Inmarsat - this piece is on the BBC website, by a science correspondent Jonathan Amos.
"This story is a mystery for sure, but information is now starting to emerge that allows us to join some of the dots.
We know the aeroplane was fitted with a satellite system that enabled it to pass information to the ground during flight. It is my understanding that this system, operated by London's big satellite telecommunications company Inmarsat, received an automated signal from Malaysia Airlines flight MH370 at least five hours after the plane was reported lost.
There is no way that signal could have been sent unless the plane was intact and powered. These types of satellite systems can pass a range of data, even voice calls.
But even if the last communication was a simple, automated ping carrying no real information, its receipt alone should make it possible to work out an approximate position for the aircraft at the time of that last signal. This may well explain why the US has now sent search teams to the Indian Ocean."
BBC News - Correspondents - Jonathan Amos