In intensity interferometry the phase is not measured, and the timing accuracy is I believe only proportional to the desired effective bandwidth of the measurement. It was done in 1950's with bandwidths ~10 MHz -> 0.1microsec accuracy, should be do-able with SDR. Intensity interferometry is a bit of mind-twister...

(Another at first surprising thing is that radiation received from celestial sources is only coherent because of their very small apparent size -- the sources themselves are not coherent at all, because their physical size is very large)