[PUP] AIS pitfalls

Mon Jun 4 13:30:31 EDT 2007

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