Managed to figure out how to get gnuradio to take two SDRs, cross correlate the signals and align the phase of the signal and combine them.
Hopefully this will help defeat all the QRM I get.
I've also got a plan to automatically detect (some? Most?) QRM and automatically set the phasing to null it out instead, although I'm worried that there are just too many QRM sources in this apt building that it's impractical to get a clean signal from the remainder.
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How do you provide a shared clock to them, so as to keep better sync than the sync between SDR's clock and absolute time? (Does the signal serve as that, or do you simply not want that?)
@robryk Today I'm using an RSPduo that has two inputs and uses a shared clock.
But I don't think I need to. I'm currently recalculating the phase difference every 512 samples. So if they're offset I'll just see that as a phase difference. If they drift then it'll correct relatively quickly. So I should be able to use two unsynchronized sources and it'll cope.