High Frequency Omnidirectional Range

The VHF Omnidirectional Range navigation system, VOR, was probably the most significant aviation invention other than the jet engine. With it, a pilot can simply, accurately, and without ambiguity navigate from Point A to Point B. The widespread introduction of VORs began in the early 1950s and 50 years later it remains the primary navigation system in the overwhelming majority of aircraft. VHF omnidirectional radio range (VOR), is a type of short-range radio navigation system for aircraft, enabling aircraft to determine their position and stay on course by receiving radio signals transmitted by a network of fixed ground radio beacons with a receiver unit.

It uses radio frequencies in the very high frequency (VHF) band from 108 to 117.95 MHz. Developed in the US beginning in 1937 and deployed by 1946, VOR is the standard air navigational system in the world, used by both commercial and general aviation. There are about 3000 VOR stations around the world. It is practically free from static and night effect therefore is a reliable navigational aid by day and night.

Radials are identified by numbers beginning with 001, which is 1° east of magnetic north, and progress in sequence through all the degrees of a circle until reaching 360. To aid in orientation, a compass rose reference to magnetic north is superimposed on aeronautical charts at the station location.

TWO CATEGORIES OF VOR

1. Normal VOR Beacon for en-route navigation- has radio frequency carrier output of about 200 Watts to provide a service range of up to 200 nautical miles. (Category A)

2. Terminal VOR- has a lower output of about 50 Watts to provide the limited coverage(25 nautical miles) required for approach and let down to an airport.

(Category B) BASIC PRINCIPLE OF OPERATION

The basic principle of operation of the VOR is very simple: the VOR facility transmits two signals at the same time. One signal is constant in all directions, while the other is rotated about the station. The airborne equipment receives both signals, looks (electronically) at the difference between the two signals, and interprets the result as a radial from the station. VOR stations broadcast a VHF radio composite signal including the station’s identifier, voice (if equipped), and navigation signal. The identifier is typically a two- or three-letter string in Morse code. The voice signal, if used, is usually the station name, in-flight recorded advisories, or live flight service broadcasts. The navigation signal allows the airborne receiving equipment to determine a magnetic bearing from the station to the aircraft (direction from the VOR station in relation to the Earth’s magnetic North at the time of installation). VOR stations in areas of magnetic compass unreliability are oriented with respect to True North.

A VOR ground station sends out a master signal, and a highly directional second signal that varies in phase 30 times a second compared to the master. This signal is timed so that the phase varies as the secondary antenna spins, such that when the antenna is 90 degrees from north, the signal is 90 degrees out of phase of the master. By comparing the phase of the secondary signal to the master, the angle (bearing) to the station can be determined. This bearing is then displayed in the cockpit of the aircraft, and can be used to take a fix as in earlier radio direction finding (RDF) systems, although it is, in theory, easier to use and more accurate. This line of position is called the “radial” from the VOR. The intersection of two radials from different VOR stations on a chart provides the position of the aircraft. VOR stations are fairly short range, the signals have a range of about 200 miles.

VOR EQUIPMENT

Course deviation indicator
-A course deviation indicator (CDI) is an avionics instrument used in aircraft navigation to determine an aircraft’s lateral position in relation to a track. If the location of the aircraft is to the left of course, the needle deflects to the right, and vice versa.

VOR Antennas

-One or two metre-long antennae sticking up and back out of the roof of the cockpit. These are the antennae for your VHF communication radios, the ones used to talk to traffic and to control towers.

VOR Receiver

– The VOR receiver converts signals from the antenna to the readings displayed on the navigation indicator.

VOR Ground Equipment

– Consist of a VOR ground station which is a small low building topped with a white disc upon which are located the VOR antennas and fibreglass cone-shaped tower.

CONE OF CONFUSION

Ground transmitters radiate signals in elevation 60° to 80° above horizon. A gap over head in the form of an inverted cone is left with no or weak radiation. Flying through this region causes confusion in indications in the airborne equipment. Passing through this zone the indications flick rapidly. To determine precisely the overhead position is difficult but positive and stable indication thereafter confirm passage of the station.

STATION IDENTIFICATION

A Morse code amplitude modulated signal comprising normally three letters is transmitted every ten seconds for positive identification of the VOR by aircrew. Some VORs carry voice transmission also for automatic terminal information service (ATIS) and identification. Limited voice communication one way facility by ground control may also be available in event of communication failure on normal VHF. The voice signal, if used, is usually the station name, in-flight recorded advisories, or live flight service broadcasts.

The navigation signal allows the airborne receiving equipment to determine a magnetic bearing from the station to the aircraft (direction from the VOR station in relation to the Earth’s magnetic North at the time of installation). VOR stations in areas of magnetic compass unreliability are oriented with respect to True North. During maintenance a test signal or no identification may be received. Pilots using VOR must always positively identify the station before use and continue to monitor the same.

MONITORING.

A monitor unit near the transmitter on ground is located within area of radiation of the VOR transmitter. The monitor continuously compares the received signal with specified parameters. In event of any of the following, the monitor switches off the VOR transmitter or withholds the identification and navigation signal transmission. (a) Received bearing is in error by more than 1°

(b) Either of the signals – master or secondary signal fall below 15% in strength (c) Monitor itself fails. A standby transmitter is provided to takeover in case of malfunction, but it takes some time to stabilize its transmission. Therefore, it is emphasized that pilots must listen for identification of the code for sake of safety.

SERVICE VOLUMES

A VOR station serves a volume of airspace called its Service Volume. Some VORs have a relatively small geographic area protected from interference by other stations on the same frequency—called “terminal” or T-VORs. Other stations may have protection out to 130 nautical miles (NM) or more. Although it is popularly thought that there is a standard difference in power output between T-VORs and other stations, in fact the stations’ power output is set to provide adequate signal strength in the specific site’s service volume. In the United States, there are three standard service volumes (SSV): Terminal, Low, and High (Standard Service Volumes do not apply to published Instrument Flight Rules (IFR) routes). US Standard Service Volumes (excerpted from FAA AIM)|

* 1 nautical mile = 1.85200 kilometers

VOR INDICATOR

A : Rotating Course Card is calibrated from 0 to 360 degrees, which indicates the VOR bearing chosen as the reference to fly by pilot. B : Omni Bearing Selector or OBS knob , used to manually rotate the course card to where the point to fly to. C : TO-FROM indicator . The triangle arrow will point UP when flying to the VOR station. The arrow will point DOWN when flying away from the VOR station. A red flag replaces theseTO-FROM arrows when the VOR is beyond reception range or the station is out. D : Course Deviation Indicator (CDI). This needle moves left or right indicating the direction to turn the aircraft to return to course. DOT : The horizontal dots at center are represent the aircraft away from the course . Each dot represent 2 degrees deviate from desired course.

Aircraft VOR Component

1. VOR RECEIVER: In many modern aircraft one control unit is used for both the VOR receiver and the VHF communications transceiver. When located together, the radio is called a NAVCOM (See Navcom Control Panels figure). The VOR signals are received on the antenna, normally located on the vertical stabilizer or on top of the fuselage. This antenna resembles a “V” lying in a horizontal plane. The VOR receiver converts signals from the antenna to the readings displayed on the navigation indicator.

2. NAVIGATION INDICATOR: The VOR navigation indicator gives the pilot aircraft position information by means of three components. The track selector, sometimes called the omnibearing selector or OBS, is used to rotate the azimuth ring, which displays the selected VOR track, (See Navigation Indicator figure). This ring may also show the reciprocal of the selected track.

The TO-FROM/OFF flag indicates whether the track will take the pilot to or from the station. If the aircraft is out of stationrange and cannot receive a reliable, usable signal the TO-FROM/OFF indicator displays OFF. Also, the OFF flag is displayed when the aircraft is directly over the station, when abeam of the station in the area of ambiguity (i.e., directly on either side of the station) or when beyond the reception range of the station selected. When the aircraft heading agrees generally with the track selector, the track deviation bar (TB) shows the pilot the position relative to the track selected and indicates whether the radial is to the right or left The TB needle has a l 0° spread from center to either side when receiving a VOR signal. The Track Bar), shows that an aircraft 5° off track would have the TB one-half of the way from center to the outside edge. If the aircraft is 10° off track the needle swings completely to one side. Each dot on the navigation indicator represents 2° when the pilot is flying VOR.

3. TRACK ARROW: Each time a track is chosen on the selector, the area around the VOR station is divided into halves or envelopes (see Left Right Envelopes figure). It is helpful to think of the dividing line as a track arrow, which runs through the station and points in the direction of the selected track. The TB shows the pilot in which of these two envelopes the aircraft is located. If the aircraft is flying along the track line, the TB needle is centered. If the aircraft flies to the left of the track arrow (as in position A), the TB needle swings to the right. If the aircraft moves to the right of the track arrow, (position B), the TB needle swings to the left. Whenever the pilot changes the track selector, he or she should visualize an imaginary track arrow over the station. In this way, the pilot can look at the TB and tell in which envelope the aircraft is located.

4. REFERENCE LINE: When the pilot selects a track, the position of another line is established, a reference line perpendicular to the track arrow and intersecting it at the station. The reference line divides the VOR reception area into two additional sectors. The area forward of the reference line is the FROM envelope and the area to the rear of the reference line is the TO envelope. The TO-FROM indicator shows in which envelope the aircraft is located. In the To-From Envelopes figure, both aircraft display a FROM reading.

5. VOR Indications figure, shows the readings that an aircraft would receive in eight different locations around the VOR station. In position A, the aircraft shows a centered TB, indicating that it is on track; the TO-FROM flag shows FROM. Position B shows a left TB and a FROM indication. Aircraft at positions C and G are in the area of ambiguity. In this area, the opposing reference signals that actuate the TO-FROM indicator cancel each other and produce an OFF Indication. The area of ambiguity widens with increasing distance from the station. The greater the distance, the longer the TO-FROM flag will indicate OFF as the aircraft moves between the TO and FROM envelopes.

DETERMINATION OF POSITION

1. HEADING: Aircraft heading has absolutely no effect on the readings of the VOR indicator. No matter which direction the aircraft is heading, the pilot receives the same indication as long as the aircraft remains in the same track envelope (see VOR Orientation).

2. POSITION FIX: To determine a fix (without DME), the pilot must use two VOR stations because the VOR gives only direction and not distance from the station. First, the pilot should tune the number one VOR to one of the desired stations and make positive identification. Unless the pilot makes positive identification, that station should not be used. If a VOR station is shut down for maintenance or the signal is unreliable because of a malfunction, the navaid identification is not transmitted. After identifying the station, the pilot should center the TB needle with the positive FROM indication on the TO-FROM/OFF flag. The pilot repeats this procedure with the other VOR station. Then, using the chart, the pilot draws a line outbound from the VORs along the radials indicated by the track selector. The intersection of these bearings is the aircraft’s position (see VOR Position Fix figure).

FLIGHT TO A VOR STATION

1. BRACKETING: Because there is generally a crosswind, the pilot rarely can intercept a radial, take up the heading of that track, and fly directly to the station. To stay on track, the pilot must make a series of small corrections. The process of intercepting a radial and making the corrections necessary to remain on track is called bracketing. The method described here minimizes the number of turns needed to determine the necessary wind correction, and requires the least attention by the pilot. Figure Bracketing a VOR Radial, below, shows the series of maneuvers that a pilot uses in bracketing a radial to a VOR station. The pilot of the aircraft in position l determines that the radial of the desired VOR station is to the right and the pilot must turn right to intercept it. In position 2, the pilot turns to an intercept angle of 30°. Since the radial is 090° to the station, the intercept heading is 120° as shown on the heading indicator. In position 3, the aircraft intercepts the radial. The pilot immediately turns the aircraft to a 090° heading to coincide with the inbound direction of the radial. While using the heading indicator to carefully bold the heading, the pilot in position 4 starts to drift off track.

The pilot then takes up a new intercept heading of 070° a 20° intercept angle. The pilot flies this new intercept heading of 070° until re-intercepting the radial, at which time (position 7) he or she divides this intercept angle by two and then turns to the new heading which is 080°. The new heading of 080° lets the aircraft drift a little north of track. This informs the pilot that the desired track heading must be somewhere between 090°, which allows the aircraft to drift south of the radial, and 080°, which takes the aircraft north of the radial. At no time from this point to the station will the pilot turn to a heading less than 080° or heading more than 090°. As shown in position 9, the aircraft takes up the heading of 090°, which allows the aircraft to drift back onto the radial. As the aircraft intercepts the radial at position 10, the pilot turns to a heading between 090° and 080°, then proceeds to the station, tracking the radial with an aircraft heading 085°.

If the pilot takes up a specific intercept angle and then divides the angle by two, as necessary, the aircraft brackets the radial with the least number of turns and holds e track with the greatest accuracy. TRACK TO THE STATION: The pilot should check the heading indicator against the magnetic compass when beginning to track. (The VOR indicator tells the pilot only the position of the aircraft relative to a certain radial and the pilot must rely upon the heading indicator for aircraft heading formation). The most common use of VOR navigation to fly on a radial from station to station. The pilot selects a radial course on the OBS and tracks that radial by keeping the TB needle centered, which occurs as long as the BS is in general agreement with the heading indicator.

For example, if the dial is to the right, the indicator will point the right, and the pilot must turn in this direction to intercept the radial. As the aircraft passes the VOR station, the VOR receives two basic indications provided that the aircraft crosses directly over the station. The most positive indication is that TO-FROM indicator changes to the opposite reading. (TO to FROM). The second, less certain indication is the fluctuation of the TB. If the aircraft passes directly over the station, the needle fluctuates from side to side and returns to its original position. If the aircraft is left of track, the needle does not fluctuate, bur continues to point to the right. Likewise, if the aircraft is right of track, the needle will point to the left and not fluctuate as the aircraft passes abeam the station.

TIME CHECK: Another use for VOR is to take a time check, which informs the pilot of the time remaining to fly to a station. For example, while inbound to the station on the 022° radial (See VOR Time Check figure), the pilot wishes to estimate the time to the station. The pilot elects to use the 030° radial to begin the time check, and turns the aircraft to a heading of 120°, which is at right angles to the 030° radial. The OBS is turned to 030° and as the needle centers, the pilot notes the time. Immediately afterward, the pilot rotates the OBS to 040°, which is the next radial to be used in the time check. The pilot then continues to bold the 120° aircraft heading and flies to the 040° radial.

As the pilot crosses this radial and the needle centers, he or she notes the time and finds that it has taken two minutes (120 seconds) to make the 10° radial change. The formula for determining the time remaining to the station is: (TIME IN SECONDS BETWEEN RADIAL CHANGE)/(DEGREES OF RADIAL CHANGE) equals TIME TO STATION IN MINUTES. Therefore, by dividing 120 seconds by 10, the pilot finds that there are 12 minutes remaining to fly to the station. Although this problem can be worked out using any degree of radial change, l0 degrees of radial change is the simplest and fastest to compute.

RANGE AND ACCURACY

Range

* The VHF signals associated with VORs are propagated through line-of-sight. Line of sight range of the transmitter located at sea level and aircraft at 5000 feet would be about 88nm and at 10000 feet about 125 nm. Accuracy

* Published VOR radials are accurate within 3°.

TYPES OF VOR

DVOR (Doppler Very High Frequency OmniDirectional Range)

A ground-based navigational aid operating at very high frequency and using a wide-aperture radiation system to reduce azimuth errors caused by reflection from terrain and other obstacles; makes use of the Doppler principle to solve the problem of ambiguity that arises from the use of a radiation system with apertures that exceed one-half wavelength; the system is so designed that its signals may be received on the equipment used for the narrow-aperture VOR (very-high-frequency omnidirectional radio range).

Test VOR (VOT)

-Transmitter installed for testing the airborne equipment during the pre-flight checks.
– This is not to be used for any navigational information.
Terminal VOR (TVOR)
* A low-powered VOR (very high frequency omnidirectional radio range) located at or near an airport for arrival and departure navigation.

Weather Broadcast VOR (BVOR)
* Transmits voice weather information of selected aerodomes in between the identification signals.

VOR Tactical Air Navigation(VORTAC)
– A ground radio station consisting of a collocated very-high-frequency omnidirectional radio range (VOR) and Tacan facility – can be used by civil aircraft as VOR/DME combination.
– Transmitted 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 that both signals being received are definitely from the same ground station. DBVORTAC (Broadcasting Doppler VORTAC)

* A weather broadcasting co-located at TACAN.

ERRORS AND LIMITATIONS

Beacon Alignment/Ground Station Errors
* Signal accuracy can be affected by error in the generation of the signal and alignment of 360 radial with local magnetic north. Regular calibration of ground equipment and alignment of signals are carried out with changes of local variations.

Site Errors

* VORs are sensitive to the interference of terrain. Even grown grass may affect the directional propagation of VOR signals.

Propagation Error

* Signals arriving at aircraft can be distorted by spurious signals that have been reflected by terrain or obstructions during the propagation. * Mixed signals received in airborne equipment causes error in display.

Airborne Equipment Error

* Manufacturing inaccuracies and imperfections in the airborne equipment produce small differences between the detected bearing and its display on the instrument. The equipment should be regularly check and error contained within ±2º.

Pilotage Error

* While calculating the total accuracy of VOR signal the difficulty in holding a radial by pilot.