Can't planes be tracked with 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.
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.
The satellite data which suggests flight MH370 flew on for several hours are basic 'pings' sent by the plane, and so far only help to identify two very approximate flight corridors north and south.
Over the next decade, a new system called ADS-B (Automatic Dependent Surveillance Broadcast) is expected to replace radar as the primary surveillance method for air traffic control
ADS-B 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.