The definition in Article 3(o) of the EU ETS Directive determines who is an "aircraft operator" for the purposes of the EU ETS. This definition refers to a natural or legal person which operates an aircraft at the time it performs an aviation activity specified in Annex I to the EU ETS Directive (i.e. a flight departure or a flight arrival at an aerodrome in the territory of the EU). If the identity of the operator cannot be ascertained then the aircraft owner is deemed to be the operator unless the owner identifies the relevant operator.