[PUP] AIS pitfalls

[John Marshall]

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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