"plano" wrote in
:

If averaged over 10 minutes or so, maybe 4800 would offer enough
bandwidth in certain locations, but you just cannot risk that say 20
(long) AIS sentences are received more or less simultaneously. There
is no way 4800 baud (1/8th!! of 38400) will handle that. Any idea
how much traffic one can expect say in the English Channel? It's the
worst case scenario that counts, not the average in Charleston harbor.
plano



You can watch the Irish Sea:
http://www.aisliverpool.org.uk/index.php
Finest AIS system on the internet.

But, of course, YOUR AIS isn't this good. Your range is about 10-12
miles with a 50' antenna listening to these 12W transmitters. This
limited range limits the number of AIS packets you must process in your
small system. That in itself reduces the load considerably.

Click on [Pan and Zoom] on the Liverpool AIS system. Zoom out until the
scale in the lower left bottom corner says 10 mi - 20 km on your screen.

Now, pan out of the Liverpool ship channel by the docks. Cruise the pan
out the channel into the Irish Sea, a busy place out from Liverpool. Go
off in the direction of the maximum density of ships you can find at the
time. Using the scale in the lower left hand corner as a RADIUS from
your boat, the actual range of your own AIS receiver in any at-sea
situation, how many ships can you get inside that 40 km circle around
your cursor? In Liverpool harbor, with a lot of ships docked but still
transmitting away on AIS, I can get, maybe 15 in range. Out at sea,
where we are concerned about this problem, the ships are spread out more.
If I center my boat 20 km N of Amtwch, the peninsula sticking out to the
East of Liverpool, at this moment I would be painting 7 or 8, tops.

Point is your boat-mounted AIS receiver's 20 km horizon ISN'T going to
paint all those ships you can see on this chart of Liverpool and the
Irish Sea, a very busy place for shipping. If 8 ships are transmitting
full AIS data into my system on 4800 baud every other second, it won't
tax the 4800 baud bandwidth anywhere near its limit to the point where it
would jam or nav data packets from the other instruments would be slowed
down to a crawl. It just won't happen, unless we put up a 1000' mast to
get more packets......

Larry
--
http://www.youtube.com/watch?v=fEJmc...elated&search=