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Old 25-05-2006, 03:34   #31
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AIS
Automatic Identification System Overview - USCG Navigation Center:
http://www.navcen.uscg.gov/enav/ais/default.htm

”... The AIS is a shipboard broadcast system that acts like a transponder, operating in the VHF maritime band, that is capable of handling well over 4,500 reports per minute and updates as often as every two seconds. It uses Self-Organizing Time Division Multiple Access (SOTDMA) technology to meet this high broadcast rate and ensure reliable ship-to-ship operation. ...”

How AIS Works:
http://www.navcen.uscg.gov/enav/ais/how_AIS_works.htm
What AIS Broadcasts:
http://www.navcen.uscg.gov/enav/ais/...broadcasts.htm
Types of Automatic Identifications Systems:
http://www.navcen.uscg.gov/enav/ais/types_of_AIS.htm

DSC
DIGITAL SELECTIVE CALLING :
http://www.navcen.uscg.gov/marcomms/gmdss/dsc.htm

”The U.S. Coast Guard offers MF/HF radiotelephone service to mariners as part of the Global Maritime Distress and Safety System. This service, called digital selective calling (DSC), allows mariners to instantly send an automatically formatted distress alert to the Coast Guard or other rescue authority anywhere in the world. Digital selective calling also allows mariners to initiate or receive distress, urgency, safety and routine radiotelephone calls to or from any similarly equipped vessel or shore station, without requiring either party to be near a radio loudspeaker. DSC acts like the dial and bell of a telephone, allowing you to "direct dial" and "ring" other radios, or allow others to "ring" you, without having to listen to a speaker. New VHF and HF radiotelephones have DSC capability ...”
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Old 25-05-2006, 03:47   #32
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AIS is VHF. DCS is also VHF. They broadcast different information. AIS is a continuous broadcast. DCS is of course only whn the button is pushed and pretty much trash VHF communications when you greceive one. We get a lot of false DCS alerts around here.

COOT, All technology is imperfect. So are all people. It's the curse of living in a modern world. Just because you know the light bulb will eventually burn out does not mean you need to live in the dark to avoid the relaty of it's imperfection.

Warships don't broadcast AIS. The idea that commercial traffic is somehow broadcasting deliberatly false AIS seems silly. People lie on a VHF radio how is that any different?

Information is often imperfect on the water. Your charts are not perfect yet you bring them with you not knowing where they are wrong. It is the sailors lot in life to survive with imperfect information. It's always been that way and it still is. Perhaps that is your intention in your post?
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Old 25-05-2006, 04:16   #33
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In the end dilligent navigation is what is going to keep you off the rocks. I navigate by paper chart most of the time and generally don't put the speed log in the water. But when I cut the docklines in the next couple of years I will have a chartplotter visable from the helm. I believe that understanding and practicing traditional navigation techniques improves the application of technology to the nav problem.
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Old 25-05-2006, 06:40   #34
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Pura Vida,

I agree. All the electronic advancements will never take the place of a competent operator and a proper lookout. The danger in small boat operation, especially in jurisdictions that require no certification or licensing, is that some will rely solely on electronics, without acquiring a basic knowledge of seamanship, piloting or navigation.
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Old 25-05-2006, 07:05   #35
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Instruments alone are not navigation or piloting. It does take a smart pilot to read all the information, look for the things a trained eye can see and determine when something is important and when something is not.

I treat it more as a process of trying to reconfirm what the instruments tell you. Anything can be wrong but if you can spot visual objects and see patterns and make different instruments agree you can have a degree of confidence that the world is as it you think it is.

Knowledge and experience are the tools that do this best. Just as owning navigation tools won't make you a pilot. The skill of piloting is perhaps the critical skill this all refers to rather than just pure navigation science. Enhancing ones piloting skills I think is the more difficult.
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Old 25-05-2006, 23:46   #36
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Having been monitoring AIS from my boat here in Europe for the past 4 months it appears that virtually all of the cargo hauling vessels are equiped with an AIS transceiver and use them. Even the small (<200') inter-island frieghters in the Azores broadcast AIS data, as do the high speed ferries. Which vessels (other than yachts) do not broadcast AIS data? Virtually all of the fishing trawlers off the coast of Europe do not. Nor do military vessels, as previously mentioned. Of course Europe has a highly standardized and integrated shipping industry, so you might expect AIS to uniformly implemented. I'm sure in the Caribbean and South Pacific the implementation is far from uniform.
Of course even in Europe the information is only as good as the instrumentation on the vessel and the attentiveness of the deck watch. Witness the cruise ship we had in sight for several hours on the way to Gibraltar that, according to its AIS broadcast, was travelling at 100 kts.
I guess the value of AIS, for me, is not that it is on every vessel out there, but that it is on so many. They also tend to be the biggest and fastest vessels out there (with the exception of the military).
The other thing about having an AIS receiver and plotter onboard is that it is fun and entertaining (maybe I'm a little too easily entertained).

John
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Old 26-05-2006, 00:07   #37
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Paul, just happened accross your analogy. It hit home. My wife's great aunt lived for years with 25 watt bulbs in her house to save money. She left a very large inheritence. I firmly believe that if you have the technology available to you, use it. There are the shopping channel goodies that are a waste of money, and there are the redundant systems that make life just that much easier. Why live with 25 watt bulbs when you can have the comfort of a 60 watt bulb. This technology is not overly expensive, and is yet another system that will make navigation easier. Even now GPS has it's naysayers, but I would not cruise without it. When my 3 GPS's fail, and I just can't get a sight with my cheapo sextant, it would be nice to have another option.
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Old 26-05-2006, 00:11   #38
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GPS. Paper Charts. And a sextant.

That's all I'm going to have. I don't need to keep up with the Jones', with all the fuddy duddy electronic gadgets. I plan to keep it simple.
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Old 26-05-2006, 04:13   #39
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Speed is sometimes an issue in safety and that is the crux of the AIS arguments if I'm reading it correctly. Being a singlehander I plan on using a chart plotter to save time in fixing a position. I may need to fix a position and keep a hand on the helm and can't go looking for nav tools. Again traditional navigation methods inform current methods and also entertain on passages (how much music can you listen to in one day?) It seems to me that if AIS can give you and other vessels information that really helps avoid collision it may be worth the effort. Whether it is ready for prime time is a decision each skipper has to make. I just don't want to find myself off some unfamiliar headland with no power wondering exactly where I am.
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Old 26-05-2006, 15:22   #40
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Pure Vida,
I agree with your assessment of electronic navigation. As a fellow single-hander (much of the time) I value the ability to use electronic aids to make accurate decisions when time is of the essence and my judgement is often impaired due to lack of sleep.
The way AIS might enhance those decisions is not just in allowing me to plot a collision avoidance strategy, but in accurately and quickly identifying the situation. When I am on a converging course with a another vessel, especially one much larger than mine, I try to raise them on the VHF radio to determine if they are aware of my presence and if they have any concerns about our mutual courses.
Without AIS this contact might sound like the following, "Freighter at the approximate position xxx N. Lat, yyy W. Long this is the sailing vessel Sarah, Wxxxx, on a bearing of 185 degrees from your position."
With AIS the contact could sound like this, "Innnn, Innnn, Cargo Ship Ocean Quest, this is the sailing vessel Sarah, Wxxxx, 3.5 nm on a bearing of 185 degrees from your current position."
Now which contact request is more likely to get a response, one that vaguely identifies a position or one that includes the call sign and name of the vessel? And if you get a response, which situation would allow you to determine that the vessel calling back is actually the one converging on your position rather than the only vessel in your area whose deck watch is awake and wants to talk ot someone?
So I think that AIS can offer assistance in collision avoidance should be a given. Whether that assistance is worth the entrance cost (<$300) is arguable. If you don't cruise in waters frequented by commercial shipping it certainly would not be. If you went to the website URL I offered at the start of this thread you will find that I really didn't need AIS to comfortably transit one of the most congested shipping lanes in the world, but then it was in daylight, in good weather and I didn't have to cross the shipping lanes. If any one of those situations had changed then my serious use of AIS would likely have increased dramatically.
My point is skepticism of AIS as navigation aid is unwarranted. A given cruising agenda may make it more or less valuable. A preference for traditional navigation doesn't change the value of AIS or electronic navigation, but rather has to do with individual risk management at sea.
I think the common point most of us share is that electronic navigation does not increase the risk of a cruise, the lack of navigational skills certainly does. So the simple is best cruisers are not lowering their risks, just their costs (if that).

John Stevenson

Sorry if this ended up sounding like a rant, it certainly wasn't directed at your post.
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Old 28-05-2006, 04:28   #41
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Quote:
Originally Posted by jstevens
So I think that AIS can offer assistance in collision avoidance should be a given. Whether that assistance is worth the entrance cost (<$300)....
Of course, the entrance cost for taking advantage of AIS capabilities is much greater than $300 if you are starting from "scratch," without already having electronic plotting capability .... (that's the position I'm in at the moment).

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Old 28-05-2006, 05:07   #42
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Tim,
You certainly are correct. In my somewhat impassioned post (I have to remember to not open that second bottle of wine if I am posting on line in the evening) I was refering to someone upgrading an existing electronic navigation system to add AIS. Since AIS is certainly not an essential part of such a system if I were just starting with electronic navigation I might not make support for AIS support an important criteria in my choice. If the chart plotting system is actively supported by the publisher it will have AIS in some form or another by the time you might consider adding it to your system. (e.g., Raymarine made it a free upgrade to the C-series plotters).

One thing that mystifies me is why the VHF radio manufacturers haven't made AIS an option with their equipment. It would seem to me to be a cheap way to churn their base and get some people to replace perfectly good VHF radios with an AIS capable one. It would also eliminate the need for a second antenna or splitter. There must be some legal or technical issue of which I am not aware.

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Old 28-05-2006, 11:22   #43
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John - I wouldn't worry until you get to the third bottle - but, of course, by then, you really wouldn't care one way or the other.

An earlier post sort of hinted at spoofing information on AIS. Another post dismissed this. We use to feel this way about the internet - but we know how that has gone.

Why would someone want to spoof their information on AIS? The same reason - to hide their identity. I think that we all know that there are a myriad of reasons why some vessel would like to hide their identity or charactoristics. I don't think it is beyond the realm of reason to presume that those involved in illegal activities would use AIS as a means of avoiding immediate recognition as either a threat or suspect vessel.

If I had a powerful cigarette boat, but my AIS said I was a sailboat and only going 5 knots and heading XXX, it might be enough of a hoax to allow them to avoid being targeted by law enforcement/Navy/CG. --- Obviously, if the agency is not being totally dependent on the AIS information (and have some common sense - Oxymoron?), they will figure it out eventually - but, that may be the difference between being caught or not.

::Tossing 3 Aus cents on the table::: (they aren't any good up here anyway)
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Old 28-05-2006, 18:27   #44
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Originally Posted by Pblais
COOT, All technology is imperfect. So are all people. It's the curse of living in a modern world. Just because you know the light bulb will eventually burn out does not mean you need to live in the dark to avoid the relaty of it's imperfection.

Warships don't broadcast AIS. The idea that commercial traffic is somehow broadcasting deliberatly false AIS seems silly. People lie on a VHF radio how is that any different?
I was commenting on a comment about homeland security. How can you improve "homeland security" by requiring all cruising boats to carry AIS?

During an attack, the security forces must rely on their own observations, not on a data stream that may be disabled or stuffed with lies by the enemy.

An honest person will install the AIS as best they can, and get hassled by the authorities if the AIS transmitter is not working properly. (Or will they? If the security forces can tell the boat is there without the AIS transmitter, then they didn't really need the AIS data, did they?)

A dishonest person will use the AIS to lie about their intentions, so that they look like an innocent bystander. Or they will transmit data intended to cause confusion among the security forces.

In that respect, there is no difference between a lie told by voice and a lie told by AIS. You wouldn't think you could improve security by passing a law that says "All terrorists must announce their presence on channel 16 when entering the harbor". How is requiring AIS any different?
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Old 28-05-2006, 18:50   #45
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Does anyone know how to decode an AIS VDM message? For example:
!AIVDM,1,1,,A,13uTAH002nJRLAHEwTi674rh04:8,0*2B
I have actually been able to google enough information to decode the AIS information in the fifth field (13uTAH002nJRLAHEwTi674rh04:8) but haven't been able to figure out the first three fields (1,1,,) nor the sixth field, which is usually zero but not always. I'm pretty sure the fourth field is the AIS Mobile Equipment Class, 'A' or 'B', of the transmitter .
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