AIS crossing Shipping Lanes

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paulburn

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Consider....

AIS says CPA is 0.25 nm. You wish to cross the shipping lane with a bigger margin of safety, say 0.5 nm. So you next need to know if you're going to pass in front or behind the ship, to know whether to increase or decrease your speed (which may be easier than changing course).

I think to do this, AIS/chartplotter needs to indicate whether you or the ship will arrive first at the point at which the respective projected courses cross. But to my knowledge, my Garmin does not display this information.

Have I got this right ? Would this extra bit of information make crossing shipping lanes a whole lot easier ? Or is it already available and I just haven't found it ?

Grateful for advice.
 
Some AIS displays have enough processing power to do this and some don't.

My C80 display will not do it whereas my Vespermarine display on the tablet will. I can also show my AIS data on OpenCPN on both PC and Android tablet but I can't remember whether OpenCPN is clever enough to do it. I suspect it is, or soon will be if it isn't now.

I should have said that you are absolutely right Paul ..... it is a really signifcant feature of AIS and is why I keep a general lookout on the C80 but as soon as I can see an issue developing I grab the tablet.

Richard
 
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RichardS said:
...I can also show my AIS data on OpenCPN on both PC and Android tablet but I can't remember whether OpenCPN is clever enough to do it....
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One of the great strengths of OpenCPN is its handling of AIS messages, it shows very clearly whether the CPA is ahead or astern of a target. It helps if you set the COG predictor line length for both own boat and AIS target to the same setting, I use 30 mins, as the default sets them differently. Although AIS handling by OpenCPN is now pretty much stable its still much easier to upgrade that software than it may be for an 'older' plotter.

Answering the OPs question - I would say that the ability to easily visualise whether a CPA is ahead or astern is an enormous step in the usability of AIS data
 
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ShinyShoe said:
If the track lines cross are you not in front? If they don't you must be behind?
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It just shows that at some point in a given time you and the target will be, say, 0.1 miles apart. If your're going to be 0.1 miles behind you could just slow down or change course slightly to increase that 0.1 mile to something bigger. However, if that 0.1 miles is actually going to be ahead and you make the same correction, you just make things worse. :(

You need a cleverer algorithm in the display processor to tell you exactly where you will both be at that moment of closest approach.

Richard
 
The Raymarine system displays 'Dangerous Targets' as a box (circle if it can't discern accurately). A red box is depicted at the relevant point along the two vectors. It is easy to use and very useful IMHO. A user defined Guard Zone can also be set:

http://www.raymarine.co.uk/view/?id=13273
 
My Furuno plotter has COG/SOG predictor that is easy to use.
Its a vector from the bow where the end is estimated position in x minutes, x is configurable.
It show this vector for own boat and AIS targets.
 
Furuno - does that mean you need to note TCPA and then manually input x to equal TCPA and look at the end position of the vector lines ? Rather than being able to press a button to automatically display boat positions at TCPA ?

Raymarine - I'm not sure if you're implying that Raymarine has the necessary functionality to easily show positions at TCPA or not ?
 
My Raymarine e7 shows it clearly if you ask it to display vectors, when a line ahead of the other vessel appears indicating its direction. It is important to realise that this vector is in relation to your vessel and not the chart datum. You can vary the vector length, but it soon becomes very instinctive.
 
johnalison said:
My Raymarine e7 shows it clearly if you ask it to display vectors, when a line ahead of the other vessel appears indicating its direction. It is important to realise that this vector is in relation to your vessel and not the chart datum. You can vary the vector length, but it soon becomes very instinctive.
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This is a user hack that you invented to overcome a Raymarine product deficiency.

Bearing at time of CPA is the vital piece of information missing in many AIS products. I have been banging on about this for 10 years, it is interesting that a minority of people get this issue straight away, I can only assume that those who do not, sail in uncongested waters or are armchair sailors who rarely get within 5 miles of a ship.
 
RichardS said:
Some AIS displays have enough processing power to do this and some don't.
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Nothing to do with processing power, it is just substandard product design and perhaps substandard consumers.

The CPU effort to produce a bearing at time of CPA is trivial.
 
jonjo5 said:
Nothing to do with processing power, it is just substandard product design and perhaps substandard consumers.

The CPU effort to produce a bearing at time of CPA is trivial.
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Totally correct. The maths needed in the processor is trivial compared to everything else going on. I agree it is a weird but critical gap in the data.
It is correct that the latest Raymarine software helps by giving relative motoon tracks which allow to deduce this. But only on Lighthouse supported devices - the same tracks on say an E90W look similar but give entirely different results
 
I take the attitude that we don't need to rely on the very exact maths of GPS and AIS stuff and enjoy the sail, I am beginning to sound like Tom Cunliffe! If you are worried lose a tenth of a knot for a few mins and the big stuff will pass.
 
Sandy said:
I take the attitude that we don't need to rely on the very exact maths of GPS and AIS stuff and enjoy the sail, I am beginning to sound like Tom Cunliffe! If you are worried lose a tenth of a knot for a few mins and the big stuff will pass.
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Lose a bit of speed and the big thing that was intending to pass behind you will hit you.
 
Sandy said:
I take the attitude that we don't need to rely on the very exact maths of GPS and AIS stuff and enjoy the sail, I am beginning to sound like Tom Cunliffe! If you are worried lose a tenth of a knot for a few mins and the big stuff will pass.
Click to expand...

I think you have rather missed the OP's point with that response. He wants to open up the CPA. But if he is actually going to pass 0.2nm ahead of the other vessel, then slowing down by 0.1kt for a few mins is probably the worst thing he can do.
 
lw395 said:
Lose a bit of speed and the big thing that was intending to pass behind you will hit you.
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How close was the big thing to the second big thing? In my experience they don't travel bow to stern, but have at lease a mile distance between them.

We are not trying to get a GPS guided missile down a building chimney half the planet away where you do need to be that accurate.
 
Sandy said:
How close was the big thing to the second big thing? In my experience they don't travel bow to stern, but have at lease a mile distance between them.

We are not trying to get a GPS guided missile down a building chimney half the planet away where you do need to be that accurate.
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There is only one "big thing" in the OP's scenario (assuming he isn't himself in a VLCC) and that is the one he is going to miss by 0.2nm if he maintains course and speed. Or hit if he slows down, but would otherwise have passed ahead.
 
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