What is the minimum separation between navaids (for VORs/DMEs and NDBs) for them to work without interfering, by regulation and by functionality?
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$\begingroup$ Do you mean navaids using the same frequency, or different frequencies? $\endgroup$– GdDCommented Jan 18, 2019 at 12:43
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$\begingroup$ Interferences are possible, and regional meetings are used to determine frequencies and power used. Such rules are found in Annex 10 of ICAO Convention, e.g. "On the assumption that a VOR having an ERP of 17 dBW (50 W) is to be protected to a service distance of 46.25 km (25 NM) and an altitude of 3 000 m (10 000 ft), protection of the VOR system requires that the ILS be at least 148 km (80 NM) from the VOR.". For NDB: "A minimum protection ratio (wanted/unwanted signal ratio) of 15 dB is to be used as the basis for frequency assignment planning". This is a broad topic. $\endgroup$– minsCommented Jan 20, 2019 at 11:15
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1 Answer
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VOR/DME and NDBs can't interfere with each other: they work on completely different wavelengths, so you could have a VOR right next to an NDB and they'll work just fine. Similarly, VORs on different frequencies won't interfere with each other; you could have them in close proximity as well. The only time you are likely to see navaids in close proximity is at an airport, which may have an NDB, VOR/DME, and multiple ILS in operation (an airport with 2 airstrips may have 4 ILS in operation: 1 per runway).
Where you do get interference is when stations are using the same frequency. A VOR has a service volume, a set distance it is designed to reach; in the US the high altitude service area is a minimum of 130 nautical miles at 45,000ft, but in practice in the right conditions you can pick them up further than that. Some stations are set at a lower power and have less range.
