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