A bit about AIS

[tome]

Rich

The AIS channels are dedicated to the task, as is ch 70. Bear in mind that distress messages (including position and nature of distress) are sent on 70 on demand. This would not be compatible with the strict time-slot (TDMA) structure of AIS.

I can see your logic as they are both data streams, and also the logic of your update plan. We would be class B vessels, and I think they may have missed an opportunity to have a shorter class B message with less demanding transmitter switching specs.

Sadly, my reading of the specs is that there is no way you could adapt a VHF set without adding significant complexity.

For what it's worth, I fear that mandatory adoption of AIS to leisure boats may well result from security rather than safety issues in the future. I don't know if you saw the article in YM, but imagine the system overload with all those boats on the Cowes start line. I'd venture that us class B's will have a dedicated set of channels so that we could be more easily filtered, in other words the big boys could turn us off if we became a nuisance.

Food for thought!

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[Rowana]

<the big boys could turn us off if we became a nuisance.>

... and then mow us down with impunity! Does that not negate the whole idea of the system?? Or am I missing something??

I'm beginning to suffer from information overload! God knows how I would cope with all this information when in the seperation lanes in a pea-souper!

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[jonjo]

> I'd still want radar for showing that my tired brain hadn't entered the
> wrong GPS datum

Sure. I don't expect my argument to win over those who have already justified an investment in radar, hence the term "poor mans radar". Whatever price trends can be expected in radar AIS is always going to be 1/3 the cost by definition.

Personally speaking if I had to make a single handled cross channel voyage using either radar with a big screen at the chart table or an AIS system that could overlay ship vectors onto a small 4" binnacle mounted GPS plotter display with zero human input, I would choose AIS.

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[jonjo]

> Used alone without radar AIS is no more use than one of the devices
> that detects that radar is being used and gives a direction for it.

I disagree. I predict that 2 years from now it will be possible to buy a £400 box about the size of a small VHF. This box will send out NMEA sentences that will describe the position, heading, speed and turning rate of any big ship within 5 miles.

The NMEA sentences will travel by wireless bluetooth signal to a handheld GPS chart plotter. By then new GPS plotter software will be able to overlay the AIS vectors of my yacht plus the big ships and predict closest approach using 'O' level trigonometry.

The same handheld plotter will also squawk loudly the second I put in a tack that will put me under the bows of the Cowes hydrofoil that is sneaking up on me at 35knots in my peripheral vision.

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[Robin]

In the meantime I will be run down by a fast sport cruiser who also had AIS but not radar. In the ensuing panic, my handheld plotter went overboard squawking madly 'should have bought radar, should have bought radar, should have glug glug......'

OK you win I've had enough, time for bed/forums/images/icons/smile.gif


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[ParaHandy]

ta, muchly ... just trying to sound knowledgable with techno bolx ..........

i'd calculated it to be twice that speed but realise only need 4 bits for 0 to 9 .. ah, well ...

once the vessels are out of range of the shore station, wonder what tells them that this upcomming 26msec slot is their's? I'm sort of assuming there's got to be a big mama somewhere keeping them in order otherwise anarchy rules ...

thought merchant navy weren't too keen on having AIS data appearing on a radar screen. they reckoned it would clutter it up. much better on separate screen (opinion in MCA paper on subject)

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[Jools_of_Top_Cat]

With all this gear running I will have to motor sail everywhere in order to keep up slightly with power consumption. Was thinking of getting an old exercise bike and fitting it with a big dynamo, would keep us fitter on longer passages.

Right now I feel like Mattingly on the appollo simulator, the first thing I look at when adding or replacing lectroniks is the power consumption. This AIS has got to be quite a drain, especially as it would probably require dumping to a laptop in order to use it on a small craft.

When I used to sail as a kid on our first bigger boat (20ft Dauntless) we had a blister pack of 3 hand held flares and a compass. How things are changing, I had a break of around 7 years from sailing and am still shocked how different things have become.

<hr width=100% size=1>Julian

<A target="_blank" HREF=http://www.topcatsail.co.uk>Top Cat Homepage</A>

 

[qsiv]

Ship to ship messaging using AIS and VHF is in the pipeline for the next revision of the standard. This will enable bridge to bridge 'messaging', perhaps similar to text.

As AIS already transmits MMSI, voice calling is already possible.

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[qsiv]

I seem to remember some west coast US port is trialling putting AIS on bouys and similar navaids. The benefit is that it will be possible to positively ID marks that are out of position.

There is a very cute site that shows all the AIS activity around vancouver (or was it Puget?) - rather like a remote VTS radar. The ability to know that a ship has initiated a turn, almost before the heading change is detectable is remarkable.

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[tome]

There's quite a clever autonomous access scheme which isn't easy to describe in a short post. The station first waits 1 minute to determine which slots are in use and builds a frame map. From this it determines 4 or more candidate slots, then selects one at random.

When within range of a base station, it will operate in assigned slot mode. You're right about using binary data: with only 168 bits available, latitude (eg) is represented by 27 bits each representing 1/10 000 min including a sign bit for E/W.

I would imagine that data will only appear on radar targets which are being tracked by ARPA, or perhaps when the cursor hovers over the target. Otherwise it would get very cluttered. I haven't seen the MCA paper but can well understand the concerns.

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[Robin]

A non-technical question

What is the military attitude to AIS use on Navy vessels? I am thinking of the various exercise areas off the UK (and especially in the Channel) where the Navy boats movements are unpredictable. I am sure they will have AIS but will their own sets be switched to passive so that their movements remain 'secret'?

The reason I ask is that we were crossing from Dartmouth to Le Four when they decided to do some 'live missile firing' and broadcast a central position/radius that was right on our track and we were already in their 'circle'. We eventually managed to get them to answer on the VHF channel they had specified and they told us to hold our course. But for the next 2 hours, every 10-15 minutes they were broadcasting similar warnings and each time the circle centre moved a bit (like a mile perhaps) and we remained in it. During this time there were two and sometimes three warships in sight, though we never saw a puff of smoke. There were ships passing up and down channel too, quite close also at times. We were very glad when they went home for tea! Visibility was good throughout this, maybe if not they would have aborted their firing who knows but their movements were VERY unpredictable.

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[tome]

I personally don't like PC's on boats and would not rely on Bill Gates limpware for safety/nav equipment. The power drain of laptops is huge (c 40W), and I think that AIS will only be of value if a simple and low powered method of displaying the data is found. AIS transponder consumption will not be significant (around 0.25W mean power by my reckoning).

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[Oldhand]

If you can use radar accurately enough to determine a correct landfall to the sort of accuracy which could identify a datum error in your GPS/route setup then all your landfalls must be at high ground with vertical cliff shorelines and few irregularities. Or perhaps I have missed some vast advance in radar technology...

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[tome]

You in a bad mood today?/forums/images/icons/smile.gif

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Being non-flippant, we use radar for gross error checks on co-ordinates and datum shifts using offshore rigs. Sure it has limitations, but has saved our bacon on occasions. You can do the same with buoys (esp with racons) in poor viz and don't need conspic land features to show that all is well 'tween GPS and chart. When you are tired, every method of cross-checking helps when approaching a landfall.<P ID="edit"><FONT SIZE=-1>Edited by tome on 18/03/2004 18:04 (server time).</FONT></P>