In article , "The
Enlightenment" wrote:

Some phased arrays rotate but and are only phase steered in the
vertical for height finding purposes. For instance the antenna on
US AWACS aircraft use this technique, though the Israeli AWACS use
fixed antena inside the dome.

This technique goes back to the 2nd world war. (I think the German
radars Wassermann and Jagdschloss used it as did the radars for the
Soviet missiles SA1 and SA2)

Alternatively and cluster of 4 antena as seen on the USN Aegis system
can be used. There must be great difficulty in integrating the 4
antena to work together.

Having a phased array is pretty important these days. Modern fighter
tactics involve splitting in such a way that the enemy can not track
the multiple targets with a mechanical antenna anymore and using this
as an opportunity to attack.

As with most things, there are pro's and con's.
AESAs let you track more targets effectively instantly, when the nose is
pointed in the right direction.
But a mechanically scanned array has a larger off-boresite look angle.
Above statements for fighter aircraft, not surveillance types.

--
Harry Andreas
Engineering raconteur