[civilitty]

No, it's not enough to know when samples from different SDRs were taken, they must all be taken within a very small interval (tens of picoseconds in OP's example). What the SDRs really need is an electrical signal called a "trigger" that tells them to read their sensors at that exact moment which is what GP meant by plugging them in. You can use a second GPS enable device to generate that trigger but synchronizing those triggers using time of flight is very hard.

[touisteur]

Maybe you can bruteforce time correlation by shifting every source slightly, trying a yuge number of shift combinations and seeing which combination gives the 'best' (most correlated) output on a known signal? I've done that for many out-of-sync systems, and since I've started using GPUs in the two Os I've become obsessed with brute force methods :-)