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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 Chart Supplement U.S.
  • 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
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 Chart Supplement U.S. 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 Chart Supplement U.S. 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
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°

The VOR Minimum Operational Network (MON):

  • As flight procedures and route structure based on VORs are gradually being replaced with Performance−Based Navigation (PBN) procedures, the FAA is removing selected VORs from service
  • PBN procedures are primarily enabled by GPS and its augmentation systems, collectively referred to as Global Navigation Satellite System (GNSS)
  • Aircraft that carry DME/DME equipment can also use RNAV which provides a backup to continue flying PBN during a GNSS disruption
  • For those aircraft that do not carry DME/DME, the FAA is retaining a limited network of VORs, called the VOR MON, to provide a basic conventional navigation service for operators to use if GNSS becomes unavailable
  • During a GNSS disruption, the MON will enable aircraft to navigate through the affected area or to a safe landing at a MON airport without reliance on GNSS
  • Navigation using the MON will not be as efficient as the new PBN route structure, but use of the MON will provide nearly continuous VOR signal coverage at 5,000 feet AGL across the NAS, outside of the Western U.S. Mountainous Area (WUSMA)
    • There is no plan to change the NAVAID and route structure in the WUSMA
  • The VOR MON has been retained principally for IFR aircraft that are not equipped with DME/DME avionics however, VFR aircraft may use the MON as desired
  • Aircraft equipped with DME/DME navigation systems would, in most cases, use DME/DME to continue flight using RNAV to their destination however, these aircraft may, of course, use the MON
    • Distance to a MON airport:

      • Within the contiguous United States (CONUS), the VOR MON is designed to ensure that an airport that has an instrument approach that is not dependent on GPS, ADF, DME or radar within 100 NM of any location
      • These airports are referred to as “MON airports” and will have an ILS approach or a VOR approach if an ILS is not available
      • VORs to support these approaches will be retained in the VOR MON
      • MON airports are charted on low-altitude en route charts and are contained in the Chart Supplement U.S. and other appropriate publications
      • Note that any suitable airport can be used to land in the event of a VOR outage
        • For example, an airport with a DME−required ILS approach may be available and could be used by aircraft that are equipped with DME
        • The intent of the MON airport is to provide an approach that can be used by aircraft without ADF or DME when radar may not be available
    • Navigating to an airport:

      • The VOR MON will retain sufficient VORs and increase VOR service volume to ensure that pilots will have nearly continuous signal reception of a VOR when flying at 5,000 feet AGL
      • A key concept of the MON is to ensure that an aircraft will always be within 100 NM of an airport with an instrument approach that is not dependent on GPS
      • If the pilot encounters a GPS outage, the pilot will be able to proceed via VOR-to-VOR navigation at 5,000 feet AGL through the GPS outage area or to a safe landing at a MON airport or another suitable airport, as appropriate
      • Nearly all VORs inside of the WUSMA and outside the CONUS are being retained
      • In these areas, pilots use the existing (Victor and Jet) route structure and VORs to proceed through a GPS outage or to a landing
    • Using the VOR MON:

      • In the case of a planned GPS outage (for example, one that is in a published NOTAM), pilots may plan to fly through the outage using the MON as appropriate and as cleared by ATC
      • Similarly, aircraft not equipped with GPS may plan to fly and land using the MON, as appropriate and as cleared by ATC
        • In many cases, flying using the MON may involve a more circuitous route than flying GPS−enabled RNAV
      • In the case of an unscheduled GPS outage, pilots and ATC will need to coordinate the best outcome for all aircraft
      • It is possible that a GPS outage could be disruptive, causing high workload and demand for ATC service
      • Generally, the VOR MON concept will enable pilots to navigate through the GPS outage or land at a MON airport or at another airport that may have an appropriate approach or may be in visual conditions
        • The VOR MON is a reversionary service provided by the FAA for use by aircraft that are unable to continue RNAV during a GPS disruption
          • The FAA has not mandated that preflight or inflight planning include provisions for GPS- or WAAS-equipped aircraft to carry sufficient fuel to proceed to a MON airport in case of an unforeseen GPS outage
          • Specifically, flying to a MON airport as a filed alternate will not be explicitly required
          • Of course, consideration for the possibility of a GPS outage is prudent during flight planning as is maintaining proficiency with VOR navigation
        • Also, in case of a GPS outage, pilots may coordinate with ATC and elect to continue through the outage or land
          • The VOR MON is designed to ensure that an aircraft is within 100 NM of an airport, but pilots may decide to proceed to any appropriate airport where a landing can be made
          • WAAS users flying under Part 91 are not required to carry VOR avionics
          • These users do not have the ability or requirement to use the VOR MON
          • Prudent flight planning, by these WAAS-only aircraft, should consider the possibility of a GPS outage
            • The FAA recognizes that non-GPS−based approaches will be reduced when VORs are eliminated, and that most airports with an instrument approach may only have GPS- or WAAS-based approaches
            • Pilots flying GPS- or WAAS-equipped aircraft that also have VOR/ILS avionics should be diligent to maintain proficiency in VOR and ILS approaches in the event of a GPS outage

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 Chart Supplement U.S.
  • 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
Figure 6: Terminal Service Volume
VOR Low Standard Service Volume
Figure 7: Low Service Volume
VOR High Standard Service Volume
Figure 8: 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
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 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
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: