VHF Omni-directional Range (VOR)

Handout

Link to Handout

Aims and Objectives

Aims

• to learn about VOR equipment based, both on the ground and airborne
• to learn how to use VOR equipment to navigate an aircraft

Aims and Objectives

Objectives

• understand the navigational purpose of VOR
• understand how VOR radio transmitters and receivers function
• obtain information on VOR ground transmitting stations
• operate and interpret airborne VOR equipment
• identify and correct for errors in VOR indications
• obtain the fundamental skills to navigate an aircraft to/from a VOR
• develop the onboard workflows associated with the operation of a VOR

What is a VOR

What is a VOR

A VHF Omni-directional Radio (VOR) is:

• A ground-based radio station used for navigating our aircraft, using our onboard navigation aids
• Operates in the VHF radio frequency range 108-118 MHz, with 160 individual channels, spaced at 50KHz
• The term omni refers to the VOR ground station radiating an infinite number of tracks in all directions
• For practical purposes, we use 360 parts of this radio signal, each separated at 1°
• These are called radials and are relative to magnetic north

What is a VOR

• a VOR ground station transmits two VHF radio signals
  1. a reference phase signal which is transmitted in all directions (omni-directional)
  2. a variable phase signal which rotates uniformly through each radial
• the phases misalign by some amount on all other radials, with full misalignment on the 180° radial
• by measuring the difference in phase alignment, a VOR receiver can determine which radial it is located on

What is a VOR

• a VOR ground station also transmits a Morse-code identifying signal
• sometimes a VOR is undergoing maintenance, and will transmit “XP” (-··- ·–·) indicating that it is unusable
• a VOR ground station may also have limited voice communication facilities e.g. ATIS

What is a VOR

The Sunshine Coast Airport VOR on the Bundaberg VNC

• Note:
  • The Sunshine Coast VOR location
  • The Sunshine Coast VOR frequency 114.2 MHz
  • The Sunshine Coast VOR identifying code ··· ··- (SU)

What is a VOR

The Sunshine Coast Airport and Brisbane Airport VORs on the En Route Chart (ERC)

What is a VOR

The ERSA also has details on navigation aids e.g. the VOR at Brisbane Airport

• Note:
  • The identifier for the VOR (BN)
  • The frequency for the VOR (113.2 MHz)
  • The latitude/longitude of the VOR (272157.5S 1530821.4E)
  • Footnote (1): the ARP is located at a bearing of 216° and 1.6 NM from the VOR

VOR Range

VOR Range

• Like other VHF radio e.g. communication, the signal is primarily determined by line of sight
• Therefore, the primary considerations for the maximum range of a VOR are:
  • the altitude of the aircraft receiving the VOR signal
  • the height of the transmitting antenna of the VOR ground station
  • the height of the terrain between the VOR ground station and the receiving station
  • VOR errors

AIP GEN 1.5 specifies Rated Coverage of a VOR for planning purposes

Airborne VOR Equipment

Airborne VOR Equipment

• The airborne VOR equipment consists of:
  • The VOR receiving antenna — typically V-shaped to receive VHF frequencies 108-118 MHz
  • The VOR receiving radio — typically combined with the COM radio into what we call the VHF NAV-COM set
  • One or more VOR indicators to interpret the received signals into a display to the pilot

Airborne VOR Equipment

VOR Receiving Antenna

Airborne VOR Equipment

VHF NAV-COM 1 and 2 onboard Cessna 172 VH-OCP

Airborne VOR Equipment

VOR indicators 1 and 2 onboard Cessna 172 VH-OCP

Airborne VOR Equipment

VOR indicators 1 and 2 onboard Cessna 172 VH-OCP

Airborne VOR Equipment

OBS Course Card

• The OBS Course Card turns by turning the Omni-Bearing Selector (OBS)
• The set course is the one that falls under the Course Index
• Note: the term “course” is not used in Australia for navigation — it means the same as “track”

Airborne VOR Equipment

Course Index

• The VOR Course Index is the currently configured radial
• It can be adjusted by turning the OBS
• This VOR is set to the 355 radial under the Course Index

Airborne VOR Equipment

Omni-Bearing Selector (OBS)

• By turning the OBS, the OBS Course Card will turn
• The configured radial can be read from the Course Index

Airborne VOR Equipment

Course Deviation Indicator (CDI)

• The CDI will be centred or deflected to the left/right, depending on:
  • which VOR radial the aircraft is positioned on
  • which radial is configured by the OBS

Airborne VOR Equipment

Deviation Scale

• The Deviation Scale indicates how far the aircraft has deviated from the configured radial on the CDI
• Each dot represents 2° of deviation, with a maximum deflection of 10°
• For example, if:
  • the CDI is positioned to the left of centre by two dots
  • the Course Index is configured on the 355 radial
  • the Indicator Flag indicates “FROM”
• then:
  • the aircraft is 4° to the right of the 355 radial
  • it is on the 351 radial

Airborne VOR Equipment

Indicator Flag

• The Indicator Flag will read one of
  • TO: we are tracking to the VOR on the configured radial
  • FROM: we are tracking from the VOR on the configured radial
  • OFF: the indication on the VOR is not reliable

Airborne VOR Equipment

Glide Slope Indicator

• Some VORs are fitted with a Glide Slope Indicator and Indication Flag
• This indicator is not a component of this briefing

Airborne VOR Equipment

The VOR Indication is Independent of our Heading

• It is important to understand that the VOR Indication gives us a position on a VOR radial
• It is independent of our aircraft heading
• If our aircraft were in the same position, but on a different heading, the VOR indication would always be the same

VOR Errors

VOR Errors

• Although the AIP specifies a Rated Coverage for a VOR, a specific VOR may be subject to limitations due to errors
• These limitations are detailed in the ERSA
• For example, Sunshine Coast Airport VOR
  • Coverage reduced in minor arc 169 to 340 due terrain shielding
  • Bending and scalloping on 224 radial at low altitudes

VOR Errors

Ground Station Error

• The accuracy and strength of the signal from a VOR ground station can be affected by equipment
  • radio transmitter
  • radio antennae
  • power supply
• This may cause an error of accuracy up to 2°

VOR Errors

Site Effect Error

• The propagation path of VOR radio signals may be affected by physical obstacles located nearby the transmitter site
  • buildings
  • fences
  • terrain
• These errors are checked when the station is first commissioned, then periodically thereafter
• Site Errors are typically not more than 3°
• However, if large errors are unavoidable, they are published in the ERSA or if temporary, by NOTAM

VOR Errors

Terrain Effect Error

• The VOR signal may be distorted by signals reflected by terrain between the radio transmitter and the receiver onboard the aircraft
• This type of error causes oscillations of the Course Deviation Indicator (CDI) of the VOR instrument onboard the aircraft
  • Rapid oscillations are called scalloping
  • Slow oscillations are called course bending
• Terrain Errors are typically not more than 2°
• However, more significant errors will be published in the ERSA

VOR Errors

Airborne Equipment Error

• Errors may arise due to imperfections of the airborne equipment
• Airborne Equipment Errors are typically not more than 2°

VOR Errors

Vertical Polarisation Error

• VOR ground station emit radio waves which are oriented horizontally, parallel to the Earth’s surface
• An aircraft’s VOR antenna is sensitive to these horizontally polarised signals
• Radio signals may be reflected from large obstacles such that they become vertically oriented radio waves
• These signals will only be received if the aircraft is banked and the VOR antenna is moved away from its horizontal orientation
• Vertical Polarisation Error will be apparent as large, rapid deflections of the CDI during a turn, that stop when returned to level flight
• This type of error is generally rare

VOR Errors

Aggregate Error

• The combined effect of all sources of error to a VOR indication is the aggregate error
• The magnitude of error will vary and is difficult to precisely determine
• Under typical circumstances, the aggregate error rarely exceeds 5°

VOR Setup Procedures

VOR Setup Procedures

To set up the onboard VOR indicator, we follow a workflow: T.I.T.

1. Tune
2. Identify
3. Test

VOR Setup Procedures

Tune

1. Set the desired VOR frequency on the NAV radio
2. Press the flip-flop button to toggle the frequency from STANDBY to ACTIVE

VOR Setup Procedures

Identify

1. Press the desired NAV button on the NAV-COM control unit
2. This will allow you to hear the Morse Code for the VOR ground station
3. Confirm that you are hearing the correct station to positively identify it

VOR Setup Procedures

Test

1. Turn the OBS until the CDI is centred and the Indication Flag reads FROM
2. Take note of the Course Index to determine which radial you are on
3. Turn the OBS 4° from the current radial
4. Confirm that the CDI deflects appropriately
5. Turn the OBS in the opposite direction approximately 20°
6. Confirm that the CDI has maximum deflection in the opposite direction
7. Turn the OBS to return the CDI to the centre
8. If the CDI deflects as expected, we have positively tested operation of the VOR

VOR Tracking Procedures

VOR Tracking Procedures

Tracking TO a VOR Ground Station

• Turn the OBS until the CDI is centred and the Indication Flag reads TO
• Read off the bearing under the Course Index
• Set the aircraft on that track
• Remember to take into account drift angle when setting a heading

VOR Tracking Procedures

Tracking TO a VOR Ground Station

• For example, an aircraft positioned on the red arrow sets track of 050° to the VOR ground station
• An aircraft positioned on the purple arrow sets track of 230° to the VOR ground station

VOR Tracking Procedures

Tracking Abeam a VOR Ground Station

• It is common to use a VOR ground station to monitor progress of a flight and note the time when passing abeam
• Determine the current track of the aircraft
• Turn the OBS so that the Course Index reads 90° to the aircraft track
• If the aircraft is not within 10° abeam the VOR, the CDI will be fully deflected to one side
• As the aircraft passes to within 10° of the VOR ground station, the CDI will begin to move toward the centre
• When the CDI is centred, the aircraft is abeam the VOR ground station — note the time
• The CDI will deflect in the opposite direction until the aircraft is outside of 10° of the VOR ground station

VOR Tracking Procedures

Tracking Abeam a VOR Ground Station

• For example, for an aircraft on a track of 010°, turn the OBS to read 100 under the Course Index
• If the VOR ground station is to the left of intended track, the CDI will be fully deflected to the left, and vice versa
• As the aircraft continues on a track of 010°, the CDI will centre on the 100 radial — the aircraft is abeam the VOR ground station
• Continuing on track, the CDI will deflect to the right

VOR Tracking Procedures

Passing Overhead a VOR Ground Station

• With the CDI centred and the Indicator Flag reading TO, track toward a VOR ground station
• As the aircraft comes closer to the station, the CDI will become more sensitive and oscillate
• During a brief time overhead the station, the Indicator Flag will switch to OFF
• This area is called the cone of confusion and is dependent on the height of the aircraft above the station — the higher, the large will be the “cone”
• After passing overhead the station, and leaving the cone confusion, the Indicator Flag will read FROM
• The CDI oscillation will further stabilise as the aircraft flies further from the station

VOR Tracking Procedures

Intercepting a Track Using a VOR Ground Station

• To intercept a track, you must first determine:
  1. Where you are
  2. Where you want to go
  3. How you get there
• Where you are: Turn the OBS until the CDI is centred and the Indication Flag reads FROM — you are on this radial from the VOR station
• Where you want to go: Turn the OBS to the planned track to or from the VOR ground station and confirm the Indicator Flag reads correctly (TO/FROM)
• How you get there: Set the intended aircraft track before intercepting the planned track

VOR Tracking Procedures

Intercepting a Tracking Using a VOR Ground Station

• Initially, the CDI will be deflect to one side
• As the aircraft tracks closer to the intended intercept track, the CDI will move toward the centre
• Prior to the CDI reaching centre, start to turn on to the intended track so that the aircraft completes the turn with the CDI centred
• Select a heading for the intercept that accounts for any crosswind

VOR Tracking Procedures

Intercepting a Tracking Using a VOR Ground Station

• For example, while heading 198°M, you intended to track inbound on the 190 radial to a VOR ground station. The wind is westerly.
• Turn the OBS so that the CDI centres — you determine you are on the 230 radial
• Given a 90° intercept to the 190 radial, the aircraft track to the intercept will be 100°M
• Turn the OBS to the 190 radial with the Indicator Flag showing FROM
• The track to the VOR will be 010°M, with a wind correction of approximately 5° to the left (HDG 005°M)
• As the CDI begins to centre, start turning on to the intended inbound track
• When executed accurately, rolling out of the turn, the CDI will be centred on the 190 radial

Airmanship/TEM/HF

Quiz on Objectives

• VOR uses the VHF frequency band, which is “line of sight” — what does this mean?
• What workflow do we use to begin using a VOR ground station?
• What are some means by which to find information on VOR ground stations?
• An onboard VOR indicator reads 230 TO — which radial is it on?