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Presumably, in order to perform their interferometry, the Event Horizon Telescope correlator needs to know (i) exactly the time of an observation, which I understand is accomplished by timestamping with an atomic clock at each location; and (ii) the exact location of each telescope with respect to each other.

How is (ii) accomplished to the necessary precision (I presume to a fraction of a mm)? Or is it just that the absolute separation is fairly unimportant so long as relative motion can be tracked to sub-mm precision?

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    $\begingroup$ Station positions are known to ~5mm across 10000km baselines, or about 20-30ps of time (multiple references via googling 'vlbi station coordinates'). Timestamping helps coordinate the data (bring it all into phase with each other), but with 3 or more stations the main reason for a local high resolution clock is to make sure that the time offset of each station is consistent across the data collected (remember, VLBI started in the early 1970's with less precise time bases). $\endgroup$
    – Jon Custer
    Commented Apr 17, 2019 at 14:04
  • $\begingroup$ Wikipedia says, "The approximate delay required can be calculated from the geometry of the problem. ... If the position of the antennas is not known to sufficient accuracy or atmospheric effects are significant, fine adjustments to the delays must be made until interference fringes are detected." I don't know how that fringe detection works... Maybe something about this closure phase? $\endgroup$
    – Mike
    Commented Apr 17, 2019 at 14:09
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    $\begingroup$ After going through some of the links obtained from my googling, I have found that the VLBI sites are the best known positions on the Earth as a result of decades of VLBI measurements. They can see continental drift, atmospheric pressure, tides, and more in their data, all of which the community knows how to correct for, and does so on a regular (daily to yearly, depending on effect) basis. $\endgroup$
    – Jon Custer
    Commented Apr 17, 2019 at 18:05

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There are many sources of errors on the scale of millimeters; electronic, physical, atmospheric, etc.

So instead of typing in precise geographic locations at millimeter levels of accuracy, they use known locations to a high but not necessarily millimeter level of accuracy (see comments below question) and then run a complicated "fringe maximization algorithm".

From this answer to Why does the Event Horizon Telescope (EHT) not include telescopes from Africa, Asia or Australia? I've found First M87 Event Horizon Telescope Results. III. Data Processing and Calibration

  1. Fringe Detection

In the limit for which all correlator delay model parameters were known perfectly ahead of time and there were no atmospheric variations, the model delays would exactly compensate for the delay on each baseline of the data, and the correlated data could be coherently integrated in time and frequency to build up sensitivity. In practice, many of the model parameters are not known exactly at correlation. For example, the observed source may have structure and may be centered at an offset from the expected coordinates, the position of each telescope may differ from the best estimate, instrumental electronic delays may not be known, or variable water content in the atmosphere may cause the atmospheric delay to deviate from the simple model. It is therefore necessary to search in delay and delay-rate space for small corrections to the model values that maximize the fringe amplitude: in VLBI data processing this process is known as fringe-fitting (e.g., Cotton 1995). In this section, we describe three independent fringe-fitting pipelines for phase calibration, based on three different software packages for VLBI data processing: HOPS (Section 5.1), CASA (Section 5.2), and AIPS (Section 5.3). (highlighting added)

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