ATA Airlines, Inc. – formerly known as American Trans Air and commonly referred to as ATA – was an American low-cost scheduled service and charter airline based in Indianapolis, Indiana. ATA operated scheduled passenger flights throughout the U.S. mainland and Hawaii and San Juan, Puerto Rico, as well as military and commercial charter flights around the world. ATA also operated flights to Portugal (LIS, OPO, TER, PDL and SMA). In its early days the airline flew charters on a worldwide basis and had bases in Chicago, Detroit, New York, Indianapolis, Oakland and Milwaukee. Later when it entered scheduled service the airline maintained focus cities at Chicago Midway International Airport, Honolulu International Airport, and Oakland International Airport.