VHF Omni-directional Range (VOR) — Page 129, Lesson 122

We're starting a new topic now: the VOR summary. This is the consolidation page for everything about VHF Omni-directional Range, so I want to walk you through it as a complete system, because this is the page that ties all the details together. Let's begin with the fundamental characteristics. VOR gives you magnetic bearings, and those bearings are valid both day and night. That's a key selling point of VOR — it doesn't suffer from the day/night propagation changes that affect some other aids. The frequency band is 108 to 117.95 MHz, and within that band there are 160 channels available. So when you tune a VOR, you're selecting one of those 160 channels. Now, what do we actually use VOR for? Four main jobs: airways, airfield let-downs, holding points, and en route navigation. So it's a versatile aid — it structures the airways you fly along, it guides you down on approach, it gives you a fix for holding, and it keeps you on track between waypoints. The principle of operation is the heart of the system. VOR works by phase comparison of two 30 Hz signals. That's the core idea — you compare the phase of two signals, both at 30 Hz, and the phase difference tells you your bearing from the station. I'll come back to this when we look at the two types of VOR, because that's where the comparison happens differently. Identification is critical in radio navigation. VOR identifies itself with a 3-letter aural Morse code, or sometimes a voice transmission, and it repeats every 10 seconds. There's a special case called VOT — that's a Test VOR — and it transmits a continuous tone instead of the Morse identifier. Also, if the station is using ATIS — Automatic Terminal Information Service — that can be broadcast using AM on the voice channel. Now, monitoring. Every VOR has an automatic site monitor, and it's set to a tolerance of plus or minus 1 degree. If the station drifts beyond that, the monitor flags it. There's also a detail about the standby transmitter: when the standby transmitter is initially switched on, the identification is suppressed. So you won't hear the Morse until the system is stable. Let's look at the types of VOR, because this is where the phase comparison gets concrete. First, CVOR — that's the Conventional VOR. In a CVOR, the reference signal is FM, and the variphase signal is AM. And it uses a Limacon polar diagram that rotates clockwise. The Limacon is the shape of the radiation pattern — it's a cardioid-like curve, and its rotation is what creates the bearing information. Then there's DVOR — Doppler VOR. DVOR is more accurate than CVOR because it has less site error. In a DVOR, the roles are reversed: the reference signal is AM, and the variphase signal is FM. And instead of a physically rotating pattern, it uses a simulated anticlockwise rotation of the aerial. So the Doppler effect creates the rotation electronically, which is why it's less affected by the terrain around the site. Next is TVOR — that's a low-power transmitter at airfields. So when you're practicing approaches at a field, that's often a TVOR. And then VOT, which I mentioned — the Test VOR. It transmits a fixed 180 radial. So when you check your equipment against a VOT, your aircraft should show an error of less than plus or minus 4 degrees. That's your pre-flight check tolerance. Now, operational range. VOR range depends on transmitter power and line of sight — you need to see the station geometrically. And the DOC — that's the Documented Operational Coverage — is valid day and night. Accuracy is affected by several things. Site error — and that's less with DVOR, as I said. Propagation error — how the signal travels through the atmosphere. Scalloping — that's bending of the signal due to reflections from terrain, so the needle can wobble. And airborne equipment error, which is plus or minus 3 degrees. So even a perfect installation has that equipment tolerance. There's a phenomenon called the cone of confusion — that's directly above the station, where the signal is weak or ambiguous. In the cone, the OFF flag may appear, and the TO/FROM display and the bearings will fluctuate. So don't trust the instrument when you're right over the top. The airborne equipment consists of three parts: the aerial, the receiver, and the display — and the display is either a CDI or an RMI. CDI is the Course Deviation Indicator. On the CDI, each dot represents 2 degrees, and the maximum deflection is 10 degrees. The relationship between the indication and your aircraft position is what you read — the needle shows how far off the selected course you are. RMI is the Radio Magnetic Indicator. On the RMI, the arrowhead gives you QDM — that's the magnetic bearing to the station — and the tail gives you QDR — the magnetic bearing from the station. And when you use the RMI, you must apply the magnetic variation at the station, not at your position. Finally, the in-flight procedures: radial interceptions, track-keeping, and station passage. Those are the three maneuvers you'll practice — intercepting a radial, holding on track, and recognizing when you pass over the station. So that's the complete VOR summary. You've got the characteristics, the types, the accuracy factors, the displays, and the procedures — all in one place.