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

We're moving into the VOR chapter now, and I want to start with a critical operational concept that you'll encounter every time you fly near a VOR station: the cone of ambiguity. As you approach the VOR, the radials converge. Think of radials as lines radiating out from the station — the closer you get, the closer together they become. This means the VOR needle becomes more sensitive to your lateral displacement. Near the overhead, the needle oscillates rapidly, the 'OFF' flag may appear momentarily, and the 'TO/FROM' display alternates. All of this is caused by the cone where there is no planned radiation — that's the cone of ambiguity, also called the cone of confusion. Once you've flown through it, your readings stabilize. 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°. In practice, modern VOR beacons can provide usable signals within 60° to 80° above the horizon. So the cone of confusion sits right above the station, where the signal isn't usable. Now let's talk about Doppler VORs, or DVORs. These are second generation VORs. Their transmission frequencies are the same as conventional VORs, but the transmitted bearing accuracy is improved because the transmissions are less sensitive to site error. There are two key transmission differences: the reference signal is AM — amplitude modulation — and the variable phase directional signal is FM — frequency modulation. To maintain the phase relationships that 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 CVOR or DVOR beacons. Now let's look at the airborne equipment. There are three main components. First, the aerial. For slower aircraft, it's a whip type fitted on the fuselage. For high-speed aircraft, it's a blade type or flush mounted on either side of the vertical fin. Second, the receiver — a box fitted in the avionics bay. Third, the indicator. Information from the VOR signal can be fed to a flight director system, or to simpler displays like the CDI — course deviation indicator — or the RMI — radio magnetic indicator. Let me 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 OBS, the Omni-bearing Selector. The indicator is typical with an azimuth scale having a circle and four dots on each side of the centre. Since 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°. 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 radial.