Signals from commercial aircraft to Inmarsat satellites always include a code confirming the identity of the plane. You'll find more on this here - It's simply fascinating...
http://edition.cnn.com/2014/03/17/world/asia/satellite-signals-plane-identity-flight-370/index.html?hpt=hp_mid
Experts piecing together the mystery of Flight 370, are using this information to try and track the path that the plane may have taken...
In a talk with Becky Anderson, Professor David Stupples explains what a satellite handshake is, and how a plane can hide radar detection.
A satellite handshake - 'Think of it like a mobile telephone,' he says. Even though the telephone is not being used to make a call, it is registering continuously with its space station to say 'I am here.' This is what the aircraft is doing with the geostationary satellite which is 23000 miles up in outer space....
Professor Stupples breaks this down at - http://edition.cnn.com/video/data/2.0/video/world/2014/03/17/ctw-malaysia-flight-370-radar-expert-stupples-intv.cnn.html
