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One thing I have yet to understand is why DME-DME is preferred over VOR-VOR. Because the latter can support unlimited aircraft, unlike DME.

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Probably the required accuracy. VOR is on the order of a degree for accuracy. DME is around 0.1nm. So if you’re 50nm from the VOR, then you may have a position fix error of 0.87nm across the radial, if I did my math right.

That's a good point. But if you're 50nm from a DME, I think you're unlikely to be able to get a lock in practice.

For altitudes above 12,900 ft AGL, the official service volume for a DME is 100-130nm.

Below that it's considered "line of sight"... and some quick math shows that you'd be able to get line of sight >50 nm for all altitudes above 1700 ft AGL (which is very low).

Source: https://www.faa.gov/air_traffic/publications/atpubs/aim_html...

Yes but only to about 100 aircraft at a time, favouring stronger (and therefore closer) signals: https://en.wikipedia.org/wiki/Distance_measuring_equipment#S...

If you're 100nm away, chances are there are more than 100 aircraft nearer to the DME than you from at least one of the two required DMEs. Especially if GPS has failed and many aircraft are trying to use backup DME-DME. Unless you're in a very sparse area.

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