Flight altitude data comes from secondary radar

CNN has now been reporting altitude changes after Flight 370 terminated contact with flight control for hours, if not days on end. Kindly, the BBC has provided a tutorial on modern aircraft tracking with particular relevance to Flight 370.

Of particular note is the method of altitude tracking. Altitude data on the controllers display comes from "secondary" radar, or signals received from the aircrafts transponder. The radar set then fuses the transponder data with primary returns ( from the skin of the aircraft.)
If there is one thing that seems certain, it is that the cockpit turned off the transponder and ACARS before it began its deviant maneuvers. It is questionable, based upon physics, that CNN could have new altitude data if the transponder and ACARS were disabled.