ana1936: Some of us, maybe myself included, have been a but loose with the term ping perhaps.
Just to clarify this a little.
My understanding is that the satellites in the INMARSAT network which deal with ACARS check on the presence of their registered aircraft every hour by initiating a ping operation. This involves the satellite asking to see if a particular aircraft will answer. Only the specified aircraft will answer. And it will only answer if its engines are operating and it hears the satellite request.
So presumably the aircraft being 'pinged' will still reply even if the onboard ACARS has been turned off (or has failed for some reason)?
It seems that a lot of commentators in the media seem to believe that it was the AIRCRAFT that was initiating a 'ping', every 60 minutes.