[mikeash]

I don’t think that’s quite right. Light travels about one foot in a nanosecond. Unless the signal was directly overhead, the distance differential would surely be at least several feet, and thus the time differential comfortably more than a billionth of a second. Maybe they meant a millionth.

[mturmon]

The wording is misleading. You need to compute the angle of incidence of the incoming wavefront to a high accuracy in order to localize the source.

The key quantity is not the absolute delay introduced by the absolute distance from antenna to antenna (which is tens of meters, http://www.atnf.csiro.au/projects/askap/ska.html). It's the differential delay as a function of incidence angle.

[mikeash]

Ah yes. The difference in arrival time was many nanoseconds but it needed to be measured to within a fraction of a nanosecond to accurately localize the source.