every SA11 launcher has own track radar and their actions depend on search radar(snow drift), SD searches and if it finds any target to fire, tells launcher. and SA11 launcher turns to target, starts emitting, finds target and fire, if you want to hit launcher(and one of the track radars for this case) you have to come closer(around 20 mile, as you said) to make sa11 look at you and fire at you. so that you harm can find the launcher's radar signal. if you hit snow drift first, the effectiveness of launchers will reduce significantly but they can still find you and track you if you are close enough.
