[PUP] AIS pitfalls

Tue Jun 5 00:37:23 EDT 2007

Dave,
	All the tugs I've come across do identify themselves as such on AIS,  
but I haven't paid attention to 31 versus 32 or looked to see if  
there is a code for pushing a barge. But when I see "Tug", I assume a  
long tow until I can get a radar or visual picture of it.

When at sea, we set our CPA alarm threshold for 2 miles on the AIS,  
and 0.5 miles in inside waters. But if you're going through some of  
the narrow passes in the San Juan islands or north, you get a lot  
cozier than that. It's real tempting to split hairs on the CPA  
("well, 0.20 mile CPA should be Ok in here") until you watch how much  
the CPA calculation is bouncing around when the boats are in weather.

The accuracy (or rather the predictability) of that number is no  
where near as good as the precision of the readout (i.e. 2.27 miles  
or 575 yards, or whatever) suggests, although it improves  
dramatically as the vessels approach each other.

I can see someone in the fog (in more than one way) someday thinking,  
"CPA is 200 yards... hmmm, no problem in passing... DGPS is far more  
accurate than that." Only to hear a really loud crunching sound soon  
after.

	Sounds like you've had some interesting voyages... not like some of  
us 'digital navigators' who would be struggling if the chart plotter  
went down (I have three levels of backup just to be sure, but would  
really struggle if I had to DR).

John

On Jun 4, 2007, at 4:11 PM, Dave Cooper wrote:

> John wrote: To your point, the other 'fun' thing about depending  
> too much on
> AIS is to make sure you can separate tugs from tows.>
>
> I can't agree with you more. I come from the days of compass, lead  
> line and
> some hand me down charts with notes scribbled all over them.
>
> I was the master of the Nina, a replica of Columbus' 15th century  
> caravel.
> We tried to use less than modern techniques to find our way around.  
> We made
> it from San Juan, PR to New York without any outside aids including  
> a modern
> day Sextant so I am aware of the "old ways" and the needs of the  
> modern day
> mariner.
>
> AIS has a one of its many ship types is 31 for towing and 32 for a  
> long tow
> so again if the operator of the AIS is on the ball then he will  
> have punched
> in the correct code for the tow. This is sort of like navigation  
> lights. A
> long tow is a different configuration than a short tow which is  
> different
> that no tow. So if the master of the vessel is responsible for the  
> proper
> light configuration why the "h..." aren't they responsible for  
> their AIS
> data.
>
> Just seems that the light configuration part of a master test is  
> front and
> center to pass. Has anybody seen the same emphasis on the proper AIS
> configurations? Again AIS is a mandatory requirement...we're not  
> talking
> about optional compliance.
>
> I sure hope compliance is bubbled up to the top layer before there  
> is an AIS
> related accident. It has great promise to help everyone as the seas  
> become
> more crowded and good clear information is available to plan a  
> course thru
> heavy traffic zones. Transponders have been on planes since the  
> late 50's.
> They slowly tricked down to a wider variety of aircraft. Today if  
> you are
> IFR and turn off the transponder you might find a note from the FAA  
> in your
> mailbox. Better yet try an approach from an offshore location  
> entering say
> the east coast of the US without squawking the correct code and not  
> filing
> 48 hours in advance your entry. You might find an F whatever off  
> your wing.
> Not so with AIS...yet.
>
> This is also when the presentation of the data becomes so  
> important. If the
> sender is generating the correct codes then the receiver gets them and
> passes them down the line to the.......SOFTWARE......Uh oh programmers
> who've never been to see are going to present me with information  
> at sea to
> tell me what I "see". Sounds like an opportunity here for good  
> stuff or
> disaster!
> Like how about a "towing symbol for a type 31 and a longer one for  
> a type 32
> or a color change or a boat with a trailing boat. I think you see  
> what I'm
> trying to say. The data is packed with all of this info and to just  
> show a
> stupid triangle when you have the richness of the types available  
> means
> they....the programmers....just don't get it, IMHO.
>
> I know that Rose Point is planning a new version that will be  
> available
> sometime later this year. I sure hope that they spend some time on the
> expansion of the simple data that they now display so we can get some
> differentiation between those ship types, better user filtering,  
> better use
> of alarms along with more information present for the data stream.  
> Lots of
> room here.
>
> Hey, I can't afford to have a copy of everyone's plotter/nav  
> software so
> maybe there is already someone out there that does all this. If so  
> I missed
> the announcement and the demo version so please let me know so I  
> can look at
> it when I get a moment.
>
> As always YMMV...
> Cheers
>
> Dave & Nancy
> Swan Song
> Roughwater 58
> Caribbean Cruise '07
> 787 990-6064 Sat
> 954 252-3830 fax
>
>
> -----Original Message-----
> From: passagemaking-under-power-bounces@lists.samurai.com
> [mailto:passagemaking-under-power-bounces@lists.samurai.com] On  
> Behalf Of
> John Marshall
> Sent: Monday, June 04, 2007 2:51 PM
> To: Passagemaking Under Power List
> Subject: Re: [PUP] AIS pitfalls
>
> To your point, the other 'fun' thing about depending too much on AIS
> is to make sure you can separate tugs from tows. AIS gives you a
> great position report on tugs, but the tow could be 600' or so
> behind. I've talked to people who are into fine tuning their passing
> in crowded areas by nit-picking the AIS CPA report and passing too
> closely in fog. If you can see, the passing requirements for tug and
> tow are obvious, as is the radar return. But watching only that
> 'omnipotent' AIS triangle and not paying attention to other
> indicators (eyes, radar, common sense) is a recipe for disaster.
>
> And not only for us little guys... an Ohio class nuclear submarine (a
> Boomer) last year snagged a tow line with its sail while submerged in
> the Straits of Juan de Fuca (we could see it from our house when it
> surfaced) while trying to pass between tug and tow. Sub turned around
> and went back to Bangor for repairs. Not an AIS issue (he was
> submerged), but an object lesson that even the First Team can make
> mistakes by relying too much on single instruments (in his case,
> probably his passive sonar that could hear tug but not tow).
>
> John Marshall
> N5520 - Serendipity
>
> On Jun 4, 2007, at 11:07 AM, Dave Cooper wrote:
>
>> <Ron wrote: The USCG announced that the Navy in our area of North
>> Carolina
>> would be conducting tests that would disrupt both GPS and AIS. The
>> latter
>> would have to be a discrete frequency interference to avoid  
>> disrupting
>> normal VHF safety communications.>
>>
>> Thanks Ron, this further confirms the point of pitfalls.
>> However the AIS transponder will receive the false or modified GPS
>> signal so
>> it will give bogus location, course and speed data.
>> The heading and other data is independent of the GPS so they would
>> show
>> normally.
>>
>> If the data is scrambled then they are using a VHF frequency
>> transmitter and
>> the VHF transmission would be affected...I doubt that this is what
>> they are
>> doing. If just the GPS data is affected then it's the satellite
>> signal that
>> they're messing with. A more likely scenario.
>>
>> This illustrates a bit more of what I've been trying to get across
>> in my
>> poorly worded rants here. The blending of these technologies, some
>> old some
>> new, into a yet newer package makes it hard for a lay person to
>> understand
>> who's on first. It looks like it also makes it hard for an AIS
>> knowledgeable
>> person to figure it out also.
>>
>> If you jam or spoof the GPS signal the whole of AIS in the area of
>> the false
>> signal is worthless along with all the other 7 GPS's we have doing
>> whatever
>> they do. If you jam the VHF AIS frequency only you only affect the
>> AIS and
>> nothing else. A doubtful exercise, IMHO.
>>
>> I guess the tows in this area will even be more confusing with their
>> projected lines, symbols and location. Better get Penny and John
>> down there
>> to direct traffic ;-)
>>
>> As always YMMV....
>>
>> Cheers
>>
>> Dave & Nancy
>> Swan Song
>> Roughwater 58
>> Caribbean Cruise '07
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