I've been wanting to experiment with SDR triangulation. There are some off the shelf options, but I think it would be fun to cobble something together using dongles.
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For triangulation though, if you have a reference signal at a known location, TDoA (time difference of arrival) requires less hardware (just a single receiver at each location, e.g. an RTL-SDR). I don't know of any open-source software which does that though I've been slowly building some for my own use (it's pretty janky at the moment).