The VOR

The VOR

byJoe CampbellDecember, 1995

The purpose of this presentation is to explain some interesting technical research into VOR technology and some informal investigation of its history. I am also pleased to describe my personal procedures for VOR navigation in the belief that others benefit from an alternate understanding.  If you wish to experiment with the ideas and methods described here, I recommend that you use Luiz Olivera's excellent VOR simulator.

Pre-VOR navigation consisted of NDBs (radio compasses) and the low-frequency "AN" system. You're all familiar with the former, so I won't bother to explain it. The AN system was the primary means of radio navigation familiar to tens of thousands of pilots who returned from WWII. In the AN system, the pilot flies between pairs of directional radio transmitters, each modulated with a Morse code identifier. When the aircraft is on one side of the course, a Morse 'A' is heard; when the aircraft is on the other side of the course an 'N' is heard. When on course, the pilot hears only a steady tone. When directly over the station, no sound is heard ("the cone of silence").

It's important to understand that the NDB and the AN system have an important characteristic in common--they are command instruments. That is, both instruments tell the pilot unambiguously whether to turn right or left to reach the desired course. This is an important point--this was the form of navigation most pilots were familiar with. Their expectations and training apparently shaped the way in which the new VOR technology was presented to the pilot population.

As prelude to studying for my instrument rating, I decided to learn how VOR transmitters and receivers worked. Rooting around in the dusty off-campus engineering archives at UC Berkeley, I located a 1949 book entitled (approximately), "The Theory and Design of Quadrature Navigational Systems." As its title suggests, this text explained the theory of quadrature modulation (which I'll summarize shortly) implemented with the vacuum-tube technology of the day--a technology I'm old enough, alas, to understand.

While puzzling over circuit diagrams for a VOR receiver, I noted an output labeled "Hemispherical Station Heading Indicator." The book often refers to it merely as the SHI. I had never heard of such a thing and was baffled at what its purpose might be. Finally it struck me--this circuit drives an indicator on the face of the instrument that always indicates the headings that lead toward the VOR station.

On the earliest receivers (apparently prototypes), were two indicator flags in the shape of arrow heads. One arrow was located in the top half of the instrument face and points upward. The other was located in the bottom half and points

downward. Only one of these indicators is activate at a time. Between the two indicator arrows was written the word STATION.

Let's see how this works. Assuming no wind, we all know that for any radial tuned on the OBS, half of all possible headings intersect it. These intersecting headings are indicated by the CDI. That is, the position of the CDI cuts a navigation problem in half. Of the headings on the side of the CDI, half lead away from the station and half lead toward it. The headings leading toward the station are always identified by the SHI. When the upper arrow is visible, the course to the station lies in the upper half of the OBS. When the lower arrowhead is visible, the course to the station lies in the lower half of the OBS.

Just as the CDI eliminates half of all possible headings, the SHI indicator eliminates half of those on the side of the CDI. The result is that the instrument always provides the course to the station within ninety degrees-- hence the word "quadrature" in the name of the technology. In other words, the VOR is essentially a station--heading indicator.

Let's look at a couple of examples.

The OBS in this example is tuned to the 150 radial. The CDI shows that all headings intersecting the 150 radial lie on the left side of the instrument, between 150 and 330 degrees. The SHI further indicates that the headings to the station lie at the bottom of the OBS between 060 and 240 degrees. Superimposing these two hemispheres, we see that the headings that intercept the 150 radial and fly toward the station lie between 060 and 330 degrees. Thus, intercepting the 150 at a 45 degree angle and flying toward the station requires a course of 015. Conversely, intercepting the 150 at a 45 degree angle and flying away from the station requires a course of 105.

This example presents the same information in a reciprocal example. This time, the OBS is tuned to the 330 radial. The CDI shows that all headings intersecting the 330 radial lie on the right side of the instrument, between 150 and 330 degrees. The SHI further indicates that the headings to the station lie in the top half of the OBS between 060 and 240 degrees. Again superimposing, we see that the headings that intercept the 330 radial and fly toward the station lie between 330 and 060 degrees. Thus, intercepting the 330 at a 45 degree angle and flying toward the station requires a course of 015. Conversely, intercepting the 330 at a 45 degree angle and flying away from the station requires a course of 105.

Note that the headings required to fly to and from the station are identical in both examples.

Further examples are possible, but the interpretation is always same--the quadrant shared by the CDI and the SHI always contains the headings to intercept the dialed--in radial and fly toward the station.

From the technical information available, it's pretty clear that the VOR was originally intended to be interpreted in the way I've just described. During certification testing in the late forties and early fifties, however, officials observed that pilots, many of whom were familiar with the AN and NDB systems, tended always to turn toward the needle, regardless of the SHI. Exactly half the time thus flying the VOR as a command instrument--that is, turning left or right toward the CDI without regard to the SHI--sends the aircraft off in the wrong direction.

Concerned about the safety of the system, the CAA (the predecessor of the FAA) decided that new procedures would have to be developed to accomodate pilots' insistence on flying the VOR receiver as a command instrument. The word STATION between the two flags was replaced with the words TO and FROM inscribed below the individual arrows. Many Cessna instruments (and doubtless others) are labeled this way. In later designs, the words TO and FROM replace the arrows. In some instruments, a single flag flips to display either TO or FROM.

Changing the names alone did not solve the problem of pilot misinterpretation because pilots apparently took the names TO and FROM literally. As one might predict, pilots were uncomfortable flying toward the station on a heading that falls in the FROM half of the OBS. This infelicity was "corrected" by a simple procedure change: when flying to the station, the OBS is tuned to the reciprocal of the desired radial. In this way, the headings to the station always lie at the top of the OBS and on the side of the CDI. No such rule exists when flying away from the station--the OBS is remains tuned to the desired radial.

This view of the VOR as a command instrument persists to this day.

Flying the SHI

In this section I intend to explain how I do VOR navigation. To simplify the discussion, I refer to my method as the SHI (Station Heading Indicator) method and to the conventional practice as the command-instrument method.

During VOR instruction for the private, every student is told how to intercept a given radial and fly to a station: tune the OBS to the reciprocal of the desired

radial, confirm a TO indication, then fly the heading at the top of the OBS. During this instruction, the student usually hears the term "reverse sensing" for the first time. Although I have never found a clear definition of revese sensing, I believe this to be a fair summary: misinterpretation of VOR indications produces a situation in which the pilot flies in the opposite direction because he is unable to center the CDI by turning the airplane toward the needle (i.e., it is no longer a "command instrument"). Trevor Thom in his excellent "Instrument Flying," ends his discussion of this situation with the lamentation, "What a pity!" It is not clear why remembering to reverse tune is less onerous than remembering to interpret the TO/FROM flag correctly.

From my earlier description of VOR technology it should be obvious that there is no such thing as reverse sensing. The instrument senses nothing in reverse. Rather, reverse sensing occurs in the pilot's mind as a result of improper interpretation of the VOR indicators.

I personally believe that if one abandons the abritrary notion that the headings one flies must lie in the top half of the OBS, the VOR becomes a vastly simpler instrument to understand and navigate with. The sole purpose of the VOR receiver then becomes to provide headings to the desired course. Whether the CDI is deflected right or left is irrelevent because only headings are important in IFR VOR navigation.

Before continuing, I'd like to resolve some issues of nomenclature. The FAA and its navigational cartographers define a radial as a course extending outward from a facility. The PILOT/CONTROLLER GLOSSARY in the US Airman's Information Manual also defines it this way. In this view, the 270 radial does not exist east of the station, and the 090 doesn't exist west of the station. In this view, one can't track the 270 to the station because there is no such thing as a TO radial. Instead, one tracks the reciprocal radial that properly leads away from the station.

Although a radial is depicted on charts as originating at the center of a facility and progressing outward, the VOR reciever ubiquitiously and simultaneously picks up the electronic signals of all radials. To the CDI, the signal for any radial and that of its reciprocal are identical (that's why the CDI happily centers on either). For the purposes of naviagation, therefore, a pilot may with perfect confidence treat every radial as proceeding outward in two reciprocal directions from the transmitter. The pilot then relies on the SHI to indicate the direction in which the station lies.

Although I now use the SHI method as naturally as I once used the command-instrument method, I freely admit that it took some practice. But much of my effort was spent unlearning the conventional mindset I had learned as a student.

I'm further convinced that a student encountering VOR naviagation for the first time would find the SHI method no harder to learn than the conventional ones. In fact, I believe the SHI method is easier to learn because there are no reverse tuning rules to master. To prove this hypothesis, I used my son Ben (age 28) as a guinea pig. While he was obtaining his Private certificate, I asked him to ignore all instruction on VOR navigation and learn the SHI method instead. Not only did he correctly answer all the questions on his written exam, his ability to instantly solve complicated navigation and position problems greatly vexed his instructors and the examiner on his check ride. (To their credit, no one failed him just because they didn't understand his methodology.)

In the SHI method there is but a single tuning rule: the OBS is always tuned to the course or radial of interest. There are no exceptions. If the approach plate shows the inbound course to be XXX degrees--set the OBS to xxx. If a controller tells me to "intecept the such-and-such radial and proceed to XYZ VOR"set the OBS to the such-and-such radial. When flying the procedure turn on a VOR or ILS--the OBS remains set to the inbound course throughout the approach. When entering a hold at a VOR--set the OBS to the holding radial (or the inbound course--your choice). With the exception of making course changes, the SHI method eliminates the TWIST from the 5 T's mantra.

Here's a simple example. Let's assume an example where you're located WSW of the station and you wish to track the 270 to the station. You wish to intercept the 270 radial and track it to the station. Now, the reverse sensing paradigm insists that you cannot track the 270 to the station because no such "TO" radial exists; you must tune the OBS to 090, instead. Let's see if this is true.

As always, tune the OBS to the radial you are intested in--in this case, 270. Your VOR receiver appears as follows:

In this example all the headings that intecept the 270 radial lies on the right side of OBS 270 and 090. The SHI shows that the headings that also lead toward the station lie in the bottom half of the OBS. A heading of 045 therefore intercepts the 270 at 45 degrees; a heading of 080 would give a ten-degree intercept, and so on. When the CDI centers, a turn to 090 takes you to the station. As in the command-instrument method, the CDI is used for tracking. The difference is that the pilot derives a heading to fly relative to the SHI as well as the OBS.

Another example: While tracking inbound on the 270 radial (OBS is 270 and heading is 090) of XYZ VOR, you receive the instructions, "Hold southwest at XYZ on the 270 radial." The radial of interest, 270, is already in the OBS. Upon passing the station, the SHI flips to show that the headings to the station are now at the top of the OBS. You now turn south to the outbound heading of 270. As you pass abeam the station, the SHI flips to show that the headings to the station are in the lower part of OBS. After one minute, you turn to an appropriate

heading given by the OBS and SHI to intercept the 270 radial back toward the station.

A final example: You're flying the ILS 30 on a transition that requires a procedure turn. Although the OBS setting has no effect on the CDI, as you near the outer marker IAF, you set 300 in the OBS as a reminder. Upon crossing the compass locator, you turn to the outbound heading of 120. The CDI is off to one side. Although SHI on a localizer is inactive, you know (or shoud know) that the headings required to intercept the localizer appear at the bottom of the OBS on the side of the CDI. You chose an intercept heading from the bottom of the OBS, and when it starts to center, turn back onto the heading at the bottom of the OBS--the outbound heading--and continue to track the localizer outbound using headings at the bottom of the OBS. After a minute or so, you turn in the protected direction indicated on the approach plate and complete the turn back toward the inbound course--the one at the top of the OBS.

The point of this last example is that on the outbound leg you derive headings in the same manner as on the inbound leg. That is, you fly headings derived from the CDI and the imaginary SHI. For a back course approach, the procedure is exactly the same, with one exception: because a localizer transmits only a single directional signal, you put the front course heading in the OBS instead of the back course heading. Since the heading you're flying is at the bottom of the OBS, you obtain tracking headings from there also.

Finding Intersections

In my Diary of an IFR Ticket, I refer to a foolproof method for navigating to an intersection without figuring out your current position. This method is based upon the prinicples explained in this paper. As many have asked for an explanation of that method, consider the following example:

For the reasons explained earlier in this paper, by definition the two dialed-in radials intersect, so they must have at least one radial in common. In the example above, 045 appears on the side of the CDI of both OBS's. Fly that heading; since you're West of VOR A, its CDI will center first. Now, just turn to and track 090, which (after passing over VOR A) will eventually center VOR B. When VOR B centers, you're at the intersection.

If you wish to experiment with the ideas and methods described here, I recommend that you use Luiz Olivera's excellent VOR simulator.

VHF omnidirectional range

From Wikipedia, the free encyclopedia

Jump to: navigation, searchThis article is about the radio navigation aid, see VOR for other uses.

D-VOR (Doppler VOR) ground station, co-located with DME.

VOR, short for VHF Omni-directional Radio Range, is a type of radio navigation system for aircraft. A VOR ground station broadcasts a VHF radio composite signal including the station's identifier in morse code (and sometimes a voice identifier), and data that allows the airborne receiving equipment to derive 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. This line of position is called the "radial" from the VOR. The intersection of two radials from different VOR stations on a chart allows for a "fix" or approximate position of the aircraft.

Developed from earlier Visual-Aural Range (VAR) systems, the VOR was designed to provide 360 courses to and from the station selectable by the pilot. Early vacuum tube transmitters with mechanically-rotated antennas were widely installed in the 1950s, and began to be replaced with fully solid-state units in the early 1960s. They became the major radio navigation system in the 1960s, when they took over from the older radio beacon and four-course (low/medium frequency range) system. Some of the older range stations survived, with the four-course directional features removed, as non-directional low or medium frequency radiobeacons (NDBs).

The VOR's major advantage is that the radio signal provides a reliable line (radial) from the station which can be selected and followed by the pilot. A worldwide land-based network of "air highways", known in the US as Victor Airways (below 18,000 feet) and "jet routes" (at and above 18,000 feet), was set up linking VORs. An aircraft could follow a specific path from station to station by tuning the successive stations on the VOR receiver, and then either following the desired course on a Radio Magnetic Indicator, or setting it on a conventional VOR indicator (shown below) or a Horizontal Situation Indicator (HSI, a more sophisticated version of the VOR indicator) and keeping a course pointer centered on the display.

VORs provide considerably greater accuracy and reliability than NDBs due to a combination of factors in their construction -- specifically, less course bending around terrain features and coastlines, and less interference from thunderstorms. Although VOR transmitters were more expensive to install and maintain, today VOR has almost entirely replaced the low/medium frequency ranges and beacons in civilian aviation, and is now in the process of itself being supplanted by the Global Positioning System (GPS). Because they work in the VHF band, VOR stations rely on "line of sight" -- if the transmitting antenna could not be seen on a perfectly clear day from the receiving antenna, a useful signal cannot be received. This limits VOR (and DME) range to the horizon -- or closer if mountains intervene. This means that an extensive network of stations is needed to provide reasonable coverage along main air routes. The VOR network is a significant cost in operating the current airway system, although the modern solid state transmitting equipment requires much less maintenance than the older units.

Contents

[hide] [hide] 1 How VORs work 2 Using a VOR 3 VORs, Airways and the Enroute Structure 4 Accuracy 5 Future 6 See also 7 References

8 External links

[edit] How VORs work

VORs are assigned radio channels between 108.0 MHz (megahertz) and 117.95 MHz (with 50 kHz spacing); this is in the VHF (very high frequency) range.

The VOR uses the phase relationship between a reference-phase and a rotating-phase signal to encode direction. The carrier signal is omni-directional and contains an amplitude modulated (AM) station Morse code or voice identifier. The reference 30 Hz signal is frequency modulated (FM) on a 9960 Hz sub-carrier. A second, amplitude modulated (AM) 30 Hz signal is derived from the rotation of a directional antenna array 30 times per second. Although older antennas were mechanically rotated, current installations scan electronically to achieve an equivalent result with no moving parts. When the signal is received in the aircraft, the two 30 Hz signals are detected and then compared to determine the phase angle between them. The phase angle is equal to the direction from the station to the aircraft, in degrees from local magnetic north, and is called the "radial."

This information is then fed to one of three common types of indicators:

1. The typical light-airplane VOR indicator is called an Omni-Bearing Indicator (OBI)[1] and it is shown in the accompanying illustration. It consists of a knob to rotate an "Omni Bearing Selector" (OBS), and the OBS scale around the outside of the instrument, used to set the desired course. A "course deviation indicator" (CDI) is centered when the aircraft is on the selected course, or gives left/right steering commands to return to the course. An "ambiguity" (TO-FROM) indicator shows whether following the selected course would take the aircraft to, or away from the station.

2. A Horizontal Situation Indicator (HSI) is considerably more expensive and complex than a standard VOR indicator, but combines heading information with the navigation display in a much more user-friendly format.

3. A Radio Magnetic Indicator (RMI) was developed previous to the HSI, and features a course arrow superimposed on a rotating card which shows the aircraft's current heading at the top of the dial. The "tail" of the course arrow points at the current radial from the station, and the "head" of the arrow points at the reciprocal (180 degrees different) course to the station.

VORTAC TGO (TANGO) Germany

In many cases the VOR stations have colocated DME (Distance Measuring Equipment) or military TACAN (TACtical Air Navigation -- the latter includes both the distance feature, DME, and a separate TACAN azimuth feature that provides military pilots data similar to the civilian VOR). A co-located VOR and TACAN beacon is called a VORTAC. A VOR with co-located DME only is called a VOR-DME. A VOR radial with DME distance allows a one-station position fix. Both VOR-DMEs and TACANs share the same DME system.

VORTACs and VOR-DMEs use a standardized scheme of VOR frequency - TACAN channel pairing so that a specific VOR frequency is always paired with a specific channel for the co-located TACAN or DME feature; on civilian equipment, the VHF frequency is tuned and the appropriate TACAN channel is automatically selected.

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.

[edit] Using a VOR

If a pilot wants to approach the VOR station from due east then the aircraft will have to fly due west to reach the station. The pilot will use the OBS to rotate the compass dial until the number 27 (270 degrees) aligns with the pointer (called the Primary Index) at the top of the dial. When the aircraft intercepts the 90-degree radial (due east of the VOR station) the needle will be centered and the To/From indicator will show "To". Notice that the pilot set the VOR to indicate the reciprocal; the aircraft will follow the 90-degree radial while the VOR indicates that the course "to" the VOR station is 270 degrees. This is called "proceeding inbound on the 090 radial." The pilot needs only to keep the needle centered to follow the course to the VOR station. If the needle drifts off-center the aircraft would be turned towards the needle until it is centered again. After the aircraft passes over the VOR station the To/From indicator will indicate "From" and the aircraft is then proceeding outbound on the 270 degree radial. The CDI needle may oscillate or go to full scale in the "cone of confusion" directly over the station but will recenter once the aircraft has flown a short distance beyond the station.

In the illustration on the right, notice that the heading ring is set with 254 degrees at the primary index, the needle is centered and the To/From indicator is showing "From" (FR). The VOR is indicating that the aircraft is on the 254 degree radial, west-southwest "from" the VOR station. If the To/From indicator were showing "To" it would mean the aircraft was on the 74-degree radial and the course "to" the VOR station was 254 degrees. Note that there is absolutely no indication of what direction the aircraft is flying. The aircraft could be flying due north and this snapshot of the VOR could be the moment when it crossed the 254 degree radial.

[edit] VORs, Airways and the Enroute Structure

The Avenal VORTAC shown on a sectional aeronautical chart. Notice the light blue Victor Airways radiating from the VORTAC. (click to enlarge)

VOR and the older NDB stations were traditionally used as intersections along airways. A typical airway will hop from station to station in straight lines. As you fly in a commercial airliner you will notice that the aircraft flies in straight lines occasionally broken by a turn to a new course. These turns are often made as the aircraft passes over a VOR station. Navigational reference points can also be defined by the point at which two radials from different VOR stations intersect, or by a VOR radial and a DME distance. This is the basic form of RNAV and allows navigation to points located away from VOR stations. As RNAV systems have become more common, in particular those based upon GPS, more and more airways have been defined by such points, removing the need for some of the expensive ground-based VORs. A recent development is that, in some airspace, the need for such points to be defined with reference to VOR ground stations has been removed. This has led to predictions that VORs will be obsolete within a decade or so.

In many countries there are two separate systems of airway at lower and higher levels: the lower Airways (known in the US as Victor Airways) and Upper Air Routes (known in the US as Jet routes).

Most aircraft equipped for instrument flight (IFR) have at least two VOR receivers. As well as providing a backup to the primary receiver, the second receiver allows the pilot to easily follow a radial toward one VOR station while watching the second receiver to see when a certain radial from another VOR station is crossed.

[edit] Accuracy

The predictable accuracy of the VOR system is ±1.4°. However, test data indicate that 99.94% of the time a VOR system has less than ±0.35° of error. Internal monitoring of a VOR station will shut it down if the station error exceeds 1.0°.[2]

ARINC 711-10 January 30, 2002 states that receiver accuracy should be within 0.4 degrees with a statistical probability of 95% under various conditions. Any receiver compliant to this standard should meet or exceed these tolerances.

[edit] Future

Like many other forms of aircraft radio navigation currently used, it is likely that some form of space-based navigational system such as Global Positioning System (GPS) will replace VOR systems. VOR is specifically in jeopardy because of the need for numerous stations to cover a large area. The satellite-based GPS is capable of reliably locating an aircraft's position within about 100 feet horizontally. Augmented by "Wide Area Augmentation System" (WAAS) currently being deployed in the U.S., the error is reduced to a cube about 10 feet on each side. This allows precision instrument approaches (with lateral and vertical guidance) with landing weather minima nearly as low as the Category I Instrument Landing System -- but with no ground-based equipment except for a relatively few units that determine the WAAS correction signals relayed through satellites to user aircraft. Further refinements include "Local Area Augmentation System" (LAAS) which will probably allow Category III approaches (practically speaking, landings in "zero-zero" weather) -- again, with minimal requirement for ground stations. LAAS is planned to use the same VHF band for its correction message. This might require some existing VOR facilities to be shut down or shifted to different frequencies to avoid interference issues.[3] As of 2008 in the USA GPS-based approaches outnumber VOR-based approaches. More and more, conventional VOR navigation equipment is being phased out or replaced by integrated avionics packages that also contain one or more VOR receivers. Old VOR navigation equipment is put to new use in experimental aircraft projects, as is shown exemplary by this Ibis canard aircraft instrument panel development report.

[edit] See also

TACAN Direction finding (DF) Instrument flight rules (IFR) Instrument Landing System (ILS) Non-directional beacon (NDB) Distance Measuring Equipment (DME) Global Positioning System (GPS) Wide Area Augmentation System (WAAS) Head-up display (HUD) Airway (aviation) (Victor Airways)

[edit] References

1. ̂ CASA. Operational Notes on VHF Omni Range (VOR)2. ̂ Department of Transportation and Department of Defense (March 25, 2002).

"2001 Federal Radionavigation Systems" (PDF). http://www.navcen.uscg.gov/pubs/frp2001/FRS2001.pdf. Retrieved on November 27 2005.

3. ̂ Department of Transportation and Department of Defense (March 25, 2002). "2001 Federal Radionavigation Plan" (PDF). http://www.navcen.uscg.gov/pubs/frp2001/FRP2001.pdf. Retrieved on November 27 2005.

[edit] External links

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Navigation aid search from airnav.com VOR Navigation Simulator Macromedia Flash 8 Based VOR Navigation Simulator

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Navigation Systems - Level 3

VOR Navigation

A. GENERAL

The airway system is based primarily on the very high frequency omnidirectional range (VOR). This extensive system consists of several hundred ground stations that transmit navigation track guidance signals used by aircraft in flight.

The VOR navigational system has many advantages for the IFR pilot. The VOR transmits in the very high frequency range of 108.1 through 117.95 MegaHertz (MHz); therefore, it is relatively free from precipitation static and annoying interference caused by storms or other weather phenomena. Accuracy is another advantage: a track accuracy of plus or minus 1° is possible when flying a VOR radial. Wind drift is compensated for by flying to center the track bar indicator.

VOR signals are transmitted on line-of-sight. Any obstacles (buildings, mountains or other terrain features, including the curvature of the earth) block VOR signals and restrict the distance over which they are received at a given altitude. This can result in a sudden scalloping fluctuation of the cockpit indicators -normally for short time intervals. Certain terrain features may produce areas where VOR navigation signals are unusable, so every instrument pilot making an "off airways" flight should be aware of the restrictions along the route.

Because of greater reception distances at higher altitudes, it is possible for an aircraft to receive erroneous indications due to the reception of two VOR stations operating on the same frequency. Stations on the same frequency are spaced as far apart as possible, but there are, nevertheless, more VORs than the 160 frequencies available. The solution has been to design and classify VORs according to the usable cylindrical service volume.

This is the system by which VOR frequencies are assigned to stations far enough apart to prevent overlapping, confusing signals. As long as pilots use the proper chart, they are protected from interference between two VORs. The pilot uses low-altitude charts below 18,000 ft and the high-altitude charts at and above 18,000 ft.

B. VOR ACCURACY

1. REQUIREMENTS FOR CHECKS: The Air Regulations and good judgement dictate that the VOR equipment of aircraft flying under instrument flight rules be within specified tolerances. Airborne VOR equipment used on IFR flights must be maintained, checked and inspected under an approved procedure. The pilot normally makes these operational checks. In preparing for an IFR flight the pilot should check the aircraft instrument, then make a physical check on the appropriate VOT (test) frequency to determine whether the VOR equipment meets accuracy requirements.

2. VOR TEST FACILITY- The first method is to use a VOR test facility signal (VOT). This is an approved test signal, located on an airport, that enables the pilot to check the receivers conveniently and accurately. First, the pilot should tune the VOR receiver to the VOT frequency. These frequencies are coded with a series of Morse code dots or a

continuous 1020-cycle tone. When the pilot sets the course selector to 0°, the track bar (TB) indicator should center and the TO-FROM indicator should read FROM. Then the pilot sets the selector to 180°. The TO-FROM indicator should read TO and the TB should be centered.

The pilot determines the exact error in the receiver by turning the track selector until the TB is centered, and noting the degrees difference between 180° or 0°. The maximum Permissible bearing error with this system check is plus or minus 4°. Apparent errors

greater than 4° indicate that the aircraft receiver is beyond acceptable tolerance. In such circumstances the pilot should determine the cause of the error and have it corrected before attempting IFR flight.

The airports with VOT facilities are listed in the Airport/ Facility Directory. Because the VOT signal is only a special test signal, it may be received and used regardless of the aircraft's position on the airport.

3. VOR CHECK POINT SIGNS: A number of aerodromes have VOR check point signs located beside taxiways. These signs indicate a point on the aerodrome where there is sufficient signal strength from a VOR to check aircraft VOR equipment against the radial designated on the sign. Frequently a DME distance will also be indicated for check purposes. The maximum permissible difference between aircraft equipment and the designated radial is 4° and 0.5 NM of the posted distance.

4. DUAL VOR CHECK.- If neither a test signal (VOT) nor a designated check point on the surface is available and an aircraft is equipped with dual VORs (units independent of each other except for the antenna), the equipment may be checked against each other by tuning both sets of the same VOR facility and noting the indicated bearings to that station. A difference greater than 4° between the aircraft's two VOR receivers indicate that one of the aircraft's receivers may be beyond acceptable tolerance. In such circumstances, the cause of the error should be investigated and, if necessary, corrected before the equipment is used for an IFR flight.

5. AIRBORNE VOR CHECK: Aircraft VOR equipment may also be checked while airborne by flying over a fix or landmark located on a published radial and noting the indicated radial. Equipment which varies more than 6° from the published radial should not be used for IFR navigation.

C. AIRCRAFT VOR COMPONENTS

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, on the right). 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, below right). 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 station range 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 Deflections  figure (left), 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, on the right). 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, on the right, both aircraft display a FROM reading.

The VOR Indications  figure, on the left, 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.

D. 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 figure, on the right).

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, on the right).

E. 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 right, 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, on the right), 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.

The material for this section is reproduced from the publication, FROM THE GROUND UP, with the permission of its copyright owner, Aviation Publishers Co. Ltd. No further reproduction is authorized, in any print, electronic or other form of media, without the prior consent of the publisher at http://www.aviationpublishers.com . Any questions

regarding this portion of the website should be directed to Dr. Claudius Carnegie. Questions regarding the publication, FROM THE GROUND UP, should be directed to the publisher at [email protected].

The format in which the material has been presented for the entire section is copyrighted by the ALLSTAR network.

OR NavigationPart I

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.

If you jumped to this point of the website without proceeding through the earlier sections, I strongly recommend that you return to the Air Navigation section and review the sections on VFR Sectional Charts, IFR enroute low altitude charts, and the basics of plotting a course. Further, you should go to the NDB

Approaches/Approach Plates section and read the basics of Instrument Approach Plates, now called Terminal Procedures.

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.

The GPS, Global Positioning System, is making inroads onto the navigation scene and offers a flexibility unavailable with either NDB or VOR systems. However, it is supplementing these systems, not replacing them.

The RMI indicator used in the NDB navigation exercises is as close to a "hands-off" indicator as you will find. In an aircraft the RMI compass card must initially be aligned with the compass before a flight begins and then rechecked every fifteen minutes or so, and that's it.

With VOR, however, course information must be manually entered into the indicator. The VOR indicator below shows an aircraft heading toward, "TO," the Omni station.

NOTE this very important fact, with more info farther down. The radial signals of a VOR always point away from the station. The indicator below shows 345°, but since we are heading toward the VOR, see arrow D, we are actually on the reciprocal radial, or the 165° radial. This aircraft is south of the station. This will become more clear in a moment.

See the text for details on the four components of the VOR Indicator.The digital indicator is a separate gauge used on the Nav Trainer Panel.

The VOR display has four elements:

A. A Rotating Course Card, calibrated from 0 to 360°, which indicates the VOR bearing chosen as the reference to fly TO or FROM. Here, the 345° radial has been set into the display. This VOR gauge also digitally displays the VOR bearing, which simplifies setting the desired navigation track.

B. The Omni Bearing Selector, or OBS knob, used to manually rotate the course card. C. The CDI, or Course Deviation Indicator. This needle swings left or right indicating the direction to

turn to return to course. When the needle is to the left, turn left and when the needle is to the right, turn right, When centered, the aircraft is on course. Each dot in the arc under the needle represents a 2° deviation from the desired course. This needle is more-frequently called the left-right needle, with the CDI term quickly forgotten after taking the FAA written exams. Here, the pilot is doing well, and is dead-on course—or maybe lazy and with the autopilot activated in the "NAV" mode.

D. The TO-FROM indicator. This arrow will point up, or towards the nose of the aircraft, when flying TO the VOR station. The arrow reverses direction, points downward, when flying away FROM the VOR station. A red flag replaces these TO-FROM arrows when the VOR is beyond reception range, has not been properly tuned in, or the VOR receiver is turned off. Similarly, the flag appears if the VOR station itself is inoperative, or down for maintenance. Here, the aircraft is flying TO the station.

Radials, Radials, Radials

To grasp the VOR system you must understanding that it is entirely based on radials away from the station.

In the Sandy Point VOR to the left, note first that the arrow on the 0° radial points away from the center of the compass rose. You'll remember that this radial points to the west of true north because of the west magnetic variation. North on a VOR is Magnetic North. So, if you overflew this VOR on the 0° radial, you would be flying away from the VOR.

Similarly, note the arrows by the 30°, 60°, 90° marks and the rest of the way around the compass rose. They all point away from the station. Radials are always away from the station.

There is only one line on the chart for each numbered radial for a particular VOR station. Whether you are flying it outbound or inbound,

or crossing it, a radial is always in the same place.

The only possible complication lies in the reciprocity of the numbers. Whenever you are proceeding outbound, your magnetic course (and heading when there is no wind) will be the same number as the radial. Turn around and fly inbound you must mentally reverse the numbers and physically reverse the OBS setting so that your course is now the reciprocal of the radial. But the radial you are flying on hasn't changed.

Some examples will cement this in your mind.

This aircraft is north of the Omni station, flying on the 345° radial away FROM the station. The left-right needle shows the aircraft on course and the FROM flag is present, pointing down, toward the station behind.

This aircraft is south of the Omni station. Its magnetic course is 345°. Walk through the steps below to understand the VOR reading.

1. The aircraft isn't on the 345° radial because that radial extends from the Omni to the northwest as shown by the arrow.

2. The aircraft is actually on the reciprocal radial, the radial pointing towards the plane. That reciprocal radial is 165°, away from the station like all radials.

3. If the 165° radial were set into the VOR, the FROM flag would properly show, because the aircraft is away from the Omni on that radial.

4. Here is the important point. If the OBS is rotated until the needle centers and the FROM flag shows, it will always show the correct radial from the Omni that the aircraft is on regardless of the aircraft heading.

5. To eliminate the confusion of location relative to an Omni, the magnetic course of the aircraft and the radial setting on the VOR should be the same.

6. Presumably the aircraft is flying in the desired course direction, so its heading will be approximately the same as the VOR setting, i.e., the magnetic course. The heading may differ slightly from the VOR because of the correction needed to correct for wind drift.

7. Thus, with the OBS set to 345° the left-right needle shows the aircraft on course and the TO flag is showing, pointing up, toward the station ahead.

Experiment with this on your FS98 or FS2K to see the effects of the OBS setting on the TO-FROM flag. Select any Omni, position the aircraft to be flying TO it, then rotate the OBS so that its reading centers the needle and the TO flag appears.

Next, rotate the OBS to the reciprocal of the course. The needle will again center, but the FROM flag will appear.

A one-line recap: to know whether you are flying TO or FROM an Omni, the OBS setting must be

approximately the same as the aircraft heading.

Where am I?

This illustration shows the confusion that can result, yes, that the VOR indicator can actually provide wrong information if the OBS isn't set properly.

Same example as before. The aircraft is south of the Omni, on the 165° radial. It is flying northwest. Observe the DG. The aircraft is heading 345° as desired. But the OBS was improperly set to 165° and the VOR is falsely informing the pilot, with a nicely centered needle, that he/she is flying away FROM the Omni. The aircraft, of course, is flying TO the Omni.

Hate to beat a dead horse, but again, the TO-FROM confusion disappears if the aircraft heading and the OBS setting are approximately the same which they weren't here. Pay attention to this and you will stay out of trouble.

This sort of error usually happens when the pilot rotates the OBS, watching only for a centered needle, not also paying attention that the setting should approximate the magnetic course, or aircraft heading.

Wandering off course?

This aircraft has drifted to the right of the desired course. To be "on course" the aircraft must be on the red line. Not paying attention to a crosswind (what other kind is there?), or simply letting the heading wander could do it. In any event, the VOR needle has swung to the left, indicating that the aircraft must move to the left to return to course. So a left turn is in order. Like the RMI, with the VOR a pilot always turns towards the needle to return to course, assuming that the OBS setting approximates the aircraft heading.

This aircraft is 4° off course. Each dot of the arc under the needle is a 2° deviation from the desired course. Don't confuse heading, the direction of the aircraft's nose, with course, the desired track along the ground. Only with no wind will heading and course be the same.

"The needle is centered, my flying is perfect"

Nice thought, but not necessarily. The VOR system operates in the VHF frequency band, from 108.0 to 117.95 MHz. Reception of VHF signals is a line-of-sight situation. Nominally, you must be 1000 ft AGL to pick up an Omni within its maximum low-altitude service range.

The VOR indicator is smart enough to know when a usable signal has not been received and displays an "OFF" flag, a red and white barber-pole striped flag in the gauge in the illustration to the left. So when you are flying to or from an Omni station and you're quite content at how

stable the CDI needle has been, it's worth taking another glance at the gauge to see if the OFF flag is staring back at you.

The OFF flag also displays if the Nav receiver is tuned to the wrong frequency or, blush, if it's properly tuned but you neglected to turn on the power switch. If you're taking your check ride with an FAA examiner for a real license, that oversight is likely to get you a quick return to terra firma. And, there's also the

possibility of a popped circuit breaker interrupting power to the Nav receiver, a connector jiggled loose, etc.

VOR RangeAh, the oft asked and seldom answered question: how far away can I pick up a reliable signal from the Omni and what altitude need I be at? The FAA neatly skirts the answer by classifying Omnis by an altitude code, with the ranges vs. altitudes as shown in the table below.

Reception Range vs. Altitude of VORs

VOR Class Rangenm

within Altitudefeet

Terminal (T) 25 1000 – 12,000

Low Altitude (L) 40 1000 – 18,000

High Altitude (H) 40100130

1000 – 14,50014,500 – 60,000,18,000 – 45,000

Data is from the Aeronautical Information Manual, AIM.

These ranges assume, please contain your laughter, that terrain plays no part in VOR ranges of reception. But terrain, of course, can greatly impact the reliable range of an Omni.

Consider the Bangor VOR, BGR, at Bangor (Maine) Int'l. Airport. Here are the comments in the Airport/Facility Directory:

"VOR unusable 342°—063° below 2500 ft."

Pretty significant terrain impact, wouldn't you say? So think of the FAA data in the table as a starting point that may be modified by terrain.

Checking VOR accuracy

The VOR is the most common navigation instrument presently on aircraft panels. We rely on it to accurately track VOR radials, whether flying between Omni stations, or locating intersections, or arriving and departing from airports. We accept at face value that what it displays is accurate. Well, on FS98 and FS2000 it is always accurate. But in the real world, not only can the gauge be wrong, but the FAA requires that a pilot check the VOR for accuracy within 30 days of an IFR flight. Even if a pilot never flys IFR, it is prudent to regularly check the VOR for accuracy.

One acceptable way to formally check VOR accuracy is with a VOR Test Facility, more commonly called a VOT. A VOT is a low-power Omni station located on many of the mid-to-large size airports. A VOT differs

from a standard Omni in that it transmits only a single radial, the 360° radial.

To calibrate a VOR, the pilot tunes in the VOT frequency while on the ground (in rare instances this check is performed in the air). Refer to the back of the Airport/Facility Directory for frequencies and whether it is a ground check (G) or an airborne check (A). See the Connecticut illustration below.

CONNECTICUTVOR TEST FACILITIES (VOT)

Facility (Arpt Name) Freq. Type VOT

Remarks

Bradley Int'l 111.4 G  

Bridgeport (Sikorsky Mem)

109.25 G  

Groton (Groton–New London)

110.25 G  

Hartford (Hartford–Brainard)

108.2 A 3 nm Radius 1200–5000 ft.

Data is from the Airport/Facility Directory.

Next, rotate the OBS until the to-from needle centers. Read the number from the Omni Bearing Indicator ring or digital display. To be legal, the gauge must be within 4° of either 180° with the TO flag showing or of 0° with the FROM flag showing.

Make note in the illustration above that the VOT at Bradley Int'l. airport is on 111.4 MHz. That information is important later while performing one of the VOR approach practice flights.

It's time to fly—All flights will be on the New York Sectional Chart.

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