Thanks Jim.  I will take a look at it.  I tried to keep my comments brief
as to the theory of how the AIS system works, without getting into Self
Organized TDMA and Carrier Sense TDMA, or the intricacies of how the
processor selects and deselects the targets of concern particularly or how
the vessels dynamic conditions affect the transmit rate.

My article, based upon my experience, for our style of cruising, is that
this is a cost efficient component of our navigational system. I can tell
many stories of how we have used it here on the Bay, and along the ICW.
Would I like to have radar? Sure.  Have there been times I would like to
have had radar?  Sure, but only 2-3 times in the past 6 years.  But radar
is not in our budget right now. AIS gives me good information and real time
information when transiting  the ports and shipping lanes. It adds to my
situation awareness every time I head out. It is not that I don't think
Radar is an important and very valuable tool. But for the 99% of cruisers
who only go out in fair weather, AIS is much more cost effective.  It
delvers a much bigger bang for the buck.

A recent example. Last week transiting Norfolk we could see the vessels
entering the harbor from the sea, in the harbor, and moving in the harbor
while we were still in Deep Creek Lock. We came thought the Southern Branch
into the harbor. We were in Lambert Bend and there was a ship ( Edward
Something) with two tugs along side ahead of us entering the port.  It was
all clearly displayed on the  chart plotter. The ship called Tadhana on
13.  He told me that he was about to execute a sharp turn to port and dock
a t the coal piers.  i thanked the pilot and said I would swing to my port
and go to the red side of Craney Island Reach.  He tanked me and
cautioned that there was a tug (Island Girl) and barge just astern of him.
The tug was out of visual and would not have been on radar either as it was
blocked by the collier.  But we already knew it was there because of our
AIS.  As we passed the collier I was able to contact the tug and ask
permission to turn to starboard in front of her and head into the Lafayette
River.  The tug knew who we were and where we were, gave us permission to
cross,and thanked us. This was a typical, but not unique, ise of AIS in our
routine cruising.  And we have many more stories like this too.

Last summer we were underway on the Bay 126 days I do not know how many
ships we passed and USCG vessels we interacted with, but it was a lot.  No,
I don't call ships very often.  We generally have a visual on them before
we see them on the chartplotter, but as we are passing or crossing it is
very nice to have information on their course and speed, as it allows us to
modify our route to stay out of the way.

Two other things I learned. First, If you have AIS you must have it on when
you are underway (actually starting 15 minutes before).  Second, I learned
that Bay pilots do want to hear from you if you are uncertain about a
course or crossing information.  The story that class A AIS recievers can
filter out and turn off Class B signals is false.  Ben Ellison was quite
helpfull  in my research as was Digital Yacht.

Cheers,
Tom
Chesapeake Bay

On Mon, May 11, 2015 at 2:12 PM, Jim Healy gilwellbear@gmail.com wrote:

Questions, comments imbedded below.

Citation, please.  Not aware of any requirement like that.  USCG?  FCC?  Manufacturer?  State?

Definitely.  That was also absolutely clear on the River System and on the A-ICW.  Professional captains appreciate hearing from us, because at least that way they know we're awake.  Not running over us saves them a lot of paperwork.

Interesting you mention Ben Ellison.  I think, though, not clearly either "true" or "false."  A complex story with multiple related issues here.  First, remember that in 2008/2009, the international standard for Class B AIS (CSTDMA) was not type-approved by the FCC for retail deployment in the US.  Many of the early Class B units that appeared in the US at that time were manufactured offshore to standards that had not yet been adopted.  AIS was clearly an emerging technology at that time.  Many of those early Class A and Class B units are still in service today.  Some AIS receivers (would have been Class A receivers because that's all that early standards design supported at that time) would have been unable to interpret the header information of Class B messages and would have thrown those messages away.  Would that be considered "filtering?"  Not explicit, but certainly implicit.  Second, it's not necessarily a Class A AIS receiver doing the "filtering" by throwing Class B messages away.  It's also the screen display software for the chart plotter.  At the time I did my research, specific Furuno chart plotter models were cited as being abe to toggle Class B targets "on" and "off."  I have no personal experience with Furuno equipment, but one of those contemporary discussions took place on Panbo (maybe 2010/2011 timeframe) between Ben and Jeff Siegel.  Today, I believe it comes down to whether or not the mandatory carry vessel is fit with a SOLAS ECDIS-certified nav system.  ECDIS-certified systems (today) do not filter.  However, some of the nav system flavors, of which there are several deployed worldwide, that are not ECDIS-certified (vs not compliant) do have the capability to filter.  It's entirely possible equipment manufactured in that early deployment period had features and behaviors that the current product lines have not continued to offer.  Then the question for us, today, is, how much of that early stuff is still deployed in the real world?  A reliable answer is unknowable.

In any case, my assessment is, reliance on Class B AIS generally promotes overconfidence in the minds of "average," non-tecchie-geek owners.  You know; ordinary people who made a lot of money but can't set the time on their VCR or sharpen a pencil.  I have seen any number of instances where I'm running along with a boat carrying Class B.  My "own-ship" icon moves smoothly on my chart plotter and iPad software.  But the AIS vessel hopscotches along at long, sometimes random, intervals, jumping over me, or dramatically changing position proximity ahead of or behind me.  Clearly, for all but the actual reporting millisecond, that vessel is NOT WHERE THE VESSEL ICON SAYS IT IS.  In busy locales with many targets, and particularly high speed targets, that makes the tool pretty much useless.  Takes your eyes off the water way too long, and distracts the mind trying to reconcile what's seen on the screen to what's seen out the window.

I am not arguing that the tool isn't valuable.  Not at all.  I just think that it does not have universal application as a nav tool, and that way too many owners do not understand it's true capabilities and nuances.  Just like RADAR.  Many people do not understand how to use RADAR properly.  And the fundamental truth is, if you don't know how the tool works and don't know how to work the tool, you will not get the most from it's operation at the very times when you most need it.

Jim

Peg and Jim Healy aboard Sanctuary, currently northbound at Oriental, NC
http://gilwellbear.wordpress.com http://gilwellbear.wordpress.com/
Monk 36 Hull #132
MMSI #367042570
AGLCA #3767
MTOA #3436