Very Long Baseline Interferometry or VLBI such as the Event Horizon Telescope or JPL's delta-DOR (see below) uses baselines of the order of the Earth's diameter.

The Earth's diameter is about 12,000 kilometers.

The Russian radio telescope satellite Spektr-R allows for an even larger baseline, with an apoapsis that can reach to 300,000 kilometers!

These result in maximum arrival times of about 42 milliseconds and 1 second, respectively.

In order to combine these signals, mathematical correlation is used, and the "beam is steered" by varying the time delay in the correlation. This can be done in real time, or during "playback". In the case of the Event Horizon Telescope (from which we are waiting for a press conference next Wednesday) suitcases full of hard drives are flown on airplanes from the various sites and collected before the correlation can even begin.

Question: Have there been observations with much longer delays used in correlation of radio signals? Minutes? Months?

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    $\begingroup$ When gravitational lensing produces multiple images there can be time delays (days) between the arrival of light, so presumably also radio waves. That is the only way I can think of that such correlations could arise. $\endgroup$ – Keith McClary Apr 6 '19 at 3:30
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    $\begingroup$ Oh, I see that's where you're coming from. $\endgroup$ – Keith McClary Apr 6 '19 at 3:38
  • $\begingroup$ @KeithMcClary That is certainly an answer, please consider posting it. There may be other, very different situations as well, perhaps multiple paths through a large cloud of (ionized?) gas acting as a refractive lens, or some kind of (as-yet unspecified) mirror in space. I can't think of anything specific right off the bat, but that's precisely why I'm asking the question. $\endgroup$ – uhoh Apr 6 '19 at 3:47
  • $\begingroup$ @KeithMcClary there's a bounty that's got 21 hours + 24 hours grace period left. Consider writing at least a short answer? $\endgroup$ – uhoh Jun 1 '19 at 16:48
  • $\begingroup$ @KeithMcClary I've added another bounty... $\endgroup$ – uhoh Jul 10 '20 at 2:17

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