bizman: "1. Airborne ACARS to transmit a simple short standardised "I-am-alive-and this is my gps position altitude and heading" message every say 5 or 10 minutes. If the traffic overhead is low, it could perhaps be done every minute."
It does. It would not have saved anybody to have the messages monitored at all times. (Although feeding them to a computer smart enough to detect anomalies such as missing messages and raise an alert might be useful for finding wreckage.)