RF phase offset distance measurement

[wbooth.clearscene]

Hi, I'm in the early stages of a product design. Part of which I need to get a (not supper accurate) distance between to moving devices up to a distance apart of around 100m.
I want to do this by measureing (at device B), the phase offset of two similar RF frequency transmitted (at the same time) by device A.
My question is: Rather than building a bespoke solution for this, does anyone know of a package that contains this or similar functionality?
Cheers.

[Andy Watson]

Tellurometer? Differential GPS?  How accurate and how fast do you want the measurement?

[KJDS]

I'm not entirely sure that what you've described will give you what you want.

You can measure the phase difference between two signals with a double balanced mixer.

[wbooth.clearscene]

Cheers for the feedback.
The measurement speed is not critical. But alignment (from device to device, or device to the horizon) is wholely indeterminate. Therefore both tellurometer or GPS are excluded.
I'd kind of settled on using the principles of RADAR (Radio Detection and Ranging), which I think I could build, but I don't really want to.
I am intreged by using a local environment reference wave, but can't see how that could be used to determine the distance between two devices? Could you illaborate?
Cheers