Aircraft are monitored using two radar systems:
Primary radar is based on the earliest form of radar developed in the 1930s. It measures the approximate position of aircraft using reflected radio signals, whether or not the subject wants to be tracked. So a radar can only indicate the presence of a plane, but not reveal which plane it is.
Secondary radar, which relies on targets being equipped with a transponder, requests additional information from the aircraft such as its identity and altitude. All commercial aircraft are equipped with transponders, which transmit a unique four-digit code when they receive a signal.
The radar stations establish speed and direction by monitoring successive transmissions. This flight data is relayed to ATCs, which only use primary radar when transponders are not fitted, are turned off or are broken.
A transponder is likely to stop working only if its electrical supply develops a sudden fault, if the pilot switches it off, or there is a collision or explosion.
Once an aircraft is more than 240 km out at sea, radar coverage fades and aircraft keep in touch with ATC and other planes using high-frequency radio or data similar to text messaging. Radar coverage also fades if the aircraft dips below 5,000 ft because of the Earths curvature.
MH370 disappeared from ATC screens when its transponder signal stopped over sea.
Worlds ATC 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 ATC.
Some of the most modern aircraft are able to uplink GPS data to satellite tracking services, but handling large volumes of data is expensive.
MH370 sent basic pings for a while, but that only helped identify two very approximate flight corridors.
Over the next decade, ADS-B (Automatic Dependent Surveillance Broadcast) is expected to replace radar. ADS-B will see aircraft work out their position using GPS but coverage does not extend over the oceans.
In the US, a satellite-based guidance and tracking system, NextGen, may be in place by 2025. The idea is for GPS to provide real-time information and to help pilots follow the shortest routes.
Other data systems
Aircraft Communications Addressing and Reporting System (ACARS) is a service that allows computers on the plane to stay in touch with computers on the ground, relaying information about the health of its systems by using satellites.
ACARS are not easy to turn off, as it requires removing circuit breakers, after reaching through a trapdoor into the planes hull. MH370 had ACARS on board.
Failsafes: In the US, aircraft are required to have an emergency locator transmitter, which transmits distress signals on a universally recognised frequency. This technology is often marketed to backpackers and adventure-seekers.