For radar only a pencil beam is needed. A regular array can be used to synthesise a raster scan beam simply by delaying rows and columns while summing the beams.
To generate an image of the source within a pencil beam requires a correlator and the ability to eliminate artefacts of the spatial array.
Early Radio Astronomy arrays of antennas were regular because all processing was narrow-band, analogue, on site, and in real time. Time delays between elements were programmed with coaxial delay lines and/or analogue phase shifters. By arranging an array with a N-S and/or E-W orientation it made it possible to scan the sky as the Earth rotated, or to track one point in the sky over a period of time.
The initial VLBI network employed what bigger dishes were available to get Earth dimension arrays for high resolution. The bandwidth was low as all data was digitised and recorded on magnetic tapes, one bit data, one track at the time, rewind the tape, record the next track, change tapes every few hours. Those tapes were sent airfreight to the correlator facility. Synchronising the data streams prior to correlation initially took most of the time because more accurate time references via GPS were not then available. A VLBI observation took 12 hours as the antenna locations rotated on the Earth, while each and every antenna tracked the same point in the sky. All spatial frequencies of the available antenna array would appear in the data record. That attenuated false correlation peaks while accumulating the hopefully true correlation image.
Waiting 12 hours means that half the observers will see the source rise, while the other half see it set. By using irregular arrays with low spatial self-correlation, but with a great total area, it is not necessary to wait for Earth rotation to spread the spatial array pattern to remove the artefacts of the array.
Small arrays, over maybe a few km span, at very short wavelengths, can be used locally for observation of absorption lines and chemical laser emissions. At longer wavelengths, those small regular arrays can combine their signals locally for use in a VLBI network.
In the last 20 years, faster A-D conversion with wider bandwidth have become available. GHz bandwidths can now be block down-converted, digitised and transmitted over fibre networks. That is what makes the SKA now possible.
Google Earth;
Australian “SKA Pathfinder” spreads out from here; -26.696971° 116.636942°
Australian “Murchison Wide Field Array” is close by at; -26.703033° 116.670424°
South Africa, the “SKA” is expanding out from here; -30.713472° 21.441458°
