All long haul overwater aircraft are still required, are they not, to be HF equipped, hence, if ACARS equipped, an Alive message could be sent, say, every 10 mins.
Ummm, have you ever used HF? It's hard to enough to talk over, passing data I would think impossible. Just listen to the bandwidth of SELCAL for data transfer - maybe 1 or 2 chs /sec
AFAIK ACARS works over VHF (los) or Satcom (when VHF unavailable).
What the ground does with the triggered alert is not unlike what happens with SAR phases, ie. uncertainty phase, distress phase, search phase etc. Obviously, there would be protocols around this.
As above, I think a sense of realism is needed over MEL requirements, reliability, Sat coverage. It sounds from this forum these messages just roll in... maybe reality is more like "occasionally, when all the holes line up, the messages come through". In practice it is probably somewhere between the 2. I just know from my little Airbus that even the "simple" VHF system frequently goes on standby, sometimes for a minute, sometimes the rest of the flight... It is not designed to be "reliable" for safety related purposes, let alone initiate an SAR alert.
It is clearly "possible". But you are needing to upgrade the aircraft systems, the comms systems, the airline systems to robust / reliable, not just "nice to have features".
If you really want such systems, why aren't you looking at, say, CPDLC? Since this is used for ATC Comms, it has a safety basis, it is "new" technology, it is being mandated effectively over the next X years, it already interacts with ATC rather than the airlines? It is a universal standard, rather then airline specific. Surely better than a system which is really using a ZX81 type audio stream to a tape recorder to transfer data
