AOT's main revenue derives from: (a) aeronautical revenue consisting of landing charge, parking charge, passenger service charge and aircraft service charge, and (b) non- aeronautical revenue consisting of concession revenue, office and real property rents and service revenues. In the management of airport operations, AOT has toacquire external operators to provide some required ground services for passengers as agreed under contracts with AOT. They are, for example, Thai Airways International Public Company Limited and Thai Airports Ground Services Company Limited. Operators of retail shops, warehouse facilities, limousines, parking services and other conveniences have to pay for concession fees, rent and service charges. Other non-contract tenants running services within AOT airports pay rent and service charges to the AOT.