Air Troopers were members of Outer Heaven. As indicated by the name, they wore jetpacks and did guard duty on the roof. They also wore yellow bodysuits.
Solid Snake encountered them on Building 1 of Outer Heaven while traversing the rooftops.
Behind the scenes
Although they don't make any reappearances in later games (future ports of Metal Gear notwithstanding), the development notes for Metal Gear 2: Solid Snake, in particular the concept map for Zanzibar Land, indicated they would have reappeared in the game as guards near the crevasse area.