The Doppler effect would only help with a moving source or target, so probably not.

I would guess that what was meant was using two (or more) frequencies at the same time would produce harmonics that reinforce and cancel the combined signal at set distances from the source that can be used to get distance info, and if you can get distance to three different points, you get a 3D location[0]. The technique dates back to WWII.

Further, I think it works best if the frequencies are each a different prime (otherwise you get many beats). There are probably additional tricks you can do by varying the precise frequency combination to extract very fine-grained location data.

[0] Technically, you get two solutions, one on each side of the plane connecting the three points, but they can be disambiguated as long as direction can at least crudely be determined within 180°.

DecaWave sell UWB modules that perform real-time location to ~10cm in a controlled environment: http://www.decawave.com/

I imagine that they use a system which is not too different to the way GPS combines the signals from multiple satellites to calculate the receiver location.

When you receive have a receive lock against multiple receivers, the spreading code allows you to determine time of flight to the nearest carrier cycle, and the carrier phase can provide even more precise timing. See also: https://en.wikipedia.org/wiki/Multilateration