Monday, May 31, 2010

The VOR Visualised (as a lighthouse)

Last night, I watched the recorded video session of network podcast, "Maxwell's House" (which is currently undergoing a slow redevelopment along strictly aviation lines).

The topic for the episode (74) was aviation navigation. Ray Maxwell tried to explain how a VOR beacon worked using a visual metaphor. I found his explanation a bit confusing in the moment, but after re-listening today, I now have his visual idea burned into my head, and it's so elegant that I now just have to write it out!

Ray kept expanding the acronym as visual omnirange, but VOR actually stands for VHF Omnirange. However, a search of the interwebs revealed quite a few aviation authorities who use the word, visual, as the first word encoded by the acronym, strangely, as the signal is entirely radio.

I suppose they're meaning "visual" in the sense that interpreting the signal in the cockpit for navigation is normally done by visual reference to an instrument (In the way way back, there were aural navaids that depended on your listening to a certain formatted sound for changes to determine your location in relation to the signal).

A VOR is an improved radio beacon which, by the way of it's construction, permits the user receiving its signals to determine with great precision their exact bearing from the beacon's perspective (called a radial in the parlance).

Before VORs, most radio navaids were NDBs (non-directional beacons). So called because they provide no directional cues. They are simple AM stations. An ADF (automatic direction finder) receiver in your aircraft (or boat/ship, they were also commonly used for coastal navigation at sea) used a directional antenna to localize the NDB, causing a pointer on your aircraft instrument to literally point to the direction from which its signal was strongest (presumably co-incident with the station's transmitting antenna...but this is radio).

Because an NDB's signal can tell you nothing about what direction you happen to be from the station, it's use for fixing your position is less accurate. Its direction from your perspective must be estimated with respect to a compass reading and the ability of your ADF equipment to precisely pinpoint the azimuth of the strongest signal.

To get a sense of how this might not be the most accurate thing, take a handheld AM radio (its internal bar antenna is directional) and swivel it around while listening to a station. Notice how the signal fades? Notice how the strongest signal occurs over a rather wide span of arc?

With a compass, you can compute a compass bearing to the NDB. And with such a reading from two NDBs, you can plot a position fix, and thereby know where you are!

Unfortunately, accuracy is limited by your compass and ADF equipment. Magnetic variation is also a factor, possibly a major one, because it varies with location. When you measure your compass bearing to two NDBs, it's with respect to the magnetic variation at your location, which cannot be known precisely because you don't yet know your location.

This is its own interesting problem to think about, but suffice it to say that if you're following a constant compass track toward or away from an NDB, the varying magnetic variation along your route will cause this line of constant bearing to be curvy, perhaps significantly so in areas on sectional charts called out with special warnings about magnetic disturbances.

With VORs, the variation programmed into the station will be the same or often similar to the actual variation at the station's location. It actually doesn't matter because the programmed variation is depicted on your chart, evidenced by the differing cant of the north-arrow for different VOR stations. Since the unique signal of a VOR lets you determine in what direction you lie with respect to that station, you can use those values to much more precisely plot your location when you take readings from two or more stations. You don't need to take magnetic variation into account (in fact you don't even need a compass) because the bearing being determined is referenced from the station, where the variation is known, to you. Not the reverse, where the variation is not known.

The way a VOR does this is where the novelty of Ray Maxwell's description comes in, re-imagined now by me into hopefully clearer terms. This is so cool, and so simple, it blew my mind!

Imagine a VOR station, not as a radio beacon, but as a lighthouse. It has a bright white rotating beacon, which emits a powerful beam in a single direction and rotates at a constant speed, say one RPM, sweeping clockwise when viewed from above. Now, imagine that as the beam rotates through north, a red beacon on top of the lighthouse flashes at the instant the rotating beam hit 360 degrees.

If you were standing due north from this lighthouse, staring south back at it, you would see the white beam rotate past you giving you a flash of white as it hit 360 degrees. You would also see the red beacon above flash at the same instant. Now, the steady white beacon is still rotating, one RPM, or in other words, six degrees per second.

If you were now at some random location nearby the lighthouse, knowing its configuration as we do, once you sighted the beacon on the horizon, you would be able to quickly determine your location relative to the beacon by timing the appearance of the red and white flashes. Start your stopwatch when you see the red beacon flash. Stop timing when you see the steady white rotating beacon sweep past you. Then, your location relative to the lighthouse is:

timefrom the lighthouse
simultaneous red+white flashdue north
white 15 seconds laterdue east
white 30 seconds laterdue south
white 37.5 seconds laterdue southwest

For each additional second between the red and white flashes, you are located someplace along a bearing from the lighthouse, 6 additional degrees from the north reference. These bearings radiating away from the station are then known as radials. If you know the timing between the red and white pulses, you know which radial you are on.

This can be plotted accurately on your chart because the depiction includes the station's north reference. You can draw a line away from the station corresponding to the radial you observed. Repeat this with a second station and you have an accurate position that's immune to the effects of compass accuracy and magnetic variation!

As you travel along, by keeping the radial indicated by a given station constant, you are assured to be travelling along that radial, either toward or away from the station.

For the real VORs, the signals are radio, not light. The signal analogous to our flashing red timing beacon is broadcast omnidirectionally, while the signal analogous to our rotating white beacon is a tight directional radio beam, rotating through a full circle, just as our light beacon was. On old VORs, there is a directional antenna inside the VOR housing that physically rotates. Newer VOR stations have a circular array of antennas that are electrically modulated to create a tight, directionally focused radio beam which rotates without moving parts.

Simplifying slightly, these two VOR radio signals are modulated differently so the VOR receiver in your airplane can tell them apart. The receiver handles comparing the two signals to determine their timing with respect to each other, as we did by looking at the lighthouse in our example. And in this way the receiver knows which radial it is located on from the VOR it is receiving. Because of this fact, no special directionally tunable reception antenna is required for a VOR receiver, unlike ADF. This makes VOR navaids more accurate, and the airborne equipment is simpler and cheaper.

A technical discussion is available on Wikipedia, but this simple and intuitive visual analogy just made so much sense to me that I decided I had to share it here.

On doppler VORs, or D-VORs, I think both radio signals are omnidirectional, but the phase of the modulation of the "rotating" signal varies around the circle of antenna elements, creating a unique phase relationship between the two signals as one travels around the circumference of the station. There is no directional beam, but the VOR receiver computes the radial you're on by referring to this unique phase relationship between the signals that couldn't be received along any other radial from the station. I think... The Wikipedia article started to get math heavy at that point.

To my knowledge, our visual lighthouse analogy for VOR operation has never actually been used for real navigation by nighttime visual reference, but it could be. An additional light signal would be required to flash out an identifier signal, so you could determine which lighthouse-VOR you were looking at on your chart.

An audio morse-code identifier is part of a radio VOR's signal, to which you listen after tuning the station to confirm the signals you are receiving are from the station you wanted (and these days, more advanced avionics monitor this morse ID code for you and present the three or four letter code on your navigation display, saving you the trouble).

The lighthouse example did remind me of my coastal navigation days, where different signal lights and lighthouses do indeed flash various ID codes at you, to which you can reference on your nautical charts to determine which light you're looking at. But in this case, the signal lights are the visual equivalents of NDBs. The flash pattern contains no embedded directional signals. Taking a bearing to the light from the pelorus, and along with the ship's heading you can draw lines of position on your chart radiating from lights sighted, a process essentially the same as using an ADF with NDBs in the air, with the same accuracy pitfalls from compass and magnetic variation variances.

Later in life I observed that the visual airport beacons for different airports in my local area tended to rotate at significantly different speeds. From my earlier nautical light knowledge, I assumed that aeronautical charts might have timing data for each airport's beacons so that one might confirm the identity of the field they have in sight at night. Alas, it's just a coincidence. The timing is crudely specified, a slower range of flash rates for airports, a faster range for heliports. There are also color codes in the flashes which serve to differentiate civil, military, land, water, and emergency services aerodromes, but specific aerodromes are not positively identified by this signal.

I should note that terrestrial radionavigation aids are effectively obsolete now by the extremely accurate, increasingly cheap, and highly available satellite navigation systems. It's expected that these satellite constellations will become more numerous and feature rich, increasing the likelihood of guaranteed signal access for civil navigation uses. It's practically this way already for lower accuracy positioning, but for the most precise uses (i.e. for instrument landing systems and automated landing) there are periods of unsuitability due to the configuration of the orbits of the satellites, or maintenance of the error correction mechanisms (check RAIM and WAAS). Most of the time though, you can get accuracies of a few feet, anywhere with a decent view of the sky. That's better than any terrestrial system.

The FAA desires to shut down most VOR stations as soon as the GPS system is deemed available and precise enough, as they are expensive to maintain. But if the decommissioning of LORAN is any guide, we'll probably have access to VOR signals for some years to come.

In the event of a major space storm disabling a good deal of orbiting satellites (plausible, but not very likely), access to such terrestrial systems would become highly desired again.

Most VORs and NDBs are used for navigating from one station to another, or along specific VOR radials. The better GPS based systems allow one to specify any GPS waypoint for use as a "virtual VOR", whereby traditional VOR-style course-deviation-indication (CDI) and omnibearing-selector (OBS) controls allow you to precisely fly a given course to or from the waypoint. Such functions can be helpful when given random holding fixes to fly, for example. Today, however, much of the emphasis is on database navigation. Where any and all possible waypoints and procedures for flying between them are encoded in a database stored inside the GPS navigator, and you fly by querying the database for the route between desired waypoints and procedures for instrument approach. The navigator, if coupled to an autopilot, can then navigate the aircraft all the way through the approach with little for you to do but monitor and assure yourself it's going where you expected and intended.

With the newest "highway in the sky" features available as part of the synthetic vision systems of avionics like Garmin's G1000, flying complex procedures with curving approach and departure paths and tight glidepath and obstacle restrictions is as easy and ubiquitous as using the flight controls to guide the little computer airplane through the series of hoops on the video display. It's becoming literally as easy as a video game. Remember Pilotwings for the SuperNES? Or Independence War on the PC? Or the dropship sequence from Aliens on the C64? Like that.

You just have to be sure you programmed your navigator correctly before you start out!

This is one area where general aviation has advanced (temporarily) ahead of the airlines. Most airliners do not have avionics as advanced as a G1000 system with synthetic vision. A rare instance were the smaller guys were able to be more nimble, thanks to ever cheapening computer technology. The airlines are catching up, however, and G1000-type systems with synthetic vision are going now into business jets, with similarly featured systems planned for the latest iterations out of Boeing and Airbus.

That's all, thanks for reading! Hope you had fun!

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