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VHF Omni-Directional Range

Introduction:

  • VHF Omni-Directional Rangers (VORs) provides 360° azimuth information in the form of radials expressed in magnetic heading, except for a few VOR stations that are located at very high latitudes
  • If collocated with Distance Measuring Equipment (DME), then resultant "VOR/DME" provides both azimuth and distance information
  • If collocated with military Tactical Air Navigation (TACAN) unit, then resultant "VORTAC" provides both azimuth and distance information
    • While transparent for the pilot, the charted symbol for a VORTAC and VOR/DME is different, despite the fact they provide the same thing
  • Ultimately, the Standard Service Volume dictates the reception limits of the NAVAID
NAVAIDS Depicted On Legend
Figure 1: NAVAIDS Depicted On Legend

Function:

  • Emits 2 signals, one for each 360° and another aligned with true north; combined they give an indication on the instrument
  • Subject to line of site limitations
  • The area directly over a VOR may cause erroneous indications and is referred to as the Cone of Confusion
  • Must identify a VOR with a Morse code identifier and/or with a recorded automated voice identifier by the word "VOR" following the range's name prior to use for navigation
    • During periods of maintenance a T-E-S-T code (-· ···-) code may radiate or the code / voice may be removed
    • Reliance on determining the identification of an omni-range should never be placed on listening to voice transmissions by the Flight Service Station (FSS) (or approach control facility) involved
      • Many FSSs remotely operate several omni-ranges with different names
      • In some cases, none of the VORs have the name of the "parent" FSS
  • Most VORs are equipped for voice transmission on the VOR frequency
    • The transmission consists of a voice announcement, "AIRVILLE VOR" alternating with the usual Morse Code identification
  • VORs without voice capability are indicated by the letter “W” (without voice) included in the class designator (VORW) as shown below as published in the Airport/Facility Directory (A/FD)
  • Some VOR equipment decodes the identifier and displays it to the pilot for verification to charts, while other equipment simply displays the expected identifier from a database to aid in verification to the audio tones
    • You should be familiar with your equipment and use it appropriately
    • If your equipment automatically decodes the identifier, it is not necessary to listen to the audio identification

    Mavrick VOR (TTT) - VORW
    Figure 2: Mavrick VOR (TTT) - VORW

  • Roughness:
    • On some VORs, minor course roughness may be observed, evidenced by course needle or brief flag alarm activity (some receivers are more susceptible to these irregularities than others)
    • Some stations, usually in mountainous terrain, may occasionally observe a brief course needle oscillation, similar of the indication of approaching the station
      • Pilots flying over unfamiliar routes are cautioned to be on the alert for these vagaries, and in particular, to use the "to/from" indicator to determine positive station passage
    • Roughness can also be caused by interference or certain propeller RPM/rotor speed settings and can cause momentary losses or irregularities in navigation signals which may be solved by slight changes in RPM settings
      • Fluctuations can be as much as &plusmn 6°
      • Pilots are urged to check for this modulation phenomenon prior to reporting a VOR station or aircraft equipment for unsatisfactory operation
    VOR Receive Only
    Figure 3: Receive Only Frequency (122.1R)
  • An R indicates you transmit on that frequency and listen on the VOR frequency
    • Example: At Monroeville VOR, you will transmit on 122.1 and receive on 116.8
  • Airplane displacement from a course is about 200' per dot per NM
  • At 30 NM out, one dot is 1 NM displacement; two dots, 2 NM
  • At 60 NM out, one dot is 2 NM displacement; two dots, 4 NM
  • Adjusted through the use of the Omni-Bearing Selector (OBS) knob

San Diego VOR
Figure 4: Mission Bay (MZB) VOR

Frequency Ranges:

  • All: 108.0-117.95 MHz (at a power output necessary to provide coverage within their assigned operational service volume)

  • LOC: 108-111.95 MHz (odd tenths)
  • VOR: 108-111.85 MHz (even tenths)
  • VOR: 112.0-117.95 MHz (all frequencies)

Application:

  • Navigation
  • Airways
  • Intercepting and tracking
  • Holding
  • Instrument Approaches
  • Homing
  • Tracking

Components:

  • VOR ground station or transmitter
  • VOR receiver
  • Aircraft display
  • Antenna

VHF Omni-Directional Range/Tactical Air Navigation (VORTAC):

  • A VORTAC is a facility consisting of two components, VOR and TACAN, which provides three individual services at a central location:
    • VOR Azimuth
    • TACAN Azimuth
    • TACAN Distance (DME)
  • Although consisting of more than one component, incorporating more than one operating frequency, and using more than one antenna system, a VORTAC is considered to be a unified navigation aid
    • Both components of a VORTAC are envisioned as operating simultaneously and providing the three services at all times
  • Transmitting signals of VOR and TACAN are each identified by three-letter code transmission and are interlocked so that pilots using VOR azimuth with TACAN distance can be assured both signals being received are definitely from the same ground station
  • The frequency channels of the VOR and the TACAN at each VORTAC facility are "paired" in accordance with the national plan to simplify airborne operation

VOR Test facilities (VOT):

  • The FAA VOR test facility (VOT) transmits a test signal which provides users a convenient means to determine the operational status and accuracy of a VOR receiver while on the ground where a VOT is located
    • The airborne use of VOT is permitted; however, its use is strictly limited to those areas/altitudes specifically authorized in the A/FD or appropriate supplement
  • Periodic VOR receiver calibration is most important:
    • If a receiver's Automatic Gain Control or modulation circuit deteriorates, it is possible for it to display acceptable accuracy and sensitivity close into the VOR or VOT and display out-of-tolerance readings when located at greater distances where weaker signal areas exist
    • The likelihood of this deterioration varies between receivers, and is generally considered a function of time
    • The best assurance of having an accurate receiver is periodic calibration
      • Yearly intervals are recommended at which time an authorized repair facility should recalibrate the receiver to the manufacturer's specifications
    • No correction other than the correction card figures supplied by the manufacturer should be applied in making these VOR receiver checks
  • Federal Aviation Regulations (14 CFR Section 91.171) provides for certain VOR equipment accuracy checks prior to flight under instrument flight rules
  • To comply with this requirement and to ensure satisfactory operation of the airborne system, the FAA has provided pilots with the following means of checking VOR receiver accuracy:
    1. VOT or a radiated test signal from an appropriately rated radio repair station
    2. Certified airborne check points
    3. Certified check points on the airport surface
  • A radiated VOT from an appropriately rated radio repair station serves the same purpose as an FAA VOR signal and the check is made in much the same manner as a VOT with the following differences:
    • The frequency normally approved by the Federal Communications Commission is 108.0 MHz
    • Repair stations are not permitted to radiate the VOR test signal continuously; consequently, the owner or operator must make arrangements with the repair station to have the test signal transmitted
      • This service is not provided by all radio repair stations
      • The aircraft owner or operator must determine which repair station in the local area provides this service
      • A representative of the repair station must make an entry into the aircraft logbook or other permanent record certifying to the radial accuracy and the date of transmission
      • The owner, operator or representative of the repair station may accomplish the necessary checks in the aircraft and make a logbook entry stating the results
      • It is necessary to verify which test radial is being transmitted and whether you should get a "to" or "from" indication
  • Test facilities transmit a test signal for ground and some airborne use
    • Allows users a convenient means to determine the operational status and accuracy of a VOR receiver
  • Identified by either a series of dots or a continuous tone with the VOT frequency on your VOR receiver:
    • With the Course Deviation Indicator (CDI) centered, the omni-bearing selector should read 0° with the to/from indication showing "from" or the omni-bearing selector should read 180° with the to/from indication showing "to"
    • Should the VOR receiver operate an RMI (Radio Magnetic Indicator), it will indicate 180° on any omni-bearing selector (OBS) setting
    • Two means of identification are used. One is a series of dots and the other is a continuous tone
    • Information concerning an individual test signal can be obtained from the local FSS
  • Information concerning an individual test signal can be obtained from the local FSS
  • Certified airborne and ground checkpoints:
    • Consist of radials that should be received at specific points on the surface or over landmarks
    • Airborne and ground check points consist of certified radials that should be received at specific points on the airport surface or over specific landmarks while airborne in the immediate vicinity of the airport
    • Approved locations can be found in the A/FD with the supplemental pages
    • Should an error in excess of plus or minus 4° be indicated through use of a ground check, or plus or minus 6° using the airborne check, Instrument Flight Rules (IFR) flight must not be attempted without first correcting the source of the error
  • If a dual system VOR (units independent of each other except for the antenna) is installed in the aircraft, one system may be checked against the other
    • Turn both systems to the same VOR ground facility and note the indicated bearing to that station
    • The maximum permissible variations between the two indicated bearings is 4°

VOR Receiver Checkpoints
Figure 5: VOR Receiver Checkpoints

Accuracy/Tolerances:

  • The effectiveness of the VOR depends upon proper use and adjustment of both ground and airborne equipment
  • The accuracy of course alignment of the VOR is excellent, being generally ± 1°
    • VOT ± 4° (Should indicate 180 TO or 360 FROM)
      Ground ± 4° (Should indicate 180 TO or 360 FROM)
      Airborne ± 6° (within)
      Airway ± 6° (within)
      Dual 4° Difference (against one another)
  • No correction other than the correction card figures supplied by the manufacturer should be applied in making these VOR receiver checks
  • Outside of these tolerances, Instrument Flight Rules (IFR) flight must not be attempted without first correcting the source of the error

  • Course alignment is generally ± 1°
  • If in error, IFR may not be conducted; test receivers yearly (recommended not required)
  • Report all errors to ATC, FSS, or FAA
  • Must be checked every 30 days for IFR flight

Limitations:

  • Line of site:
    • The range varies proportionally to the altitude of the receiving equipment
    • This means the farther from the station, the higher you must be
    • See Standard Service Volume (SSV) for more range restrictions
  • Misinterpretation:
    • Receiving two signals on same frequency
  • Reverse sensing:
    • Reverse sensing flying TO a station with a FROM
      indication or a FROM with a TO indication
  • Bending & Scalloping:
    • Built to provide maximum error of 2.5°
    • Signal reflects off terrain and buildings
  • Polarization:
    • Error up to 10°
  • Cone of Confusion
  • CDI sticking
  • OBS Calibration
    • Error between radial selected and the one shown in the course selection window
  • Propeller Modulation:
    • If signal passes through prop, arc modulation can be changed
    • CDI needle may fluctuate as much as plus or minus 6°

Standard Service Volume (SSV):

  • Most air navigation radio aids which provide positive course guidance have a designated standard service volume (SSV)
  • The SSV defines the reception limits of unrestricted NAVAIDs which are usable for random/unpublished route navigation
  • Each NAVAID operates at the power necessary to provide coverage within their assigned operational service volume
  • A NAVAID will be classified as restricted if it does not conform to flight inspection signal strength and course quality standards throughout the published SSV
    • However, the NAVAID should not be considered usable at altitudes below that which could be flown while operating under random route IFR conditions (14 CFR Section 91.177), even though these altitudes may lie within the designated SSV
  • Service volume restrictions are first published in Notices to Airmen (NOTAMs) and then with the alphabetical listing of the NAVAIDs in the A/FD
  • 3 Classes of VOR (Terminal, Low, High)
  • The SSV of a station is indicated by using the class designator as a prefix to the station type designation
    • Example: TVOR, LDME, HVORTAC
  • Standard Service Volume (SSV) does not apply to airways as they've been certified at their respective distances
VOR Terminal Standard Service Volume
VOR Low Standard Service Volume
VOR High Standard Service Volume
Figure 6:
Figure 7:
Figure 8:
Terminal Service Volume
Low Service Volume
High Service Volume

NOTE:
All elevations shown are AGL and the cone of confusion still applies

VOR/DME/TACAN Standard Service Volumes
Figure 9: VOR/DME/TACAN Standard Service Volumes

Service Volume Lower Edge Terminal
Figure 10: Service Volume Lower Edge Terminal

Service Volume Lower Edge Standard High and Low
Figure 11: Service Volume Lower Edge Standard High and Low

Time to Station:

  • 60 x minutes flown between bearing change
  • Degrees of bearing change

Distance to Station:

  • TAS x minutes flown
  • Degrees of bearing change

RECEIVER OUTPUTS TO DISPLAY VOR MODES
DISPLAY OPERATE FAIL OFF
BEARING TO STATION HSI VOR Bearing Pointer, and Digital Bearing Displayed VOR Bearing Pointer, and Digital Bearing removed VOR Bearing Pointer, and Digital Bearing removed

HUD Command Heading Marker displayed Command Heading Marker displayed Command Heading Marker Displayed
CDI COURSE LINE HSI CDI Course line displays deviation Center portion of CDI course line removed Center portion of CDI course line removed

HUD Command Steering Marker shows VOR Bearing Command Heading Marker removed Command Heading Marker removed
VOR SIGNAL VALID: VOR Bearing Pointer, Digital Bearing, and Command Heading Marker displayed VOR Bearing Pointer, Digital Bearing, and Command Heading Marker displayed VOR Bearing Pointer, Digital Bearing, and Command Heading Marker displayed

NOT VALID: VOR Bearing Pointer, Digital Bearing, and Command Heading Marker removed VOR Bearing Pointer, Digital Bearing, and Command Heading Marker removed VOR Bearing Pointer, Digital Bearing, and Command Heading Marker removed


Conclusion:

  • Pilots should be aware of the possibility of momentary erroneous indications on cockpit displays when the primary signal generator for a ground-based navigational transmitter is inoperative
    • Pilots should disregard any navigation indication, regardless of its apparent validity, if the particular transmitter was identified by NOTAM or otherwise as unusable or inoperative
  • Additionally tools are available to better increase your knowledge of navigation including VOR/NDB Simulators

References: