VHF Omni-directional Range (VOR) — Page 116, Lesson 103

We’re starting the VOR chapter now, and the first thing I want to do is get you familiar with the different types of VOR you’ll meet, because each one has a specific job and a specific limitation. The first is the CVOR, the Conventional VOR. This is the standard workhorse. It’s used to define airways and for en-route navigation. So when you’re flying along a published airway, the navaids you’re tracking are typically CVORs. Next is the BVOR, a broadcast VOR. This one gives weather and airfield information between the beacon identification. So in the gaps between the Morse code identification, it broadcasts voice or data with weather and airfield details. Then we have the DVOR, the Doppler VOR. This is the one that overcomes siting errors. I’ll come back to siting errors in a moment, but the key point is that a Doppler VOR is engineered to be less affected by the terrain around it. Next, the TVOR, the Terminal VOR. This has only low power and is used at major airfields. It’s there to help you with terminal area navigation, not for long-range en-route work. Then there’s the VOT. This is found at certain airfields and broadcasts a fixed omni-directional signal for a 360° test radial. Now, this is important: the VOT is not for navigation use. It’s used to test an aircraft’s equipment accuracy before IFR flight. The way you use it is you tune it, and the instrument should indicate a bearing of 360° from the station. If the error is more than plus or minus 4°, that indicates the equipment needs servicing. So that’s your pre-flight accuracy check. Next, the VORTAC. This is a co-located VOR and TACAN, and TACAN is the military DME. So you get both VOR bearing information and DME distance from the same site. And finally, the DBVORTAC — that’s simply the combination of those systems. Now, let’s talk about what affects the operational range of a VOR. The first factor is transmitter power. The higher the transmitter power, the greater the range. So an en-route VOR with a 200-watt transmitter will have about a 200 NM range, while a TVOR will normally transmit at 50 watts. The second factor is the height of both the transmitter and the receiver. VOR transmissions are line-of-sight, plus a slight increase due to atmospheric refraction. We can assess this with a formula: Maximum theoretical reception range (NM) = 1.23 × (√h1 + √h2) Where h1 is the receiver height in feet AMSL — that’s Above Mean Sea Level — and h2 is the transmitter height in feet AMSL. So you take the square root of each height, add them, and multiply by 1.23 to get the range in nautical miles. Now, the third factor is terrain. Uneven terrain, intervening high ground, mountains, and man-made structures can cause VOR bearings to be stopped — that’s called screening — or reflected, or bent, which is called scalloping. All of these give rise to bearing errors. Where such errors are known, AIPs — the Aeronautical Information Publications — will publish details. For example, they might say: “Errors up to 5.5° may be experienced in sector 315° to 345° to 40 NM.” So you know where to expect degraded accuracy. Finally, there’s the Designated Operational Coverage, or DOC. This is about guaranteeing no co-frequency interference. There are 160 frequencies available worldwide, and to avoid two beacons on the same frequency interfering with each other, co-frequency beacons must be separated by at least twice their anticipated line-of-sight range. Let me walk you through the example. Say an aircraft is at 25,000 feet and the VOR is at mean sea level. The reception range is: 1.23 × √25,000 = 194.5 NM Then, to get the required separation, you double that: Separation = 389 NM So two VORs on the same frequency need to be at least 389 NM apart to guarantee no interference. That’s the DOC concept — it defines the protected coverage area for each beacon. So to summarise the key points: know your VOR types — CVOR for airways, BVOR for broadcasts, DVOR for siting errors, TVOR for terminals, VOT for testing, VORTAC and DBVORTAC for combined systems. Remember the 4° VOT tolerance. Remember the range formula with the 1.23 factor and heights in feet AMSL. And remember that DOC separation is twice the line-of-sight range.