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The VHF Omnidirectional Range (VOR) is the backbone of the National Airway System, and this radio aid to navigation (NAVAID) provides guidance to pilots operating under visual flight rules as well as those flying instruments.
On Sectional Aeronautical Charts, VOR locations are shown by blue symbols centered in a blue compass rose which is oriented to Magnetic North. A blue identification box adjacent to the VOR symbol lists the name and frequency of the facility, its three-letter identifier and Morse Code equivalent, and other information as appropriate. See the “Radio Aids to Navigation and Communications Box” information in FAA Legend 1.
Some VORs have a voice identification alternating with the Morse code identifier. Absence of the identifier indicates the facility is unreliable or undergoing routine maintenance; in either case, it should not be used for navigation. Some VORs also transmit a T-E-S-T code when undergoing maintenance.
The VOR station continuously transmits navigation signals, providing 360 magnetic courses to or radials from the station. Courses are TO the station and radials are FROM the station.
TACAN, a military system which provides directional guidance, also informs the pilot of the aircraft’s distance from the TACAN Station. When a VOR and a TACAN are co-located, the facility is called a VORTAC. Civil pilots may receive both azimuth and distance information from a VORTAC.
At some VOR sites, additional equipment has been installed to provide pilots with distance information. Such an installation is termed a VOR/DME (for distance measuring equipment).
Cockpit display of VOR information is by means of an indicator.
The omni bearing selector (OBS) is an azimuth dial which can be rotated to select a course or to determine which radial the aircraft is on. The TO/FROM indicator shows whether flying the selected course would take the aircraft to or from the VOR station. A TO indication shows the radial selected is on the far side of the VOR station, while a FROM indication means the aircraft and the selected course are on the same side.
The course deviation indicator (CDI), when centered, indicates the aircraft is on the selected course, or, when not centered, whether that course is to the left or right of the aircraft. For example, Figure 10-2 is indicating that a course of 030° would take the aircraft to the selected station, and to get on that course, the aircraft would have to fly to the left of 030°.
To determine position in relation to one or more VOR stations, first tune and identify the selected station. Next, rotate the OBS until the CDI centers with a FROM indication. The OBS reading is the magnetic course from the VOR station to the aircraft. Repeat the procedure using a second VOR. The aircraft is located at the point where the two lines of position cross.
To determine the course to be flown to a VOR station on the sectional aeronautical chart, first draw a line from the starting point to the VOR symbol in the center of the compass rose. At the point where the course line crosses the compass rose, read the radial. The course to the station is the reciprocal of that radial.
The routes established between VORs are depicted by blue-tinted bands showing the airway number following the letter “V,” and are called Victor airways.
When approaching a VOR where airways converge, a pilot must exercise extreme vigilance for other aircraft. In addition, when climbing or descending VFR on an airway, it is considered good operating practice to execute gentle banks left and right for continuous visual scanning of the airspace.
VOR receiver accuracy may be checked by means of a VOR Test Facility (VOT), ground check points, or airborne check points.
VOTs transmit only the 360° radial signal. Thus, when the OBS is set to 360°, the CDI will center with a FROM indication; while the reciprocal, 180°, will cause the CDI to center with a TO indication. An accuracy factor of plus or minus 4° is allowed when using a VOT facility.
GPS is a United States satellite-based radio navigational, positioning, and time transfer system operated by the Department of Defense. The system provides highly accurate position and velocity information and precise time on a continuous global basis to an unlimited number of properly equipped users. The GPS constellation of satellites is designed so that a minimum of five are always observable by a user anywhere on earth. The GPS receiver uses data from a minimum of four satellites to yield a three dimensional position (latitude, longitude, and altitude) and time solution.
The GPS receiver verifies the integrity (usability) of the signals received from the GPS satellites through receiver autonomous integrity monitoring (RAIM) to determine if a satellite is providing corrupted information. Without RAIM capability, the pilot has no assurance of the accuracy of the GPS position. If RAIM is not available, another type of navigation and approach system must be used, another destination selected, or the trip delayed until RAIM is predicted to be available on arrival.