I think two things should be required. There should be some indication if
the display unit/software has lost communication with the AIS talker,
whether there are currently targets or not. There should also be a simple
indication of lost signal from a particular target, such as the red line you
describe. This would provide the confidence in the system needed to make
navigation decisions.


"Peter Bennett" wrote in message
...
On Fri, 20 Oct 2006 21:16:22 +0300, Tapio Sokura
wrote:

sw wrote:
Thanks for the info. It's too bad in a way that you get the red X even
though the AIS receiver is hooked up and running. It strikes me as
confusing. My guess is that there will be improvements in the software
down
the road.

The problem is that when no AIS transmitters are in range, the AIS
receiver isn't receiving anything and thus it doesn't output anything on
its serial port. This means that the device on the other end of the AIS
serial cable has no way of knowing whether there simply is no AIS
traffic within range or whether the cable is cut, or if the receiver is
malfunctioning. There is no "keepalive" so that the AIS receiver could
tell serial port listeners that it is still alive and kicking.

Tapio

I wonder if the commercial-grade AIS units do send a "keep alive"
message, and if there is a requirement for commercial systems to alarm
when no message is received from the AIS receiver for some time.

I've been beta-testing a charting program - initially, it not only
alarmed when it didn't receive AIS data, but it also DISABLED the AIS
input!! After some complaints, they changed things so it no longer
disabled the input, but still alarmed if data was not received for 10
sec., and required a manual click to acknowledge the alarm and remove
the alarm message box. The current version of the program has an
option to disable the alarm, so it is finally usable.

(The same program also disabled output to the autopilot if it didn't
get anything back from the autopilot. They removed that problem, but
I never did find out what they expected to get back from the
autopilot.)

One of the AIS-capable chart programs I have places a red line across
any vessel it hasn't heard from for some time (I forget the interval).
It doesn't sound alarms for this, so I consider this action
acceptable, and even desirable.

--
Peter Bennett VE7CEI
email: peterbb4 (at) interchange.ubc.ca
GPS and NMEA info and programs:
http://vancouver-webpages.com/peter/index.html
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