To add to the AIS weirdness, I encountered a USCG buoy tender in
Clarence Strait in Alaska whose AIS wasn't outputting SOG or COG, but
just position and heading (presumably from a flux-gate or SAT
compass). My Nobeltec screen showed their position accurately, but
didn't give me any alarm or any prediction information (as it usually)
does.

I had the radar zoomed down to 1 mile range as I was scanning for
smaller boats in the clutter (visibility wasn't great) and the cutter
called me at fairly close range to negotiate passage, which was a big
surprise.

When I zoomed out and saw him bearing down on me, the CPA was
essentially zero and he was coming fast.

Given I have Nobeltec set to warn me about boats whose AIS CPA
prediction is less than 0.5 miles when they are 3 miles or less range,
I was puzzled by the lack of the alarm. Also, Nobeltec shows a dotted
line predictor that crosses my screen if the AIS target is outside my
set range. But no predictor.

My Furuno radar also shows a predictor extending across my screen when
an AIS target is outside my current range, but it didn't show a
predictor either. But once I zoomed radar out to 2 miles, there he
was, big as life.

My guess is that Nobeltec and Furuno both use the broadcasted SOG and
COG as part of their collision detection and predictor algorithms.

I called the cutter back and told him his AIS was putting out a bunch
of garbage that disabled my collision avoidance software, and he
thanked me, saying he'd get a technician right on it.

On the other side of the coin, once when I was skirting the north side
of Whiskey Gulf torpedo range in Georgia Strait, BC (range was active)
just south of the Balenas Islands, I had two big, fat AIS targets
steaming right through the range, heading pretty much my way. I
checked their data, and found they were labeled as passenger ships,
with ship names and data that sounded reasonable.

My first reaction was, "how come they get to drive through the torpedo
range when its closed to us?" I grabbed my glasses to check them out,
but saw no ships. I looked back at the display, and they were moving
closer to me, with all the data still looking just perfect for real
ships. But they weren't there, visually or on radar. The LCD screen on
the Furuno AIS didn't show them either, but Coastal Explorer was
showing them clear as day, with tracks and predictors and everything.
They were just "Phantoms in Whisky Gulf" (sounds like a book title). I
had to shut down CE and restart it to erase the phantoms. Not sure if
the was spoofing me or I found a really bizarre CE bug.

All of this reminds me of something a former United Airlines chief
Boeing 777 pilot, and also an active trawler driver and a member of my
yacht club, once told me..."radar is truth, all the rest is just
theory."

To another question that was asked about heading sources, the Furuno
GPS that uses rate gyros is the SC50, which I own, often called a SAT
compass. Its an excellent TRUE heading source as its unaffected by
boat motion, magnetic anomalies, deviation, variation, declination or
anything else associated with a real magnetic compass.

Which I suppose makes it a really BIG pile of theory.

John Marshall
Nordhavn 55-20 Serendipity

John brings up an interesting issue. What sensors should provide what input
to which device? Until the advent of AIS, a Furuno radar could display
MARPA/ARPA information with 3 inputs: radar data, rate compass/GPS data, and
an ARPA board. The compass can be anything from a KVH electronic to a Furuno
satellite compass based upon GPS.  The other two components do not have
substitutes. In that mode, the radar's ARPA/MARPA represents ground truth
and computes everything based upon radar returns related to the other two
inputs.

Radars can display AIS vessel symbols or data rectangles. I do not know if
there are pure radars that can interpret AIS message content such as type,
course, and speed; but right now that is data that is not always present.
When it is present, it is dependent upon the GPS/AIS installation on the
other vessel. Your radar is independent of outside variables. The AIS data
is food for your brain (type, course, and speed) and perhaps navigation
software. The vessel's name displayed readily allows you to contact vessels
whose CPA requires attention.

I would not seek to have the radar compute tracks based upon AIS information

• unless the radar could ID AIS targets which were not providing a return.
  Probably not that hard to compute. As far as spoofing goes, I would not
  underestimate the military's ability to simulate whatever they wanted to on
  civilian equipment. If I doubted the presence of a ship, I'd deactivate the
  filter on my radar which suppresses the display of other vessel's radars and
  see if those targets were emitters. Depending upon the band used, my radar
  could probably reveal whether this was a real target or not. In a busy lane,
  we don't have that luxury.

BTW, when I was following the GSSR, USCG vessels in the vicinity of Dutch
Harbor, Alaska often suppressed selected elements of their AIS data. I know
this because two of them seemed to be orbiting in the same place over two
days and sometimes they suppressed their names and USCG identity. I could
see their doing this when they are in their fisheries' enforcement mode or
other law enforcement mode. "Get a technician" my posterior.

Ron Rogers

----- Original Message -----
From: "John Marshall" johnamar@mac.com
To: trawlers-and-trawlering@lists.samurai.com
Sent: Friday, September 25, 2009 6:01 AM
Subject: T&T: Other AIS weirdness

To add to the AIS weirdness, I encountered a USCG buoy tender in
Clarence Strait in Alaska whose AIS wasn't outputting SOG or COG, but
just position and heading (presumably from a flux-gate or SAT
compass). My Nobeltec screen showed their position accurately, but
didn't give me any alarm or any prediction information (as it usually)
does... Snip

Even Radar is not what it used to be.  I was about 2nm off this warship
http://telstarlogistics.typepad.com/telstarlogistics/2007/08/britains-stealt.html
and she didn't show on  my 4kw Furuno radar, not even the faintest trace.  I
thought about firing up the 12Kw array but decided discretion was in order.
I didn't want anyone suddenly taking an interest in why I was trying to
paint her :)

Paige