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)
(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)