[bigiain]

Have you seen this: http://www.rtl-sdr.com/passive-radar-dual-coherent-channel-r...

I can't find the link here right now, but there's some more recent work that's doing that without needing the two clock synced receivers too...

[audi100quattro]

Looking at http://www.rtl-sdr.com/tag/passive-radar/ and youtube, it seems like some progress has been made. Not sure if any of the code is open sourced in GNU Radio, Desktop SDR or http://www.rtl-sdr.com/big-list-rtl-sdr-supported-software/ Will look into it more, it's been 2+ years since the last time.