Precise Event Horizon Telescope locations? 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?
 A: 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



*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)

