The black hole at the center of M87 imaged last year by the Event Horizon Telescope, using Very Long Baseline Interferometry, at Earth subtends an angle of about 20 microseconds of arc. This happens to be the maximum resolution obtainable with a baseline of the diameter of the Earth and a resolution of one wavelength at 230 GHz, 1.3mm, which happens to be the frequency at which the Event Horizon Telescope imaged the M87 black hole. (I don't know why this frequency was chosen; maybe there is a strong spectral line there, or maybe it's the maximum frequency useable with current technology.) I have a question about how adequate time and space synchronization to make this image possible could be available. I have found one previous similar question, Precise Event Horizon Telescope locations?, but the answers to it seem to me inadequate, including the linked article about Roger Jennison's closure phase technique, which seems to require already having mostly solved the time and space synchronization problems. Its description, however, may have been inaccurate.
To obtain the claimed resolution of the M87 black hole, which was about its diameter (of the region within its event horizon), positions of the various receiving antennas relative to some plane at approximately right angles to the line from Earth to the BH would have to be known to within about 1.3mm, the working wavelength. (I think this is a requirement, but whether this is really so is part of my question). One of the answers to the above-mentioned question said that the relative positions of the receiving antennas were known to about 5mm- not good enough. Also, the time (relative, not standard such as UTC), of arrival of the waves at the various distant EH sites would have to be known to about the period of a 230GHz EM wave, 4.3pS (4.3 x 10^-12 seconds). Not long ago, NIST said the time keeping system which they rented, for $1,000/mo, could give UTC within 10nS, not nearly good enough, but this was standard, UTC time, not relative. They also said that their single-shot timing resolution, not time distribution, was about 30pS. However, technology has progressed since then.
I can think of 3 ways in which time synchronization within about 4pS between distant points on Earth could be obtained: Distribution by transported atomic clocks, by direct coaxial or fiber-optic cables, or by satellite. The atomic clocks would have to be transported very quickly, probably flown, since the time interval accuracy with them decreases with elapsed time since they were set, but some of the telescopes making up the EH system don't have a nearby airport. Also, probably some of the sites don't have communication with a central EH site by coaxial or fiber-optic cable. This leaves satellite time synchronization. This would require less than 4pS total variation between the relay between a central EH site, various moving, probably GPS, satellites, and finally downlink to the (8?) EH telescope sites. Is this possible? Finally, I don't know that even the fastest current electronics has 4pS resolution. [Added 2020-10-21: I thought of some that does- The fastest Tektronix and Keysight real-time digital oscilloscopes have bandwidths of about 70GHz, so probably have sampling rates of at least 200GHz, so (probably) time resolution of 5pS or less. This still leaves the more difficult problem of worldwide time synchronization within 4pS.] Can all these requirements be met, or are they somehow not necessary to get the (actually achieved) BH image?