from the beacons. 2 The target bearing is reached to or from the beacon. 3 Passing the beacon. 4 Beacon ID (3 Morse code characters).
The VOR beacon can work with the DME distance measuring device to form a VOR / DME system, as described below.
The principle of operation of the VOR is similar to that of a lighthouse. The beacon transmits a radio signal beam which rotates around the beacon 30 times per second. At the same time, a constant phase is transmitted in all directions, modulated with a 30 Hz tone. When the variable phase beam passes through the magnetic north, the maximum level of solid phase modulation occurs. Determining the azimuth consists in measuring the time that passes between the highest level of the solid phase and the receipt of the variable phase signal. VOR beacons are usually equipped with complex signal control and maintenance systems. The device has additional batteries (UPS) and air conditioning to maintain a favorable operating temperature of the device.
The beacon operating frequency is 108 - 117.9 MHz. In the range of 108 - 112 MHz every 200 kHz. The remaining channels are used by the ILS system. The power of the beacons is 100 - 200 W. There are also low power beacons 50 W, so-called T-VOR, ie beacons with reduced power due to the high density of devices in the vicinity of the airport. Beacons are accurate from plus / minus 5.2 degrees to plus / minus 2.5 degrees. The range of the beacons is short or medium and, depending on the signal strength, reaches 25 - 200 NM (46.30 km - 370.40 km).
VOR beacons are the most popular angular system in aviation navigation, but they are not without their drawbacks. The main disadvantage of this system is high susceptibility to signal interference due to terrain obstacles. The answer to this is the DVOR (Doppler VOR) beacon with a modified principle of operation.
Currently (2022) we have 23 DVOR / DME beacons in Poland, which are regularly deployed throughout the country. 10 years ago, there were only 12 VOR and DVOR beacons. Most of the new DVOR / DME beacons are located in the landing areas of commercial airports.
VOR beacon work.
The beacon in the signal sends a Morse code every 10 seconds, thanks to which the device emits the Morse code through the loudspeaker in the cockpit. After listening to the transmitted signal, the pilot can be sure that he has set the correct frequency and is going to the correct device, if there are several nearby. The beacon identification mark shall consist of three Morse code letters. The next part of the message is the SP bearing from the beacon to magnetic north, information on reaching the desired bearing to or from the VOR beacon, signal of the aircraft passing the beacon.
Typically, there are three different HSIs (Horizontal Situation Indicators) on board an airplane that represent data obtained from VOR beacons. The information that can be displayed by the HSI is the information "from-to", ie information whether we are flying from the device or to the device and the bearing direction to the VOR. Due to the device there may be two TDI (Track Deviation Indicator) or CDI (Cross Deviation indicator). We also know from the indicators when we will fly over the beacon. So when we approach the VOR the pointer begins to swing in one direction, when we fly over the device, the pointer returns to the vertical and after flying over the device the pointer will swing in the other direction.
DVOR beacon.
DVOR or Doppler VOR is an improved version of the conventional VOR, which has solved the problem of signal reflection from terrain obstacles. It is composed of 48 antennas that are smaller on the outside and distant from the central antenna by about 6.5 m.
In order for airplanes to use the DVOR system, they must be at least 1000 ft (304.80 m) high. The DVOR uses the Doppler effect, which is a frequency shift in one direction and emits a signal at different frequencies. The DVOR beacon has specific requirements for obstacles in the vicinity of the device, so that there is no interference from these obstacles. The protection zone requirements extend up to 500 m from the antenna. Although already from a distance of 300 m, there are no restrictions, except for high-voltage power networks and electric railway traction.
DME device.
DME (Distance Measuring Equipment) is a device for providing information about the distance from a DME device. The range of the device can be up to about 300 NM (555.60 km). DME operates in the bands 978 - 1,215 MHz, with a 1 MHz interval, uses two different frequencies for query and response, and they are shifted by 63 MHz in relation to each other. And on the same frequency, two signals are placed, the first channel labeled "X" and the individual pulses are 12 microseconds apart and the reception of a second channel labeled "Y" which has pulse intervals of 30 microseconds. It operates on a query and response basis between the DME and the "interrogator" in the airplane. The plane sends the query and the ground device receives the signal and identifies whether it is a query from the plane or a random signal. On request and response, the signal takes approximately 12.36 microseconds to travel 1 NM between the aircraft and the device.
The DME device supports up to 100 aircraft at the same time. The DME system also complements the ILS system by installing the antenna on the glide path. The DME device has terrain obstacle restrictions. No buildings, structures, trees, fences or other physical obstacles are allowed up to a distance of 5 m. Up to a distance of 2 500 m, single terrain obstacles are allowed, with the exception of high voltage power networks and electric railway traction.
There is a DME receiver in the cockpit of the aircraft. The receiver gives three values. The first value is the distance of the aircraft from the DME device (called NM), the second value is the speed of the aircraft (called KT) and the last value is the number of minutes left for the aircraft to travel to the transmitter (number of minutes).
Sometimes a TACAN system is used instead of DME, then it is called a VORTAC or VOR / DMET system. The VOR signal is transmitted on a 1,020 Hz tone with an audible Morse code, and the DME on a 1,350 Hz tone. It has a range of 160 NM (296.32 km). Historically, the TACAN system was first introduced to Western Military Aviation.
Written by Karol Placha Hetman