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

Right, let’s pick this up where we left off. We’ve just seen how the VOR beacon transmits its bearing information continuously, even during the identification period. Now I want to walk you through the terminology and the transmission details, because this is where the professional precision really starts. First, the key definition you must lock in: A Radial, also called QDR, is a magnetic bearing FROM a VOR beacon. That “FROM” is the critical word. When you’re sitting in the aircraft and your instruments tell you you’re on the 227° radial, that doesn’t mean you’re heading 227°. It means the beacon is bearing 227° magnetic from you — in other words, if you drew a line from the VOR out to your position, that line points 227° magnetic. The radial is always defined as the bearing from the beacon, not to it. That’s the QDR convention, and it’s a distinction that will save you from a lot of confusion later. Now, let’s get into the transmission details, because a VOR is a very specific piece of radio equipment. VOR beacons operate within the VHF band, which is 30 to 300 MHz, but specifically between 108.0 and 117.95 MHz. And that range is split into two parts. The first part is 40 channels between 108 and 112 MHz. Now here’s the interesting bit: this is primarily an ILS band — that’s the Instrument Landing System — but ICAO has allowed it to be shared with short range VORs and Terminal VORs, which we call TVORs. The channel spacing here is not uniform in the way you might expect. The channels are at even decimals and even decimals plus 0.05 MHz. So you get 108.0, 108.05, 108.20, 108.25, 108.40, 108.45, and so on up to 111.85 MHz. Notice what’s missing — the odd decimals like 108.10 or 108.30 are not used, because those are reserved for the ILS localizer. So the VOR channels in this band are interleaved with the ILS channels. The second part is 120 channels from 112 to 117.95 MHz, and here you get a channel every 0.05 MHz, uniformly spaced. So that’s the full picture: 40 shared channels in the lower band, 120 dedicated channels in the upper band. Now, the emission characteristics. This is written as A9W, and each character tells you something specific. The A means the main carrier is amplitude modulated, double side-band. The 9 means it’s a composite system — that is, it combines multiple signals into one transmission. And the W means it’s a combination of telemetry, telephony, and telegraphy. So when you hear that VOR signal, you’re actually receiving a composite of the navigation signal, the voice channel, and the Morse identification all together. Let’s talk about identification, because this is something you’ll be checking constantly in the cockpit. UK VORs use a three-letter aural Morse code identification, sent at approximately 7 groups per minute, and it’s transmitted at least every 10 seconds. That’s your positive check that the beacon you’re tuned to is actually the one you want. The ident can also be in voice form — for example, “This is Miami Omni” — immediately followed by the Morse ident. The voice channel is also used to pass airfield information via ATIS, which is the Automatic Terminal Information Service. That voice information uses AM, amplitude modulation, and it’s transmitted at the same time as the bearing information. So the voice and the navigation signal share the same carrier. There’s a special case you need to know: a continuous tone, or a series of dots, identifies a TEST VOR, which we call a VOT. If you hear that, you know the beacon is on test and you should not use its bearing information. Now, monitoring — this is where the safety net comes in. All VOR beacons are monitored by an automatic site monitor. This monitor will warn the control point and either remove the identification and the navigational signals, or switch off the beacon entirely, in the event of any of three things. First, if the bearing information changes by more than 1 degree. Second, if there’s a reduction of more than 15% in signal strength — and that applies to both or either of the two 30 Hz modulations, or of the RF carrier frequency. Third, if the monitor itself fails. Here’s the operational consequence you need to remember. When the main transmitter is switched off, the standby transmitter is brought on-line, and it takes time to stabilise. During that changeover period, the bearing information can be incorrect, and no identification is transmitted until the changeover is completed. So the rule is simple and absolute: do not use the facility when no identification is heard. That’s not a suggestion — it’s a vital safety check, especially when you’re monitoring 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. So to tie it all together: you have a VHF beacon transmitting a composite signal, you identify it positively by Morse or voice, you trust the bearing only when the ident is present and the monitor is happy, and you always remember that a radial is the magnetic bearing from the beacon. That’s the foundation of VOR navigation.