How do you track a plane?

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MH370 mystery

• What we know
• Search logistics
• Passport fraud
• The passengers

Mystery continues to surround the fate of Malaysia Airlines flight MH370, which disappeared without trace more than a week ago.

The Boeing 777-200ER plane, with 239 people on board, vanished from air traffic control screens at approximately 01:20 local time on Saturday 8 March - just an hour after leaving Kuala Lumpur bound for Beijing.
Concrete facts about what happened remain elusive. A variety of theories have been put forward, from hijacking or sabotage, to a slow loss of cabin pressure, which in turn could cause the crew and passengers to become disorientated.
But, with no debris yet identified, much of the search has been focused on the last-known movements of the aircraft.
So, how do you track a plane and what do we know about the movements of flight MH370?

How are aircraft normally tracked?

Air traffic control standard international practice is to monitor airspace using two radar systems: primary and secondary.
Primary radar, based on the earliest form of radar developed in the 1930s, detects and measures the approximate position of aircraft using reflected radio signals. It does this whether or not the subject wants to be tracked. Secondary radar, which relies on targets being equipped with a transponder, also requests additional information from the aircraft - such as its identity and altitude.
All commercial aircraft are equipped with transponders (an abbreviation of "transmitter responder"), which automatically transmit a unique four-digit code when they receive a radio signal sent by radar.
The code gives the plane's identity and radar stations go on to establish speed and direction by monitoring successive transmissions. This flight data is then relayed to air traffic controllers.
However, once an aircraft is more than 240km (150 miles) out to sea, radar coverage fades and air crew keep in touch with air traffic control and other aircraft using high-frequency radio.
Flight MH370 disappeared from air traffic control screens when its transponder signal stopped. The last definitive sighting on civilian radar showed the plane flying north east across the Gulf of Thailand.
The final radio message received by air traffic control - "Alright, roger that" - suggested everything was normal on board.

Don't planes have GPS? Yes, but while GPS (Global Positioning System) is a staple of modern life, the world's air traffic control network is still almost entirely radar-based.
Aircraft use GPS to show pilots their position on a map, but this data is not usually shared with air traffic control.
Flight MH370's last known location

Some of the most modern aircraft are able to "uplink" GPS data to satellite tracking services, but handling large volumes of flight data is expensive and such systems are usually only used in remote areas with no radar coverage.
However, there are plans for air traffic controllers to replace radar as their primary surveillance method over the next decade.
The new system - ADS-B (Automatic Dependent Surveillance-Broadcast) - will see aircraft work out their position using GPS and then relay data to the ground and other planes.
But, as with existing secondary radar, ADS-B coverage does not extend over the oceans.
ADS-B is already used by flight-tracking websites, but the Malaysian aircraft disappeared from these at the same time it vanished from air traffic control screens.
The disappearance of flight MH370 has already brought renewed focus on whether mid-air tracking should be improved.

Could other data systems provide clues? When Air France flight 447 crashed into the mid Atlantic in 2009, its onboard data system - Aircraft Communications Addressing and Reporting System (ACARS) - gave investigators an early insight into what had gone wrong.
ACARS is a service that allows computers aboard the plane to "talk" to computers on the ground, relaying in-flight information about the health of its systems.

ACARS data from Air France Flight 447 revealed how things started to go wrong.

Messages are transmitted either by radio or digital signals via satellites, and can cover anything from the status of the plane's engines to a faulty toilet.
This provides ground crews with vital diagnostic information, allowing maintenance to be carried out more quickly.
In the Air France case, ACARS highlighted faulty speed readings, which caused the air crew to become disorientated.
The Wall Street Journal has reported that US investigators believe that aircraft manufacturer Boeing received ACARS updates for around five hours after flight MH370 disappeared.
That led searchers to believe the plane could have flown more than 1,600 km (1,000 miles) beyond its last confirmed radar sighting.
But Malaysian authorities deny this, saying the last ACARS transmission came less than half an hour after take-off.

The BBC's Richard Westcott takes a look at the gadgets used to track a plane in flight

What about the 'black boxes'? The mystery offlight MH370 may only ever be solved when the aircraft's "black box" flight recorders are recovered.
However, recovering them from the sea is not easy. In the case of Air France flight 447, it took nearly two years.
If under water, the boxes emit ultrasonic signals - but these signals have a limited range, and search crews may not detect them unless close to the crash site.

"Black boxes" may be the only way of establishing what went wrong.

Black boxes - described by aviation reporter Stephen Trimble in The Guardian as "one of the most galling anachronisms of modern aviation technology" - are not currently equipped with any form of GPS location transmitter.