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.