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A look at the different types of signals jetliners give off

The missing Malaysia Airlines Boeing 777-200 sent signals to a satellite for four hours after the aircraft went missing, an indication that it was still flying for hundreds of miles (kilometers) or more, according to a U.S. official briefed on the search for the jet. This raises the possibility that the plane may have flown far from the current search areas.

A look at three types of signals planes give off, and how they relate to the missing jetliner:

TRANSPONDERS:

Transponders are electronic devices that automatically identify commercial aircraft within air traffic control radar range and transmit information on the plane's identity, location and altitude to ground radar stations. Beyond radar range, they enable planes to be identified and tracked anywhere in the world by satellite. Transponders can be turned off by pilots.

The missing jet's transponder last communicated with Malaysian civilian radar about an hour after takeoff, when the plane was above the Gulf of Thailand between Malaysia and southern Vietnam.

ACARS:

ACARS — or Aircraft Communications Addressing and Reporting System — is a data link system used to transmit short messages such as weather updates and status reports between aircraft and ground stations via radio or satellite.

According to the U.S. official, ACARS messages sent by the missing plane continued after its transponder went silent, although he wasn't certain for how long.

OPERATING DATA SENT VIA SATELLITE:

Boeing offers a satellite service that can receive data during a flight on how the aircraft is functioning and relay the information to the plane's home base. The idea is to provide information before the plane lands on whether maintenance work or repairs are needed. Even if an airline does not subscribe to the service, planes still periodically send automated signals — or pings — to the satellite seeking to establish contact.

Malaysia Airlines did not subscribe to the satellite service. The U.S. official said automated pings were received from the jetliner for four hours after it went missing, indicating it probably flew for hundreds of miles (kilometers) beyond its last confirmed sighting on radar.