[PUP] AIS pitfalls

[John Marshall]

Actually, the FA-150 is a transponder... our buddies track us via AIS  
when we are in the area long before they can raise us on VHF. Which I  
find interesting given AIS uses VHF as well.

We also are encouraged when approaching VTS areas as we often hear  
VTS controllers giving updates to other ships where they mention our  
boat by name, location, destination and course. That's done prior to  
making contact with us and sometimes just before we enter the  
controlled areas. They could only be getting this from our AIS.   
Sometimes they also initiate calls to us to confirm the data they  
have (which I think is to your point about stale data loaded into  
some AIS units).

Here's the link to the unit we have installed: http://www.furuno.fi/ 
sw2983.asp

Our AIS is technically installed in violation of the 04 guidelines as  
our AIS antenna has too much gain. That helps us see targets further  
out (and presumably be seen), but opens the envelope larger than I  
think was intended.

To your other point, for course input, we do have a GPS compass  
system... the dome for that is mounted high above everything else at  
about 38' elevation to make it work well. I understand it uses 3  
GPS's all by itself. It broadcasts heading to all devices on our NMEA  
bus. It worked great coming up the Pacific coast from Dana Point to  
Victoria, BC, even in large, steep head seas when we were pitching  
like crazy. The swing at 38' above the water must have been dramatic.

John Marshall
N5520 - Serendipity

On Jun 4, 2007, at 10:30 AM, Dave Cooper wrote:

> <John wrote: The Furuno FA-150 on my boat includes a 12 channel DGPS
> receiver to ensure it always has correct internally-generated COG,  
> SOG and
> location data. It does not need any external heading input as it
> computes it all from its GPS.>
>
> The FA-150 is only an AIS receiver and I have been talking about  
> what we
> receive and the transponders that generate this info to transmit.
>
> BTW, eight must be a common number as we have 8 distinctly separate  
> GPS's
> also. Some of the clearance forms now ask the specific question  
> here. How
> many GPS's aboard...screwy question for them to ask but it's there!
>
> The GPS can only give course not the ships heading unless you have  
> a GPS
> compass in which case you'd have 10 AND need another several square  
> meters
> of EMF free space to mount them ;-). Heading is which way the bow  
> is pointed
> whether the ship is moving or not. Course, in this case, in the  
> course over
> the ground as derived from the GPS.
>
> As we have seen and has been discussed many time on both the PUP  
> list and
> the T & T list GPS's can and do lose signals and also can be  
> spoofed. In the
> US this is prevalent around warships and other areas of high need to
> disguise/cloak the location.
>
> We have seen several examples on this on our displays when the GPS  
> signal is
> either lost or reports a bad location. We were anchored about a  
> mile from a
> Mega yacht a few weeks ago at anchor. His AIS location would jump  
> from a
> mile or more from his real location several times an hour. One would
> conclude that his GPS was having difficulty locking onto his  
> location as the
> heading input wouldn't do this. However it was not in error  
> sufficiently
> long enough to generate the "loss of signal" that is built into the  
> AIS
> transponders....another minor oversight of the developers, IMO.
>
> So unless you have a separate and unrelated source to the GPS  
> course input
> then you don't have a heading which is what drive the ships symbol  
> on the
> chart plotter/PC or is at least supposed to be what drives the symbols
> according to the AIS spec. Awfully easy for a programmer to pick up  
> the GPS
> info and use it for both :-(
>
> This is why the two pieces of data will always be different...two  
> different
> sources. This difference may be correct as caused by current, wind  
> or other
> external factors influencing the ships heading or they may be in error
> caused by an installation error or an alignment error of the GMDSS
> equipment.
>
> Again with no easy way to determine any of this other that with an  
> external
> receiver one never knows if the data that is being transmitted is  
> correct or
> bogus.
>
> I have a Tropical Shipping container ship from Florida 8 miles away  
> at the
> present docking at Road Bay, Anguilla. His heading was 27 degrees  
> different
> from his course and his ETA of his destination of Anguilla was 1 June
> 0300!!! This on the 4th of June! So I contacted him via VHF and he  
> wouldn't
> discuss anything about his AIS. Not my concern in his opinion!!
>
> Hmmmmm, with this attitude on the bridge it will be a long time  
> before one
> can discuss a software upgrade that he needs to see us as a Class B  
> unit
> I'll bet ;-)
>
> As with all the black boxes we as operators/navigators/masters need  
> to be
> aware that they are all run by software. This software has been  
> written by
> folks who have never been at sea or outside of the sight of land.  
> They have
> little concept as to the use of it nor the repercussions of a  
> bug...i.e.
> we'll patch it on the next upgrade. AIS is no different than any  
> other of
> these packages. Trying to integrate a space based position system  
> with a
> ship based heading system, transmitting a digital signal via an  
> scheme that
> was developed for hardwired communications originally is no mean  
> feat. I tip
> my hat to the folks who have got it this far and have spent a fair  
> number of
> boat units.
>
> We are still on the bleeding edge of this technology even though it  
> is a
> mandatory carriage requirement of 90% of the worlds merchant fleet at
> present. The failure of the authorities has been to insure that  
> there is a
> robust installation criteria that is followed along with an update  
> program
> and an enforcement program to insure that what they have developed and
> mandated is in fact what is in place and use.
>
> Anyone who has an AIS system installed should at least read the AIS
> guidelines that were published in '04. Not as good or as tight as they
> should be but it gives you a better idea of why installations are  
> not done
> per the book and perhaps why we get so much bogus info.
>
> I get the feeling here that I'm alone in this area of concern about  
> the data
> from the targets I see on our AIS's. Am I also alone about the  
> quality and
> correctness of the data we broadcast almost continuously from our AIS
> transponders whether they are A's or B's?
>
> Jeff Siegel wrote re the broadcasting of the AIS signal. "The issue is
> "automated" transmission.  There are many international issues with  
> this
> too.  I need a $25/year radio license from the Bahamas in order to  
> key my
> mic on the HF band (with a valid FCC amateur license).  How come an  
> AIS can
> just blast data out?"
>
> This is another good point...perhaps in some countries one will  
> require
> additional licensing who knows? It sure is indicative of the left  
> hand not
> agreeing with the right in the US with the delay of the FCC  
> approvals of
> Class B. What would happen if every country took this approach....a
> nightmare as we travel!!!
>
> As always YMMV.....
>
> Cheers
> Dave & Nancy
> Swan Song
> Roughwater 58
> Caribbean Cruise '07
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