Figure 11-96.  V-shaped, horizontally polarized, bi-pole antennas are commonly used for VOR and VOR/glideslope reception. All antenna  shown are VOR/glideslope antenna.  Figure 11-95.  A VOR ground station. transmitter stations, or VORs, that communicate with radio  receiving equipment on board aircraft. Many of the VORs  are located along airways. The Victor airway system is built  around the VOR navigation system. Ground VOR transmitter  units are also located at airports where they are known as  TVOR (terminal VOR). The U.S. Military has a navigational  system known as TACAN that operates similarly to the VOR  system. Sometimes VOR and TACAN transmitters share a  location. These sites are known as VORTACs.  Early flight instruments contributed greatly to flying when the  ground was obscured by clouds. Navigation aids were needed  to indicate where an aircraft was over the earth as it progressed  towards its destination. In the 1930s and 1940s, a radio  navigation system was used that was a low frequency, four- course radio range system. Airports and selected navigation  waypoints broadcast two Morse code signals with finite ranges  and patterns. Pilots tuned to the frequency of the broadcasts  and flew in an orientation pattern until both signals were  received with increasing strength. The signals were received as a blended tone of the highest volume when the aircraft  was directly over the broadcast area. From this beginning,  numerous refinements to radio navigational aids developed.  The position of all VORs, TVORs, and VORTACs are  marked on aeronautical charts along with the name of the  station, the frequency to which an airborne receiver must be  tuned to use the station, and a Morse code designation for  the station. Some VORs also broadcast a voice identifier  on a separate frequency that is included on the chart.  [Figure 11-95] Radio navigation aids supply the pilot with intelligence  that maintains or enhances the safety of flight. As with  communication radios, navigational aids are avionics  devices, the repair of which must be carried out by trained  technicians at certified repair stations. However, installation,  maintenance and proper functioning of the electronic units,  as well as their antennas, displays, and any other peripheral  devices, are the responsibilities of the airframe technician. VOR uses VHF radio waves (108–117.95 MHz) with 50 kHz  separation between each channel. This keeps atmospheric  interference to a minimum but limits the VOR to line-of- sight usage. To receive VOR VHF radio waves, generally  a V-shaped, horizontally polarized, bi-pole antenna is  used. Other type antennas are also certified. Follow the  manufacturer’s instructions for installation location.  [Figure 11-96]  One of the oldest and most useful navigational aids is the  VOR system. The system was constructed after WWII and  is still in use today. It consists of thousands of land-based  11-43 VOR Navigation System