(CNN) -- Cars have Global Positioning System devices to pinpoint where drivers are when they get lost, so why can't GPS be used to locate the exact position of planes when the worst happens?



There is currently no way to constantly track planes as they fly over oceans.



It took search and rescue teams over 30 hours to locate the wreckage of the Air France plane that crashed in the Atlantic on Monday. It appears that the aircraft's onboard GPS system was little help to rescuers in the mission.



Although details of Air France flight 447's fate remain uncertain, in some air accidents, this critical time could mean the difference between life and death for any survivors.



Michel Roelandt, aviation expert for Eurocontrol, a European air navigation safety organization, told CNN that nearly all modern planes are fitted with GPS navigation systems. How sophisticated those the communications and navigation systems are is down to individual airlines when they fit out their planes, an Airbus spokesperson told CNN.



On board GPS systems are primarily navigation tools for the crew, and do not currently enable a constant tracking of a jet by a ground crew at all times.



Planes receive a position signal, but don't transmit that back. This is not a problem when a jet is flying over land or in coastal areas, as it is tracked by radar. Over oceans radar does not work as it relies on line-of-sight tracking from a ground station.



On oceanic flights a flight crew will know its exact position via GPS, but it is not able to be tracked by air traffic control. It is left to someone in the cockpit to relay information via satellite communication to air traffic control or a ground station.



"Airlines often have a contract with a private operator to provide their satellite communications. Some companies pay for it, some have free contracts," Roelandt said.



Modern jets equipped with an Aircraft Communication, Addressing and Reporting System (ACARS), like the A330-200, are also able to send automatic digital signals. These are not instigated by the crew but report automatically on the plane's systems and relay information to a receiving station. In AF447's case that was Air France's head-quarters in France.



According to an Airbus spokesperson, it is up to an airline to determine how much information is sent in a data signal; it may or may not include location details.



Shortly before the AF447 disappeared, its automatic system initiated a four-minute exchange of messages to Air France's maintenance computers, indicating that "several pieces of aircraft equipment were at fault or had broken down," Air France CEO Pierre-Henri Gourgeon said on Monday.



When the Air France Airbus A330-200's systems failed these messages would no longer have been able to be transmitted. They did relay the plane's location, but the plane continued flying for an unknown period of time, so the precise location where it went down could not be known.



Once a plane loses its electronic system, the automatic data messages can no longer be sent, and in those circumstances no one on the flight deck would be able to transmit a message either.



Aviation experts have stated that the idea of GPS tracking has not been part of any safety review within the aviation industry. The general consensus has been that a trained flight crew is on hand that always has the capability to be in contact with someone on the ground should an emergency arise. In the case of Air France Flight 447, it appears that the crew had no time to relay an emergency message.



As well as searching for wreckage, a crashed plane can be found by pin-pointing the location of the in-flight black box recorder.



Black box flight recorder devices contain flight deck data and crew voice recordings. When a plane has crashed black box recorders automatically emit a honing signal, located via radar. Emitting a regular ping for 30 days it can be picked up via radio waves, meaning there is a limit to the distance from the device search and rescue teams can be before they can pick up the signal.



According
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not currently enable a constant tracking of a jet by a ground crew at all times. GPS primarily navigation tools for the crew,
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