VHF Omni-directional Range (VOR) — Page 116, Lesson 100

Right, let's pick this up with the VOR. We've already got the basic idea of the phase comparison, so now we're going to nail down exactly what that phase difference means in practical terms. Look at Figure 8.3. It shows one full revolution of that limaçon pattern, and it's marked with the phase differences at the four cardinal points. The blue sine wave on that figure is the reference signal. So here's the key relationship: the phase difference you measure at the aircraft is your radial. If you measure a phase difference of 227 degrees at the aircraft, that means you are on the 227-degree Radial. If you measure 314 degrees, you're on the 314 Radial. That's the whole trick — the VOR beacon is continuously transmitting your bearing information, and it does this all the time, even during the identification period. The bearing never stops. Now, let's get the terminology exactly right, because this is a classic trap. A Radial, which is abbreviated QDR, is defined as a magnetic bearing FROM a VOR beacon. I want you to hold onto that word "FROM". It's the bearing pointing away from the station, measured from the beacon out to you. That's the opposite of a bearing to the station, which would be the reciprocal. So when you're on the 227 Radial, the beacon is saying "you are 227 degrees magnetic FROM me." Let's move on to the transmission details, because there are some very specific numbers here that you need to know cold. VOR beacons operate within the VHF band, which is 30 to 300 MHz, but specifically they use the range from 108.0 to 117.95 MHz. And this is split into two groups. First, there are 40 channels in the 108 to 112 MHz range. Now, this is primarily an ILS band, but ICAO has allowed it to be shared with short range VORs and Terminal VORs, which we call TVORs. The channels there are things like 108.0, 108.05, 108.20, 108.25, 108.40, 108.45, and so on up to 111.85 MHz. Notice the pattern — it's even decimals and even decimals plus 0.05 MHz. Then, the second group is 120 channels from 112 to 117.95 MHz, with a channel every 0.05 MHz. The emission characteristics of the VOR are designated as A9W. Let me break that down for you. The "A" means the main carrier is amplitude modulated, double side-band. The "9" means it's a composite system. And the "W" means it's a combination of telemetry, telephony, and telegraphy. So that's the official designation of the type of radio emission. Now, identification. UK VORs use a three-letter aural Morse code ident, sent at approximately 7 groups per minute, and it's sent at least every 10 seconds. The ident can also be in voice form — for example, you might hear "This is Miami Omni" and then immediately followed by the Morse ident. That voice channel is also used to pass airfield information via ATIS. That information uses AM, amplitude modulation, and it's transmitted at the same time as the bearing information. One more thing on identification: if you hear a continuous tone or a series of dots, that identifies a TEST VOR, which we call a VOT. That's a special test facility. Finally, let's talk about monitoring, because this is a safety-critical part of the system. All VOR beacons are monitored by an automatic site monitor. This monitor will warn the control point and then either remove the identification and the navigational signals, or switch off the beacon entirely, in the event of certain failures. Those failure conditions are: a bearing information change exceeding 1 degree; a reduction of more than 15% in signal strength of both or either of the 30 Hz modulations, or of the RF carrier frequency; or a failure of the monitor itself. Here's the operational consequence. When the main transmitter is switched off, the standby transmitter is brought on-line, and it takes time to stabilise. During that period, the bearing information can be incorrect, and no identification is transmitted until the changeover is completed. So the rule is absolute: do not use the facility when no identification is heard. It is vital to monitor a terminal VOR let down into an airfield. And if a VOR is transmitting the identification "TST", that indicates the VOR is on test, and the bearing information should not be used.