# How does the Event Horizon Telescope implement the interferometry?

The BBC article Event Horizon Telescope ready to image black hole describes the Event Horizon Telescope, a coordinated observing technique with several radio telescope arrays across the globe forming a synthetic aperture with an Earth-sized baseline.

$$\frac{\lambda}{r_{Earth}} \sim \frac{r_{Sag A*}}{D_{Sag A*}} \sim 10^{-11}$$

...when plugging in 1 millimeter for $\lambda$, and with $r_{Sag A*}$ and $D_{Sag A*}$ the radius of, and distance from Earth to Sagittarius A*, the black hole in the center of the Milky Way Galaxy of 20 million km and 26,000 light years, respectively. (values taken from the article).

The equation I've written shows that millimeter wavelength interferometry with an Earth-sized baseline has the possibility to resolve the existence of some structure with scale of the black hole's event horizon.

My question is How does the Event Horizon Telescope implement the interferometry? It would certainly be impossible to bring all signals together to a central site and perform the interference there in real time as down-converted analog signals, and quite difficult/expensive to do it with dedicated, synchronized digital optical fiber lines. Is the massive amounts of data sent as IP packets over the internet to a central correlator (numerical interferometer)?

The article mentions atomic clocks and lots of hard drives, and I have a hunch these have something to do with it.

above: "The eventual EHT array will have 12 widely spaced participating radio facilities". From BBC.

• If I had to guess - since you can't synchronize these telescopes in real time - it is that each telescope is programmed to take measurements at predefined points in time on the atomic clock and then the interference is computed on a central server (or rather several servers) for all the data recorded. Would be a very complex setup that involves some pretty nasty calculations, but could actually be possible. – Adwaenyth Feb 17 '17 at 9:44

As in actually mentioned in the article, all data collected are stored on hard drives, timestamped with an atomic clock, and then flown to a central location where the interferometry actually takes place.