That all depends on the type of Radar being used HD. The Ground Movements radar used at airports will have a higher rpm than say a big 23cm HSA used for Area type applications, as well as different PRI (Pulse repition interval) and lots of other bits and bobs which help provide the desired resolution for the desired application. Standard 23cm area Radars, as you rightly say, rotate at between 8 and 15 rpm.