|There are missions that require you to take an outpost. Once you pick a faction you want to capture the outposts for, Click the back button. Then Go over the faction's bases, and hit A on their outposts . (The little icons of the faction that are around there HQ.) It should allow you to see what contract work is available. Mark it and go there. Keep beating these missions and it will unlock it.|
Make sure you clear the base before you call in the chopper. Sometimes the A.I will fire at each other and blow up a fuel tank next to the base. If there is one near the base, call your chopper to pick it up.