Intuitively, the array gain is realized by the fact that the signal is coherently added from N array elements, while the noise is incoherently added from those same elements.
[2] In radio astronomy it is difficult to achieve a good signal-to-noise ratio because of background noise from modern communications.
Even for strong astronomical radio emission it is typical for SNR levels to be below 0 decibels.
To counter this problem exposure of the antenna to the source over large periods of time are needed just as in visible sky viewing.
Array gain is achieved using multiple, even dozens of radio receivers to collect as much signal as possible.