Very High Frequency (VHF) Omni Range - VOR

Very High Frequency (VHF) Omni Range - VOR

Principle

VOR radiated VHF energy modulated with Reference signal (omni directional flash) and Variable phase signal (rotating beam) and the bearing of the VOR station is the phase difference between reference and variable phases (time difference between flash and beam).

The principle of VOR navigation involves a comparison of phase difference between the two transmitted signals.One of the signals is termed as the reference signal and the other one is variable signal.The use of AM and FM with a wide frequency separation between them prevents cross modulation between the signals which would otherwise cause bearing errors in the system. VOR transmitter radiates both unidirectional and omni directional signals.

VOR Transmitter

The transmitter is crystal controlled and is amplitude modulated by A 30 Hz AM referred to as variable phase signal (directional signal) and A sub carrier AM of 9960 Hz with a deviation of plus minus 480 Hz which is frequency modulated at 30 Hz. (omni directional signal).A 1020 Hz AM is also transmitted for the station identification (morse code of 2 or 3 letters).The VOR beacon transmits a signal in all directions (omni directional).We call these signals “Radials”. The transmitted radials contain different information to separate the radials from each other. The VOR beacon radiates the first radial in the direction of magnetic north of the station (called zero radial).

The radial is measured from the difference in phase between the reference and variable signal. The 30 Hz reference signal always has the same phase whatever  aircraft’s position with respect to the station. The phase of the variable signal varies with the aircraft’s position. VOR receiver receives the ground signal, detects and separates the phase and compares the phase of the two signals. The receiver is able to gives the magnetic and relative bearing of the transmitter, Deviation of the aircraft from a selected course and To-From information.

VOR radiated VHF energy modulated with Reference signal (omni directional flash) and Variable phase signal (rotating beam) and the bearing of the VOR station is the phase difference between reference and variable phases (time difference between flash and beam).

The principle of VOR navigation involves a comparison of phase difference between the two transmitted signals. One of the signals is termed as the reference signal and the other one is variable signal. The use of AM and FM with a wide frequency separation between them prevents cross modulation between the signals which would otherwise cause bearing errors in the system. VOR transmitter radiates both unidirectional and omni directional signals.

The VOR display has four elements:

A.    A Rotating Course Card, calibrated from 0 to 360°, which indicates the VOR bearing chosen as the reference to fly TO or FROM. Here, the 345° radial has been set into the display. This VOR gauge also digitally displays the VOR bearing, which simplifies setting the desired navigation track.

B.   The Omni Bearing Selector, or OBS knob, used to manually rotate the course card.

C.    The CDI, or Course Deviation Indicator. This needle swings left or right indicating the direction to turn to return to course. When the needle is to the left, turn left and when the needle is to the right, turn right, When centered, the aircraft is on course. Each dot in the arc under the needle represents a 2° deviation from the desired course. This needle is more-frequently called the left-right needle, with the CDI term quickly forgotten after taking the FAA written exams. Here, the pilot is doing well, and is dead-on course—or maybe lazy and with the autopilot activated in the "NAV" mode.

D.     The TO-FROM indicator. This arrow will point up, or towards the nose of the aircraft, when flying TO the VOR station. The arrow reverses direction, points downward, when flying away FROM the VOR station. A red flag replaces these TO-FROM arrows when the VOR is beyond reception range, has not been properly tuned in, or the VOR receiver is turned off. Similarly, the flag appears if the VOR station itself is inoperative, or down for maintenance. Here, the aircraft is flying TO the station.