[rlpb]

> if it's using a VOR/DME at the moment

This is the crucial thing. The article says

> The pilot will usually tune the radios to the stations that are part of the procedure that the aircraft is flying (although the pilot is free to tune to other stations as a cross check), so the kind of aircraft that we expect to see in the recording are those operating on the Madrid Barajas airport, not those flying high en route.

The article author has it right. Nowadays most aircraft are using GPS to navigate, and only use DME if on a specific approach procedure that requires it. In practice, this has far narrower scope than ADS-B.

Another commenter has it right - if instead of an experiment you actually want to locate aircraft without (or not) using ADS-B, you're far better off doing MLAT on Mode S, though you do need multiple spatially separated receivers for that. Aircraft are far more likely to have a Mode S transponder and have it switched on than they are to be using DME on the frequency you choose to monitor.

[ominous_prime]

Even if an approach uses DME, depending on the aircraft or company operating procedures they still may not be using DME, because GPS is a valid substitute for DME in an approach and more convenient if you’re already otherwise using GPS RNAV.

In fact it’s quite common to shoot approaches that have DME specified fixes in an aircraft that doesn’t even have a DME transceiver.