Question:
When is AA fire against DAS resolved? Rule 20.G.2.cpart 3 says “Each DAS operation follows the standard air sequence, until the mission resolution step is reached. ” That step (#5) occurs after the AA Fire step (#4), so that means that AA fire should be resolved when the mission is flown. However, this is impossible, because at this point no one knows whether the hex will be attacked, much less what the AA strength of the attacking units will be.

Answer:
Resolve AA fire versus GS and DAS during the mission resolution step. Fire AA for individual combats immediately before resolving the individual combat.

Source:
TEM 59/60