sw wrote:
It would seem an easy task of the receiver designer to output data sentences
with just the header information when no transmitter is in range. Really
though, I would blame the Raymarine implementation. When you run AIS in
various PC charting software, you either see targets or not. No red X's, no
problems.

I can see the logic in Raymarine software here, as there are two
possible scenarios: a) there is no AIS traffic in range or b) something
is broken with the AIS receiver setup. Situation a) is normal, but
situation b) needs to be looked at. There's no way an AIS serial stream
listener can deduce which one is the case, so to err on the side of
caution, an alarm is raised. Of course if you cry wolf too often you
might end up in trouble anyway..

Tapio