In civil aviation, a data-link system (known as Controller Pilot Data Link Communications) is used to send information between aircraft and air traffic controllers when an aircraft is too far from the ATC to make voice radio communication and radar observations possible. Such systems are used for aircraft crossing the Atlantic and Pacific oceans. One such system, used by Nav Canada and NATS over the North Atlantic, uses a five-digit data link sequence number confirmed between air traffic control and the pilots of the aircraft before the aircraft proceeds to cross the ocean. This system uses the aircraft's flight management computer to send location, speed and altitude information about the aircraft to the ATC. ATC can then send messages to the aircraft regarding any necessary change of course.