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VOR facilities are aurally identified by Morse code, or voice, or both. The VOR can be used for ground-to-air communication without interference with the navigation signal. VOR facilities operate within the 108.0 to 117.95 MHz frequency band and assignment between 108.0 and 112.0 MHz is in even-tenth increments to preclude any conflict with ILS localizer frequency assignment, which uses the odd tenths in this range. 

The airborne equipment includes an antenna, a receiver, and the indicator instrument. The receiver has a frequency knob to select any of the frequencies between 108.0 to 117.95 MHz. The ON/OFF/volume control turns on the navigation receiver and controls the audio volume. The volume has no effect on the operation of the receiver. You should listen to the station identifier before relying on the instrument for navigation. 

VOR indicator instruments have at least the essential components shown in the instrument illustrated in Figure 9-12. 

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## Omnibearing Selector (OBS) 

The desired course is selected by turning the omnibearing selector (OBS) knob until the course is aligned with the course index mark or displayed in the course window. 

## Course Deviation Indicator (CDI) 

The course deviation indicator (CDI) is composed of an instrument face and a needle hinged to move laterally across the instrument face. The needle centers when the aircraft is on the selected radial or its reciprocal. Full needle deflection from the center position to either side of the dial indicates the aircraft is 12° or more off course, assuming normal needle sensitivity. The outer edge of the center circle is 2° off course; with each dot representing an additional 2°. 

## TO/FROM Indicator 

The TO/FROM indicator shows whether the selected course, if intercepted and flown, takes the aircraft TO or FROM the station. It does not indicate whether the aircraft is heading to or from the station. 

## Flags or Other Signal Strength Indicators 

The device that indicates a usable or an unreliable signal may be an “OFF” flag. It retracts from view when signal strength is sufficient for reliable instrument indications. Alternately, insufficient signal strength may be indicated by a blank or OFF in the TO/FROM window. 

The indicator instrument may also be a horizontal situation indicator (HSI), which combines the heading indicator and CDI. [Figure 9-13] The combination of navigation information from VOR/Localizer (LOC) with aircraft heading information provides a visual picture of the aircraft’s location and direction. This decreases pilot workload especially with tasks such as course intercepts, flying a back-course approach, or holding pattern entry. (See Chapter 5, Flight Instruments, for operational characteristics.) [Figure 9-14] 

## Function of VOR 

## Orientation 

The VOR does not account for the aircraft heading. It only relays the aircraft direction from the station and has the same indications regardless of which way the nose is pointing. Tune the VOR receiver to the appropriate frequency of the selected VOR ground station, turn up the audio volume, and identify the station’s signal audibly. Then, rotate the OBS to center the CDI needle and read the course under or over the index. 

In Figure 9-12 , 360° TO is the course indicated, while in Figure 9-15 , 180° TO is the course. The latter indicates that the aircraft (which may be heading in any direction) is, at this moment, located at any point on the 360° radial (line from the station) except directly over the station or very 

