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

Right, let's get into the VOR summary. This is the consolidation page for everything we've covered on VHF Omni-directional Range, so I'm going to walk you through it as a complete system, from its characteristics right down to the cockpit displays. First, the fundamental characteristics. VOR gives you magnetic bearings, and these are valid both day and night. The frequency band is 108 to 117.95 MHz, which gives us 160 channels. Its uses are broad: airways, airfield let-downs (that's the approach phase), holding points, and en route navigation. Now, the principle of operation. VOR works by phase comparison of two 30 Hz signals. One signal is the reference, the other is the variable, and the angular difference in their phase tells you your bearing from the station. Let's talk about identification. The station identifies itself with a 3-letter aural Morse code or voice, transmitted every 10 seconds. There's a special case: a VOT, which is a test VOR, transmits a continuous tone instead. Also, some stations use ATIS — Automatic Terminal Information Service — on the voice channel using AM. Monitoring is critical. There's an automatic site monitor that checks the station's accuracy to within plus or minus 1 degree. If the station drifts beyond that, the identification is suppressed. Also, when a standby transmitter is initially switched on, the ident is suppressed until it's stable. Now, the types of VOR. We have CVOR, which is the conventional type. Its reference signal is FM and its variphase signal is AM. It uses a Limacon polar diagram rotating clockwise. Then there's DVOR, the Doppler VOR, which is more accurate than CVOR due to less site error. Its reference signal is AM and its variphase signal is FM, and it uses a simulated anticlockwise rotation of the aerial. Finally, TVOR is a low power transmitter at airfields, used for local approaches. The VOT is the test VOR. It gives a 180 radial, and when you check your aircraft against it, you should have less than plus or minus 4 degrees of error. Operational range depends on transmitter power and line of sight. The DOC, or Document of Operational Capability, is valid day and night. Accuracy is affected by several errors. Site error is less with DVOR. There's propagation error. There's scalloping, which is bending due to reflections from terrain. And there's airborne equipment error, which is plus or minus 3 degrees. There's also the cone of confusion, which is directly above the station. Here, the OFF flag may appear, and the TO/FROM display and bearings fluctuate. That's normal; you're in the dead zone. The airborne equipment consists of the aerial, the receiver, and the display, which is either a CDI or an RMI. The CDI, the Course Deviation Indicator, has a sensitivity of 2 degrees per dot, with a maximum of 10 degrees full-scale deflection. It shows the relationship between the indication and your aircraft position. The RMI, the Radio Magnetic Indicator, has an arrowhead that gives QDM — that's your magnetic bearing to the station — and the tail gives QDR, the magnetic bearing from the station. When using the RMI, you must apply the magnetic variation at the station. Finally, the in-flight procedures: radial interceptions, track-keeping, and station passage. Let me show you what this looks like on the instruments. Here's the RMI with the VOR QDM displayed. And here's the CDI showing the course and the deviation. That's the complete VOR system in summary. Everything from the ground station's phase comparison down to the 2 degrees per dot on your CDI.