Target the air defenses whose *threat ranges* come closest to friendly
bases rather than the closest sites themselves. In other words, the
SA-10 that is 5 miles behind the SA-6 will now be the priority.
This also treats EWRs a bit differently. If they are not protected by a
SAM their detection range will be used for determining their "threat"
range. Otherwise a heuristic is used to determine whether or not they
can be safely attacked without encroaching on the covering SAM.