AIS & ARPA ...

[ParaHandy]

Several exhibitors at LIBS seemed to be of the opinion that generating ARPA data, ie closest point of approach etc, from AIS information would confuse the user ...

I think their argument was that an essentially simple system (such as NASA's) would not lend itself to such complications - it being designed for small boats.

However, calculating the vectors of ships heading towards you is relatively simple trigonometry which can easily be calculated and the information displayed in as simple or sophisticated manner as desired. So I can not see any reason why such should not be as accurate and, crucially, perhaps more reliable than a radar system?

This isn't a whinge at NASA, just peeved and wondering why not ...

 

[Talbot]

Cynic mode:

expect they are tryng to sell as many simple AIS as posssible, and then bring out the new system with ARPA style data and get us all to through away the simple AIS and buy the new gizmo. /forums/images/graemlins/shocked.gif

 

[tome]

There's a bit more number crunching involved than simple trig as it involves converting geographic coords into rectangular for the display and then CPA calcs for all targets within range. Bear in mind that most leisure radar ARPAs are limited to 10 targets.

There's also the need for a graphics screen, similar to radar. This is expensive and there's the 'real-estate' problem of competing for space at the nav table/instrument pod.

Probably the best solution would be a feed into an extra screen page on something like the Raymarine C or E series displays?

 

[jamesjermain]

I am not entirely clear in my own mind what all the issues are with AIS. But three of them are these:

Very few yachts and not all ships have (or at least use) AIS. There could be many vesels out there on a collision course with you which you would still know nothing about and you could be lulled into a false sense of security.

It seems to me, but I can't be certain, that in crowded areas there is a danger of AIS overload which could make the real situation unclear.

Current yachtie AIS sets are not entirely compatible with all plotters and PC software.

I would be interested to read other peoples views on this subject

 

[Gunfleet]

Just had a failed graphics card on my laptop. It manifest itself as screen freezing randomly - a day, a week, a month or an hour between freezes, it was unpredictable. When I got it going again none of Maptech products would work. I think we're a long way from allowing ourselves to depend in any significant way on pc based products on a small boat. So any AIS or Arpa displayed on a computer would have to be non critical... which makes you wonder why you've got it on your small boat! I use the Maptech stuff and Neptune for passage planning, by the way. I'd never want to rely on the PC running all the time.

 

[ParaHandy]

I thought the mathematics to convert lat-long onto a rectangular grid fairly straightforward - fundamentally conversion of radial data to a linear position and if absolute precision was not required (eg unwise to calculate with a greater accuracy than GPS can give), a reasonable longtitude conversion approximation could be derived? All of which would give the X-Y displacement from your current position.

AIS transmits heading and speed so the resultant vector is given which leaves you to interesect your vector with the target's.

Screen depends on how sophisticated you want to go. Would have thought a normal lap-top screen would suffice?

mini-arpa can only run 10 at a time because, I assume, the algorithm might be the same as big-arpa which requires a high percentage of hits for so many revolutions of the antennae which must limit the number of targets which can be tracked when the vector computation is included .. none of these limitations would apply to AIS?

 

[ParaHandy]

I suppose you could argue (if you survived) that the vessel which clattered into you without a working AIS was liable ....

I am not clear whether all vessels over 150 tonnes (or some other figure) are required to have AIS and I am assuming that in reduced visibility common sense would dictate that they are switched on. A yacht at 5 knots is a very different proposition to a 100,000 ton ship at 25 knots.

If the update frequency that the AIS standard seems to imply is possible then, mid-channel, one would think that enough information should be available to pinpoint targets ...

Ignoring dedicated chartplotters, the gist of my argument is that getting such information onto a pc isn't (or, to me, doesn't appear to be) too difficult ...

 

[Talbot]

Dont really need the maths to convert the lat/long into a grid. Just a look up table using the information in Norries tables (Traverse Table) - you are familiar with these tables of course! /forums/images/graemlins/laugh.gif

 

[npf1]

Norries tables? I'm not familiiar with these. Can you please enlighten me?

Thanks

 

[jonjo]

[ QUOTE ]
Several exhibitors at LIBS seemed to be of the opinion that generating ARPA data, ie closest point of approach etc, from AIS information would confuse the user ...

[/ QUOTE ]
Maybe R&D experiments of overlaying AIS and MARPA CPA's highlight how inaccurate MARPA has always been.

 

[Robin]

[ QUOTE ]
A yacht at 5 knots is a very different proposition to a 100,000 ton ship at 25 knots.

[/ QUOTE ]

How about a Squadron 55 at 30kts? /forums/images/graemlins/shocked.gif

 

[david]

[ QUOTE ]
Very few yachts and not all ships have (or at least use) AIS. There could be many vesels out there on a collision course with you which you would still know nothing about.
Current yachtie AIS sets are not entirely compatible with all plotters and PC software.

[/ QUOTE ]

Interestingly in your first sentence you quoted exactly the same word quoted to me by a cargo ship captain you even emphisised the "or at least use" tha same as he. Surely to conform to law shipping should/must use the equipment or why bother having it in the first place.

My Seapro 2000 has the facility for AIS but to buy a reciever is £600 which ain't on. My dislike is some companys are advertising AIS as AIS radar, no way is it radar or even close.

 

[ParaHandy]

Mr Norie? Capt Hopkins & no, I'd not thought of him! I wonder if the gps programmes which are freeware use these tables (as a look-up table)? They're mighty quick converting lat/lon to a grid ....

ta!!

 

[ParaHandy]

you sat on it?!

 

[ParaHandy]

I did tot up how many pieces of kit (in a mini-arpa installation) had to be set-up just so otherwise significant errors would occur which I think is its weakness. But then, I am rather cynical of the best efforts of the marine industry hereabouts having compared my ST60 wind reading with half dozen others, all professionally installed, alongside me one day - they were +/-1.5 F (ie 7-8 kn) different.

AIS is very different in that respect .... just plug it in ....

 

[Robin]

[ QUOTE ]
Interestingly in your first sentence you quoted exactly the same word quoted to me by a cargo ship captain you even emphisised the "or at least use" tha same as he. Surely to conform to law shipping should/must use the equipment or why bother having it in the first place.

[/ QUOTE ]

There are circumstances, other than just failure of the AIS transmitter, where the ship using it may be allowed to have it switched off. I believe that applies (at least) to Navy vessels, presumably Customs & Excise vessels and those who wish to remain anonymous for reasons of their cargo such as a radioactive material carrier being sought by the likes of Greenpeace and those that might be subject to terrorist attack. I can imagine that fishing vessels might also not want their position broadcast to competitors either.

It is my understanding too that pleasure vessels will be discouraged or forbidden to have transmitters also, because the whole receiving screens would just become completely clogged, information overload if that were to happen.

So as long as you don't mind being hit by any of the above or a 299ton vessel and below, by any kind of leisure vessel from a sailboard to a Superyacht, or run into any land or rocks you should be OK!

 

[Superstrath]

Dep = dlong x cos lat

Calculators, once again, make Traverse Tables obsolete. This part of your calculation is a doddle for the computer, so no need for look up tables. Still keep a copy of Norie's somewhere though....


Alistair

 

[tome]

Agree that this simple formula or Norries would do the trick here, I'm used to survey accuracy where the algorithms are considerably more complex and have to take into account the spheroid, projection and datum shift.

 

[AlexL]

for close quarters stuff - like within a few miles collision avoidance - could you ignore the spheriod and just assume the AIS provided lat / long and heading, along with your own GPS info were all on a flat plane? that would consideralby simplify the trig.

 

[tome]

Yes, that would be adequate (provided you converted to rectangular coords, ie Dep = dlong x cos lat as previously stated) as long as you kept to reasonable CPA minima, eg more than 1 mile.