AIS crossing Shipping Lanes

RichardS said:
As you seem determined to continue to your aggressive stance, despite all the posts from other forumites supporting my premise, please let me help you with your grammar.
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I don't know why you need to be so consistently grumpy and rude to others repeatedly in this thread Richard but perhaps you might want to chill a bit, or take your debate to the lounge
 
RichardS said:
Lol .... are you smoking something very strong, perchance Jonjo? :)

My re-phrasing was intended to improve your grammatical understanding ..... but that's clearly been a fail. :(

As for my "churlish dismissal" of Maxi77's wikipedia extract:

That's all understood ... but the question syc1 is asking is whether the exact position of the GPS sensor relative to the rest of the vessel's structure is taken into account by the receiving AIS software when calculating the CPA and that question is not answered by the Wikipedia extract. :)

That spliff must be getting stronger and stronger. Perhaps you should go off-line until you've recovered your composure? :encouragement:
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I seem to have been attacked by an army of emoticon smilies in this thread, so perhaps I need to log off and track down the source of the psychoactive drug causing this hallucination.
 
paulburn said:
Does a system exist whereby large ships have to have both a sensor on the bow and the stern and then the receiving AIS unit can simply calculate CPA data from the nearest sensor ? That way avoiding problems with unreliable human programming errors or heading data ?
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No doubt they're not reliant on a single sensor, but there's only room in the AIS messages to transmit one position.

The unreliable heading data is primarily in the small craft which is going to be far more affected by wind/waves/inattentive helmsmen.

Not much you can do about avoiding problems with unreliable human programmers. Their products are everywhere.
 
RichardS said:
I don't think so .... but the GPS sensor on the boat certainly knows the direction of travel of the ship (COG) as it shows that on the receiving screen. I can't imagine that with large ships, which is what we are mostly concerned about, the ships heading will be that different from the COG.

The ships length and beam must presumably be assumed by the receiver to be a large rectangle with the sensor somewhere within the rectangle. It can't be that difficult for the software to draw a theoretical line from the sensor to the receiving ship's position when at the CPA and measure where that line crosses the rectangle. All it has to do then is add that distance to the CPA measured from the sensor.

Richard
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Tides could make even a big boat go sideways, at least a bit.
The static AIS data contains length, beam, distance from bow to GPS receiver and distance off beam (+ for starboard, - for port) of GPS receiver. You knew that already?

The problem with that solution is that if the GPS receiver is at the stern and you approach at an acute angle from the rear then you will pass and be retreating from the calculated CPA as you sail right into the bow of the ship. Again a situation where you really ought to see that, just picking the maths apart.
 
Dougal Tolan said:
Tides could make even a big boat go sideways, at least a bit.
The static AIS data contains length, beam, distance from bow to GPS receiver and distance off beam (+ for starboard, - for port) of GPS receiver. You knew that already?

The problem with that solution is that if the GPS receiver is at the stern and you approach at an acute angle from the rear then you will pass and be retreating from the calculated CPA as you sail right into the bow of the ship. Again a situation where you really ought to see that, just picking the maths apart.
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Yes, as per post #90. :)

Your second point is interesting as I had banked on the CPA being a single definitive point. But you're right .... the CPA to the sensor is not the same as the CPA to the hull.

The calculation algorithm would therefore have to take your course (which it has calculated already) and project that past the position of the sensor to see if there is going to be an even closer CPA than the distance of your vessel from the sensor which would mean taking into account that sensor-to-rectangle-perimeter distance along the full length of your crossing course to determine where the "definitive" CPA is.

It would be a tricky repetitive calculation for a person (I think it's called an iterative calc?) but a computer could do it instantly, of course .... but I'm now suspecting that they probably don't bother.

Richard
 
Much simpler to just look at the size of the ship and mentally subtract 0.05nm to 0.25nm as appropriate from the indicated CPA distance.
 
Dougal Tolan said:
Tides could make even a big boat go sideways, at least a bit.
The static AIS data contains length, beam, distance from bow to GPS receiver and distance off beam (+ for starboard, - for port) of GPS receiver. You knew that already?

The problem with that solution is that if the GPS receiver is at the stern and you approach at an acute angle from the rear then you will pass and be retreating from the calculated CPA as you sail right into the bow of the ship. Again a situation where you really ought to see that, just picking the maths apart.
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How about sticking yer head out of the hatch & take a series of HB compass readings, or is that too simple
 
sailorman said:
How about sticking yer head out of the hatch & take a series of HB compass readings, or is that too simple
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But then you would take your eyes off the radar/chart/AIS/engine/nav instrument/multi-function display and who knows what disaster would befall?

I used to be able to safely negotiate fishing fleets at night - now I have AIS, the multiple minimal CPAs are terrifying. It is less stress inducing to just turn off the alarms.
 
syc1 said:
Moving back to AIS, do AIS systems make allowance for the size of the vessel and the location of the GPS on that vessel?

There is a lot of talk of CPAs of .2 or .25 nm being considered acceptable, but there are container ships of 400 metres LOA almost every day in the Solent, so there must be loads of them in the shipping lanes. 400m is more than .2 nm, so if your AIS is showing you crossing ahead at .2 nm, and his GPS is on the bridge, at the stern of the vessel, have you got a problem?
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I have noticed what I thought was strange happenings on my AIS rcvr, two contacts with the same name ish! It answers your question, the big uns have two AIS transmitters by the looks of it to take in to account their length?
Stu
 
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