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

I want to walk you through the VOR — the VHF Omni-directional Range — and I want to start with something that pilots often overlook: the protection of the beacon. You see, a VOR transmitter doesn't just broadcast into an unlimited void. Because of transmitter power, propagation paths, and the degree of co-frequency interference protection required, co-frequency beacons — that is, two VORs transmitting on the same frequency — must be separated for planning purposes by an extra 100 NM to about 500 NM. In practice, a beacon is protected as far as is deemed necessary, and this is not always the anticipated line-of-sight reception range. So the protection is a planning decision, not a physics guarantee. In the UK, this protection is denoted by a DOC — that's the Designated Operational Coverage — specified as a range and an altitude. For example, a DOC of 50/25 published in AIPs — the Aeronautical Information Publications — means that an aircraft should not experience co-frequency interference within 50 NM of a VOR beacon, up to a height of 25,000 ft. So the first number is the range in nautical miles, the second is the altitude in feet. The DOC may also vary by sectors, and it is valid day and night. Use of a VOR outside its DOC can lead to navigation errors, so you must refer to the latest AIC — the Aeronautical Information Circular. And there's an important note: when super-refraction conditions exist — that's an atmospheric condition that bends radio waves abnormally — 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 ft. VOR 2 has a DOC of 100/50 — no interference within 100 NM range up to 50,000 ft. So you can see the DOC scales with the protection required. 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 such as hills, and man-made structures, trees, and even long grass, in the vicinity of the transmitter. The error to radiated bearings is termed 'VOR course-displacement error'. Ground VOR beacon site error is monitored to ±1° accuracy. So the site itself introduces up to one degree of error, and that's monitored. Second, propagation error. This is caused by the fact that, having left the VOR site with ±1° accuracy, the transmissions are further affected by terrain and distance. At considerable range from the VOR, 'bends' or 'scalloping' can occur. VOR scalloping is defined as 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 it causes the Course Deviation Indicator — the CDI — to slowly or rapidly shift from side to side. So scalloping is a signal distortion that shows up as an unstable needle. Third, airborne equipment errors. These are caused by aircraft equipment assessing and converting the phase differences to 1° of bearing. The maximum aircraft equipment error should be ±3°. So the airborne side can add up to three degrees. Now, here's the aggregation. The above errors are aggregated to give a total error of ±5°. So site error, propagation error, and airborne equipment error combine to a total of five degrees. And in addition, there is pilotage error, due to the fact that as an aircraft approaches the VOR, the 1° radials get closer together. So near the beacon, a small lateral displacement corresponds to a larger angular error — the radials converge, and that's the pilotage error you have to account for. So to sum up the accuracy picture: site error ±1°, airborne equipment error up to ±3°, aggregated total ±5°, plus pilotage error near the station. That's the complete error budget for a VOR.