Item — HN Reader

> Accurate positioning is needed

Accurate timing is also needed to shift and combine the signals from the various dishes.

> often be achieved to within a couple of meters using google maps and a sat image of your back yard.

Position accuracy requirements are fractions of the wavelength being observed. So for this 11.2 GHz dish (26.8 mm), the position of the dish needs to be known to much better than 2mm.

In practice this can be done by observing a bright source with a well-known sky position and then solving for the antenna positions. Aspects of this are discussed in Memo 503 from the Atacama Large Millimeter Array Memo series. http://legacy.nrao.edu/alma/memos/html-memos/alma503/memo503...

Yes, super-duper accurate timing. A nanosecond of timing error means your focal point moves by an angle equal to 1 foot divided by the distance between dishes.

You can calibrate out static timing errors, like due to different lengths of wire. But if the delay is varying for any reason your focal point will be slewing all over the heavens.