VOR (VHF Omnidirectional Range) Basics

VOR, which stands for VHF Omnidirectional Range, is a short-range radio navigation system used by aircraft.

The VOR system provides an aircraft’s radial with respect to a ground station. Essentially, it tells us the aircraft’s location as “seen” by the VOR transmitter. However, it doesn’t tell us anything about the aircraft’s heading. The aircraft’s radial is determined by measuring the phase difference between two signals transmitted by the ground station: the Reference Phase Signal (R) and the Variable Phase Signal (V).

Here are some common applications of VOR:

• V-Airway: Used to define “highways in the sky” for air traffic.
• Track Intercept: Allows pilots to return to a designated track if blown off course by crosswinds. By flying towards the needle on the VOR display (the Course Deviation Indicator or CDI), the aircraft moves toward the VOR ground station.
• Triangulation Position Fix: By tuning the VOR receiver to at least two VOR ground stations, a pilot can obtain a 2D position fix, pinpointing the aircraft’s location.

Figure 1 illustrates a typical VOR transmit system. The ground-based VOR has the following specifications:

• Frequency of Operation: VHF, ranging from 108 to 118 MHz.
  • 108 to 112 MHz is used for Approach NAV.
  • 112 to 118 MHz is used for Short Range NAV.
• Number of Channels: 100

Two signals are transmitted simultaneously, with the following characteristics:

Reference Phase Signal (R):

• Symbol: ‘R’
• Modulation: FM
• Rate: 30 Hz
• Type: Non-directional

Variable Phase Signal (V):

• Symbol: ‘V’
• Modulation: AM
• Rate: 30 revolutions per second (rps), equivalent to 30 Hz
• Type: Rotational

The VOR transmitter also sends a Morse code ID to the aircraft’s VOR receiver for station identification. Additionally, the VOR transmitter can be used for two-way voice communication.

The VOR receive system in the aircraft has the following specifications:

• Frequency of Operation: VHF
• The phase difference between the ‘R’ and ‘V’ signals is calculated.
• The aircraft’s Course Deviation Indicator (CDI) displays the VOR radial reading.
• If the CDI malfunctions, a red flag appears to alert the pilot.

Advantages of VOR

VOR is more efficient than ADF (Automatic Direction Finder) because its indicator points directly to the ground-based VOR transmitter. ADF, on the other hand, provides a relative bearing, which is the angle between the aircraft’s longitudinal axis and the NDB (Non-Directional Beacon). This means ADF doesn’t directly point to the ground beacon, requiring more interpretation.

Disadvantages of VOR

• Error: Accuracy is limited to approximately +/- 2 degrees.
• Line-of-Sight: VOR is limited to line-of-sight due to its use of the VHF band.
• Signal Interference: VOR signals can be reflected, blocked, or distorted by buildings, mountains, fences, and power lines.
• Interference at High Altitude: Interference may occur between two ground stations operating on the same frequency.
• No Heading Information: VOR doesn’t provide the aircraft’s heading; it only indicates the direction to the ground station.

Although GPS is increasingly prevalent, VOR remains in use as a backup navigation system in case of satellite link failure.