VHF Omni-directional Range (VOR) — Page 116, Lesson 105

Let's pick this up right where the VOR picture gets practical — the protection and accuracy side of the system. First, the coverage question. VOR beacons transmit on the same frequencies, so if two beacons are too close, their signals interfere. That's called co-frequency interference. To prevent it, planners separate co-frequency beacons by an extra 100 NM up to about 500 NM. But here's the key point: in practice, a beacon is protected only as far as is deemed necessary — and that is not always the line-of-sight reception range you might expect from VHF. In the UK, that protection is defined by something called a DOC — Designated Operational Coverage. It's specified as a range and an altitude. For example, a DOC of 50/25 published in the AIPs — the Aeronautical Information Publications — means an aircraft should not experience co-frequency interference within 50 NM of the VOR beacon, up to a height of 25,000 feet. So the first number is the range in nautical miles, the second is the altitude in thousands of feet. The DOC can also vary by sector — different directions from the beacon can have different coverage. And it's valid both day and night. Now, the critical operational point: using a VOR outside its DOC can lead to navigation errors. So you must refer to the latest AIC — Aeronautical Information Circular — for current information. There's an important note here: when super-refraction conditions exist — that's an atmospheric condition where the radio waves bend more than normal — interference may be experienced even within the DOC. So the DOC is a planning figure, not an absolute guarantee. Let me give you two concrete examples. VOR 1 has a DOC of 50/25 — no interference within 50 NM range up to 25,000 feet. VOR 2 has a DOC of 100/50 — no interference within 100 NM range up to 50,000 feet. So you can see the DOC scales with the protection needed. Now let's move to the factors affecting VOR beacon accuracy. There are three main error sources, and they aggregate. First, site error. This is caused by uneven terrain — hills, man-made structures, trees, even long grass — in the vicinity of the transmitter. The error this introduces to the radiated bearings is termed 'VOR course-displacement error'. Ground VOR beacon site error is monitored to ±1° accuracy. So the beacon itself is held to within one degree. Second, propagation error. Having left the VOR site with that ±1° accuracy, the transmissions are further affected by terrain and distance. At considerable range from the VOR, 'bends' or 'scalloping' can occur. Let me define scalloping precisely: VOR scalloping is an imperfection or deviation in the received VOR signal. It causes the signal to 'bend' as a result of reflections from buildings or terrain. And critically, it causes the Course Deviation Indicator — the CDI — to slowly or rapidly shift from side to side. So the needle on your instrument wanders. Third, airborne equipment errors. These are caused by the aircraft equipment assessing and converting the phase differences to 1° of bearing. The maximum aircraft equipment error should be ±3°. Now here's the aggregation. The above errors are added together to give a total error of ±5°. So site error ±1°, propagation error, and airborne equipment error ±3° combine to a total of ±5°. And there's one more — pilotage error. This is due to the fact that as an aircraft approaches the VOR, the 1° radials get closer together. Think about it: radials are lines of bearing from the beacon, and they converge as you get nearer. So a small lateral displacement near the beacon corresponds to a larger angular error. That's the pilotage error component. So to summarise the accuracy picture: the beacon is monitored to ±1°, the airborne equipment adds up to ±3°, the total system error is ±5°, and pilotage error increases as you close in on the beacon because the radials converge. That's the full protection and accuracy story for VOR.