The search for MH370 would have been hopeless had it not been fitted with a system called Classic Aero, a type of Aircraft Communications Addressing and Reporting System (ACARS) which transmits data on location, altitude, heading and speed.
ACARS, as we now know, can be turned off manually, via a switch on the ceiling of the cockpit or behind the throttles between the pilot and co-pilot. MH370’s ACARS system was switched off at 1.21am on March 8, two minutes after the pilots’ last verbal communication with the ground. It is this deliberate act that has convinced investigators the pilots were on some form of suicide mission.
But Classic Aero also has a second terminal that operates independently of ACARS and cannot be switched off while the aircraft still has power.
Once every hour the system sends out a “ping” to satellites operated by Inmarsat. The pings play no part in ACARS, and merely serve to synchronise timing information and keep the connection to the satellite network alive.
https://www.telegraph.co.uk/news/worldnews/asia/malaysia/10720009/MH370-Britain-finds-itself-at-centre-of-blame-game-over-crucial-delays.html