One potential problem with a very simple "AIS Received" alarm
is that under certain radio
propagation conditions the range for AIS reception
can extend to hundreds of miles. On many occasions I've received signals from boats in excess of a 200 NM range -- this is with a spreader-mounted antenna
. With a (for example) stern rail-mount antenna
my maximum receive range still exceeds 100 NM.
The guaranteed range is essentially line of sight, but this extreme range propagation is fairly common during certain weather
My concern is that an alarm
that triggers at 50+ NM will become a nuisance and not a help. You can of course design (or buy) a system that uses a GPS
input to calculate the ship's range and trigger an appropriate alarm, but this involves more than just an activity detector.