VHF Omni-directional Range (VOR) — Page 124, Lesson 107

I want to walk you through the VOR — the VHF Omni-directional Range — and we're going to look at three things: the cone of ambiguity, the coverage of the beacon, and then the Doppler VOR, which is the second-generation system. After that we'll get into the airborne equipment and the deviation indicator. Let's start with the cone of ambiguity, sometimes called the cone of confusion. As the aircraft approaches the VOR, the radials — those are the 360 lines of bearing radiating out from the station — they converge. They get closer together as you get nearer the beacon. Because of that convergence, the VOR needle becomes more sensitive. The closer you get, the more a tiny lateral displacement swings the needle. Near the overhead of the VOR, the needle oscillates rapidly, and the 'OFF' flag may appear momentarily. Also, the 'TO/FROM' display alternates — it flips back and forth. All of this is caused by the cone where there is no planned radiation. Directly above the station there's a cone-shaped volume where the antenna doesn't radiate a usable signal, and that's the cone of ambiguity or confusion. Once the aircraft has flown through this cone, the readings stabilize. So the practical lesson: don't chase the needle when you're overhead the VOR — it's meaningless there. Now coverage. The VOR shall provide signals to permit satisfactory operation of a typical aircraft installation at the levels and distances required for operational reasons, and up to a minimum elevation angle of 40°. That 40° is the guaranteed minimum — the beacon is certified to give usable signals at any angle up to 40° above the horizon. In practice, modern VOR beacons are capable of providing usable signals within 60° to 80° above the horizon. So the guaranteed figure is 40°, but the real capability is higher. Now the Doppler VOR — DVOR. These are second-generation VORs. Their transmission frequencies are the same as a conventional VOR, so you tune the same frequency. But the transmitted bearing accuracy is improved, because the transmissions are less sensitive to site error. Site error is the distortion caused by the terrain and obstacles around the beacon — hills, buildings, trees — that bend or reflect the signal. The DVOR reduces that problem. Here are the transmission differences. In a conventional VOR, the reference signal and the variable phase signal are both amplitude modulated — AM. In the DVOR, the reference signal is AM, but the variable phase directional signal is FM — frequency modulated. That's the key difference. And to maintain the phase relationships which exist in conventional VOR transmissions, the apparent or simulated rotation of the directional signal is anti-clockwise. As a result, the same airborne VOR equipment can be used with either a CVOR — conventional VOR — or a DVOR beacon. You don't need different equipment in the aircraft; the receiver can't tell the difference, and that's by design. Now let's move to the airborne equipment. There are three main components of the VOR equipment in the aircraft. First, the aerial. For slower aircraft, the aerial is a whip type fitted on the fuselage — that's the long flexible rod you see on light aircraft. For high-speed aircraft, it's a blade type, or it's flush mounted on either side of the vertical fin — that's the tail fin. Second, the receiver. This is a box fitted in the avionics bay. Third, the indicator. The information derived from the VOR signal received at the aircraft may be fed to a flight director system, or to the more simple displays such as the CDI — the course deviation indicator — or the RMI, the radio magnetic indicator. We'll describe those. Let's focus on the VOR deviation indicator. This instrument displays VOR information and is widely used in light aircraft. It indicates the displacement of the aircraft with respect to a bearing — to or from the VOR station — which has been selected on the Course Selector Knob, or the OBS, the Omni-bearing Selector. The indicator is typical with the azimuth scale having a circle and four dots on each side of the centre. As the circle itself counts as the first dot, this is a five-dot display, with each dot indicating approximately a 2° displacement from the selected VOR bearing. Full-scale deflection therefore represents 10°. So if the bar is at the first dot, you're about 2° off the selected radial; at the second dot, 4°, and so on, up to 10° at full deflection. This displacement — or deviation — is presented by a deviation bar on the indicator. The displacement of the bar depends on the angular position of the aircraft relative to the selected bearing. That's the core of how the VOR deviation indicator works — the five-dot scale, the 2° per dot, the 10° full-scale deflection, and the deviation bar showing your angular displacement from the bearing you've selected on the OBS.