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Old 18-12-2010, 21:08   #1
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AIS - Need to Take it with a Grain of Salt

Current AIS data shows the U.S. Coast Guard Cutter Aspen, of the Juniper Sea Going Buoy Tender class, is moored at Yerba Buena Island (in San Francisco Bay), making 102.3 knots.
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Old 18-12-2010, 21:13   #2
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Quote:
Originally Posted by markpierce View Post
Current AIS data shows the U.S. Coast Guard Cutter Aspen, of the Juniper Sea Going Buoy Tender class, is moored at Yerba Buena Island (in San Francisco Bay), making 102.3 knots.
Wow. Just think how fast it could go if it wasn't moored!

-dan
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Old 18-12-2010, 21:33   #3
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The military has a different system from commercial ships. I've had trouble getting anyone to talk about specifics but, from what I hear, they can basically manipulate all the data they transmit... so my guess is the Jupiter Sea was training a new operator.
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Old 18-12-2010, 23:19   #4
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AIS glitches

Actually, all that data is supposed to be input to the system automagically. There are obviously glitches sometimes, but not usually from the Coasties - that's funny!

I recently saw a freighter reporting a speed in excess of 500 knots (while anchored off Port Kelang, Malaysia). I've also seen an AIS signal with a name (humorous & possibly incorrect) but no nav data. Would have been funny but it was 100 miles from shore, at night, & we were crossing busy shipping lanes obliquely. So we knew this guy was close, but not how close, & we were trackiing several other ships at the same time.

But yes, the military (including the Coasties) are allowed to turn off their xmitters, & usually do. I had not heard that they were allowed to xmit incorrect (manipulated) data. That's a different kettle of fish entirely...
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Old 19-12-2010, 11:51   #5
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I looked into this further via an expert on AIS and his comments, as follows, make a lot of sense;

" those are not errors with peoples' AIS units, but instead are bad
interpretations of the values that come back:

1 023 = not available, 1 022 = 102.2 knots or higher

So 102.3 knots means that AIS unit on the vessel is not able to calculate
speed. That probably means they don't have a GPS lock."

He also tells me he will be making a follow-up post about this on his blog; http://schwehr.org/blog/
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Old 19-12-2010, 16:42   #6
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AIS misconceptions - a walk through of position and static data for CG Aspen

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I looked into this further via an expert on AIS and his comments, as follows, make a lot of sense;

...

He also tells me he will be making a follow-up post about this on his blog; Kurt's Weblog
Hi All,

I'm a researcher at the University of New Hampshire and funded between 1/2 to 2/3 of my time by a NOAA Office of Coast Survey grant and I maintain several NAIS feeds. There is a lot of confusion with AIS and the specifications are very difficult to follow correctly. I went into a lot of detail in my walk through of the position and static data messages in my blog post, but I left out tons. There are all sorts of funny cases. As for the comment about 500 knots for a vessel speed, there is no way to encode speeds over 102 knots, so this was likely a bug in the software you were viewing AIS. Remember, AIS is not a very reliable medium for sending messages. Messages are lost and corrupted all the time. The checksum scheme in the messages are weak and occasionally junk can get through.

I used the USCG Aspen as the example...

Kurt's Weblog: December 2010 Archives

I have been struggling to understand all the implications of AIS since 2005 and am continually surprised by new issues I discover.

-Kurt
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Old 19-12-2010, 18:12   #7
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I don't have AIS on board Exit Only, and I doubt if I will ever get it. I trust my eyeballs and my radar, but especially I trust my eyeballs.

I don't sail in areas where there is much fog, so 99.9% of the time, my eyes do just fine.

I trust my eyes implicitly. I don't ever worry about whether they are giving inaccurate data.

Maybe it's a generational thing. I don't want to turn my sailing into a digital experience. A GPS is great, but I don't rely on GPS. I still rely on my eyes.

It's easy to slip into digital overload and suffer from digital confusion trying to figure out what's reliable and what isn't. Computers and digital overload drive many cruisers crazy. I have seen yachties paralyzed by digital grib files - waiting for their digital horoscope to turn in a favorable direction. As if some computer really did have the ability to predict the future wind and weather.

I'm not interested in virtual sailing reality. I like the real deal stuff that my eyes can see.
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Old 19-12-2010, 18:23   #8
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I view AIS displays of the greater San Francisco Bay Area on my home computer. It is interesting. (Wow, a large pleasure yacht went up the Napa River today; why is there lately always a congregation of several tugs near The Sister's, even at night?; there seems to be dredging going on just south of Roe Island; the cruise ship Amsterdam is "in town", there are four pilot boats active today, etc.)

http://www.boatingsf.com/ais_map.php

Don't plan to have AIS on my boat, however.
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Old 19-12-2010, 20:44   #9
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Part of the problem with things like AIS is that when I need them most, I'm not sure that I can rely on them, and I'm too busy trying to keep things under control that I don't have time to focus on the AIS because things are happening in the real world that could kill me if I don't do the right thing.

One of the scariest moments on Exit Only was in the Gulf of Suez when I was on watch at 2 AM. We were motorsailing into a thirty knot headwind without a moon. Looking to port, there were a baziiion lights from the shore. A tug boat with a megaphone pulled along side and shouted that there was a survey ship dead ahead that was pulling several kilometers of survey cables, and the ship was going to execute a turn right in front of my path. He told me I had to get out of the way to avoid disaster. To starboard, there were two shipping lanes running north and south with running lights showing that there were ships headed in both directions. Exit Only was bouncing around in all the chop. There was chaos ahead, chaos to port, and chaos to starboard with a thirty knot headwind. At that moment, I had extremely busy eyes. I turned across two shipping lanes and headed for the Sinai side of the Gulf of Suez. Very scary stuff, and when I got to the Sinai side, the lights of all the oil rigs compounded the confusion.

I would be afraid to trust my safety to AIS in chaotic situations like that. Not to mention the fact that there are other vessels - dhows and yachts that don't have AIS.

When I don't really need AIS, it's there providing information that is not that important to me. When I really need it, and split second decisions need to be made, I don't have the time to focus on the AIS, and not all vessels may have it installed on board. That night there were at least ten yachts moving through the area, and none of them had AIS. It's all hands on deck and lots of eyeballs looking all around.
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Old 19-12-2010, 21:41   #10
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I'll take a sharp lookout over AIS any day of the week.
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Old 19-12-2010, 22:00   #11
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Presently we have the 748-foot tanker Overseas Pearlmar proceeding north offshore from Halfmoon Bay (a few miles south of San Francisco) showing an Ecuadorian estimated arrival of Sept 13, 2010.

I have the sense that no one provides any feedback/penalty for false AIS reporting. AIS has a ways to go. Perhaps the most benefit presently is providing the name of the vessel so one can radio and say "what the ..."
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Old 19-12-2010, 22:35   #12
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I recently installed an AIS transmitter on my boat. I don't look at it often either... eyes are the best collision avoidance tool and I do trust mine.

I have an AIS transmitter for the following reasons:

Being a ship's officer (master unlimited) myself I can tell you it's often very hard for a sailboat (or any slow boat) moving at 5 knots to get out of the way of a ship going 16-26. So it doesn't matter how good my eyes are, it's more important how good the ship officer's eyes are.

I use it, as markpierce says, as a way to make vhf calls easier (I can tell you it's much more likely the vessel will answer if you use their name).

Also, most fo the young ship officers I work with who are right out of the academy are much too reliant on electronic devices. Many have trouble with basic "steady bearing decreasing range" from a visual sight. If you don't show up on their computer screens then they are going to have difficulty reacting to you.

So for these reasons I carry an AIS-B transmitter is to help the other guy notice and navigate around me... not the other way round.

There is a common perception in the boating world that ships drive oblivious to small boat traffic. While this is often true in crowded harbours it's not the case at sea. I (and most conscience mariners on ships) do alter course to avoid boats. (caveat: don't assume we will) But we only avoid boats that we can see and have time to react to. The problem is that most boats are very dimly lit and hard for us to see... AIS changes that.

So, my advice is, if you want to avoid being hit by a ship buy and AIS transmitter and, if you're under 35' loa, a good radar reflector first then ignore them and drive with your eyes wide open.
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Old 19-12-2010, 23:01   #13
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Quote:
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Presently we have the 748-foot tanker Overseas Pearlmar proceeding north offshore from Halfmoon Bay (a few miles south of San Francisco) showing an Ecuadorian estimated arrival of Sept 13, 2010.

I have the sense that no one provides any feedback/penalty for false AIS reporting. AIS has a ways to go. Perhaps the most benefit presently is providing the name of the vessel so one can radio and say "what the ..."
There are 2 types of data that have to be inputed into the AIS-A units - static data (MMSI #, length, vessel type...) and dynamic data (destination, navigational status, eta) the former get's inputed by the installer and is usually accurate. The later get's updated by the ships crew and is often forgotten about or ignored. The other data (position, speed, heading...) is calculated by the computer and, apart from gps anomalies and computer bugs, is 99% on accurate.

So just because a ship has the wrong ETA or destination port... don't assume the important information (CPA, speed...) is wrong.
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Old 19-12-2010, 23:32   #14
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If possible, I'd transmit "destination random, watch out!" because that would always be accurate.
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Old 20-12-2010, 00:14   #15
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... the cruise ship Amsterdam is "in town
And will be leaving LA Dec. 22 for Florida via the Panama Canal, and then making an around-the-world cruse.
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