Under the Open Skies Treaty, a period of time during an observation flight, specified by the observing Party, when a particular sensor installed on the observation aircraft is operating. There is no limit on the number or duration of such observation periods during an observation flight. Sensors may be operated for the entire duration of a flight, as long as the observation aircraft does not deviate from the agreed flight path and the flight altitude is appropriate for each sensor. Information on such sensor operating periods is required because the film or other recording medium has to be annotated with such information. Such information permits all States Parties to know when and where data was collected, and permits third Parties to request data taken during specified periods.