AA
From MechWarrior: Living Legends Wiki
Anti-Air, often abbreviated to AA, refers to assets that are able to effectively combat aerospace fighters or VTOLs.
One of the biggest challenges to driving air assets off the field is being able to hit them. Regardless of whether you are using a rapid-fire weapon such as UAC2s or a powerful, single-shot weapon like a Gauss Rifle, the trick is a matter of finding the correct distance by which to lead the target and also knowing how to compensate for the convergence as well as location of your guns.