You can take a GV, drive right under the point where the drones fly and you'll notice that the distance is D1.0 (actually D999) as it passes over. Since the drones always fly at 3K above the field, that means the ack can actually reach up fairly far. I've seen manned ack kills out to about D3.0, which would be 9K straight up. I think manned ack can fire out to about 4K (~12K straight up).
Honestly, anything over D1.2 or so is a pretty lucky shot on a crossing target. At least that's my experience. Anything with D800 or so though can be easy pickings... lotsa people can make you pay for flying within D1.0.
-Soda
The Assassins.