Figure 7-11. Figure 9-11. VOR transmitter (ground station). Figure 9-12. The VOR indicator instrument. OBS N E S W 3 33 24 21 15 12 30 6 FR TO N A V OBS knob CDI needle Course index Unreliable signal flag FROM indicator Approximately 2 degrees in the VOR mode TO indicator 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. 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. 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°. 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. 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] 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  Function of VOR Orientation Flags or Other Signal Strength Indicators TO/FROM Indicator Course Deviation Indicator (CDI) Omnibearing Selector (OBS) 9-11