[ammar2]

The VOR Minimum Operation Network[1] in the US is basically supposed to be that. They're decommissioning a lot of the VORs but at least guaranteeing that you'll be 100NM away from a working VOR and an airport with an approach that can be accomplished with VORs for the initial fixes.

Still definitely feels like putting a lot of reliance on GPS but at least there's a backup for the worst case.

[1] https://www.faa.gov/about/office_org/headquarters_offices/at...

[tjohns]

There's also a DME Minimum Operational Network, for airliners that can use DME-DME RNAV. (That's too expensive for smaller aircraft to install, though.)

[cameldrv]

It's too bad that DME/DME RNAV isn't more widely available. The only real reason it's so expensive is that there isn't much demand for it since GPS (usually) works fine. Electronics-wise, it's not much more complicated than a transponder. Unlike a GPS, it does have to transmit, so it will always be somewhat more expensive than GPS.

The other problem is that there's a limit to how many aircraft a DME station can serve at a time (about 100), but I believe that could be greatly expanded if aircraft weren't pinging the DME so often. A position fix every second is generally fine, and it could be even more infrequent if you have a cheap inertial system to fuse with it that can fill in the track for a few seconds between pings.

[rlpb]

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.

[chipsa]

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.

[rlpb]

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.

[tjohns]

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...

[rlpb]

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.