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

All right, let's get into the VHF Omni-directional Range — the VOR. This is the backbone of short-range navigation, and I want to start by walking you through the different types of VOR you'll encounter, because each one has a specific job. First, the CVOR — the Conventional VOR. This is the standard one, used to define airways and for en-route navigation. Think of it as the workhorse of the system. Next, the BVOR — a broadcast VOR. This one gives weather and airfield information between beacon identifications. So in the gaps where it isn't sending its Morse code identifier, it's broadcasting useful information to you. Then we have the DVOR — the Doppler VOR. This is a clever design that overcomes siting errors. Siting errors are bearing errors caused by the terrain or structures around the beacon, and the Doppler technique cancels those out. The TVOR — Terminal VOR — has only low power and is used at major airfields. It's there to guide you in the terminal area, not for long-range work. Now, the VOT. This is found at certain airfields and broadcasts a fixed omni-directional signal for a 360° test radial. This is not for navigation use at all — it's purely a test facility. You use it to check your aircraft's equipment accuracy before IFR flight. If your equipment reads more than plus or minus 4° off that 360° radial, it indicates your equipment needs servicing. That's your pre-flight check. Then there's the VORTAC — that's a co-located VOR and TACAN beacon. TACAN is a military system that provides DME, so a VORTAC gives you both VOR bearing and distance measuring in one location. And finally, the DBVORTAC — that's simply the combination of those two, the Doppler VOR with the TACAN. 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 nautical mile range, while a TVOR will normally transmit at 50 watts. The second factor is height — both the transmitter and receiver height. VOR transmissions give line-of-sight ranges, plus a slight increase due to atmospheric refraction. We can assess this with a formula: Maximum theoretical reception range in nautical miles equals 1.23 times the square root of h1 plus the square root of 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 add the square roots of those two heights and multiply by 1.23. Now, terrain matters a lot. Uneven terrain, intervening high ground, mountains, man-made structures — all of these cause VOR bearings to be stopped, which we call screened, 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 Publication Supplement — will publish details. For example, it might say "Errors up to 5.5° may be experienced in sector 315° to 345° to 40 NM." So you know exactly where to expect trouble. Finally, there's the DOC — the Designated Operational Coverage. This is about guaranteeing no co-frequency interference. There are 160 frequencies available worldwide, and to keep two beacons on the same frequency from interfering with each other, you need to separate them by at least twice their anticipated line-of-sight range. Let me show you the worked example. Say an aircraft is at a height of 25,000 feet and the VOR is situated at Mean Sea Level. The reception range is 1.23 times the square root of 25,000, which equals 194.5 nautical miles. Then the separation required is twice that — 389 nautical miles. That's the minimum distance you need between two co-frequency beacons to guarantee clean reception. So to sum up the key numbers: 200 watts for en-route, 50 watts for terminal, plus or minus 4° for the VOT test, 1.23 as the constant in the range formula, and the 160 frequencies worldwide. These are the figures you'll be working with.