What is VOR? How does the system work?
Adopted by ICAO in 1960, the VHF Omni-directional Range (VOR) is a standard and highly reliable navigation aid.
The equipment operates within the frequency range of 108 ~ 117.95 MHz. An aircraft’s VOR Rx receives a 1020Hz ID tone signal, which the ground station uses to identify itself. An aircraft’s VOR receiver measures the phase difference between 2 signals from the VOR transmitter:
- A 30Hz FM-modulated Reference Signal produces a constant phase regardless of a receiver’s bearing from the VOR.
- The rotation of the transmission antenna creates modulated directional signal at 30Hz AM modulated directional signal.
VOR stands for “VHF Omnidirectional Range.” It is a radio navigation system used to determine the bearing or direction of a particular radio station. The VOR system transmits a continuous signal modulated with a reference frequency called the “Omni-bearing.”
By comparing the reference frequency phase with the received signal phase, a receiver can determine the angle or bearing to the transmitting station.
Aircraft use VOR systems for navigation, typically found at airports and along airways.
The VOR system is used in conjunction with other navigational aids, such as a heading indicator, to help pilots determine their position and maintain the course.
In addition to providing navigation information, VOR stations may also transmit other information, such as the station’s identifier, the frequency of the transmission, and the station’s location. This information is transmitted in Morse code.
How does the VOR navigation system work?
The 30Hz FM reference signal is synchronized with the 30 rev/sec rotating directional AM signal such that:
- The two 30Hz modulations are IN-PHASE when it’s due magnetic NORTH of the VOR beacon.
- The phase difference measured at any other point will equal the A/C’s magnetic bearing from the VOR.
Transmission Details (Frequency Range)
VOR beacons operates within the VHF band (30-300MHz) between 108 ~ 117.95MHz, as follows:
40 Channels (108 ~ 112MHz):
- This band is shared with ILS as it’s primarily an ILS band assigned by ICAO. The EVEN decimal digits channels are for VOR (108.0, 108.05, 108.20, 108.25, 108.40….. 111.85MHz)
120 Channels (112 ~ 117.95MHz):
- 112.0, 112.05, 112.1, 112.15 …….. 117.95MHz
- Total No. of Channels: 160 Channels.
What are the different types of VOR?
1. BVOR: Broadcast VOR, which gives weather and airfield info between beacon Identification.
2. TVOR: Terminal VOR, which has only low power, is used at the major airfields.
3. VOT: VOR on Test, broadcasting a fixed Omnidirectional signal.
4. VORTAC: Co-located VOR and TACAN (DME) beacons.
VOR Airborne Equipments are:
- The aerial.
- The Receiver
- The indicator
There are essential VOR Terminologies that must be defined and clearly understood. These are radial bearing, heading, and course.
- Radial: The line from a VOR station to a particular magnetic direction. These radials are assigned numbers that pertain to their situation around the magnetic compass card.
- Bearing: from an airplane to a VOR station. If you know the radial on which the aircraft is located, you see the bearing of the station (its reciprocal). This has nothing to do with the heading of the airplane, only its position.
- Heading: A compass functions only. It refers only to the direction in which the aircraft is pointing.
- Course: This terminology confuses maintenance people because the Course Select Knob functions are two wholly separated systems (Compass System & VOR system).
Course = Path + Direction of Travel. Or we can state that a course is a path consisting of 2 reciprocal VOR radials & the direction of travel on that path.
Difference between the Heading concept & Position Concept
One Operation (Heading Concept) gives a “Reading Reference” on that particular course. So, the signal we get from the compass system is “HDG ERR Signal.”
The other Operation (Position Concept) is sent to the VOR system to determine our position referring to that particular course.
VOR RMI needle can point to a VOR station providing that both types of information are available.
Position info + Heading info à added electrically in a Differential Synchro located in the VOR system à the output is fed to the stator of the free-swinging rotor synchro in the RMI.
VOR station antenna is a directional antenna that rotates its signal at 30 revolutions per second.
The direction in which the antenna points at any instant is carefully coordinated with the phase of the 30Hz FM signal at that instant.
Ex: When pointed North, the phase of the 30Hz FM signal is pointed positive.
This coordination enables the Rx to establish its Position, and the antenna doesn’t physically rotate.
It transmits a directional beam, rotates with 30 revs/s, and has the same effect as the rotating antenna.
