marpa

[chrisb]

I am in looking to fit radar to my yacht. although more expensive I like the raymarine version-connects to existing plotter/instruments/autopilot and has more pixels.The factors which influences me most is the MARPA collision avoidance system.Can i have comments from other users as to usefullness of marpa-or is it in practice too complicated and of little use

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[snowleopard]

i have used it on a big ship and it's magic. to be able to put the cursor on a target and know its closest point of approach is extremely useful, especially when crossing the channel.

big downside is the cost of the system to support it. if you can afford it, don't hesitate.

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[davidwf]

Got it on my Raymarine Radar its fantastic but in order for it to work reliably you have to have a Gyro Stabilised compass at around £450 for the gyro bit which you connect to the standard raymarine fluxgate compass. I have just a simple fluxgate compass and it works fine in light seas not sure how good its going to perform in rougher conditions when the boats heading is fluctuating.

Having said the above I think its absolutely fantastic and I will fit a gyro stabiliser if I see any problems in rougher seas as I don't want to be without it.

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[Dave99]

Great!

-Speed, course, estimated time of collision, all on the click of a cursor

-may also be worth considering a unit which can be used for chartplotting aswell. The GPS is all contained in the aerial so by buying a colour screen with radar, you could add in GPS for plotting at a later date I beleive.

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[Fill]

I've got this with the gyro stabiliser (allows for yaw) and it is very reassuring. I've never had to use the MARPA bit for real but have practiced when we get to the shipping tracks on cross channel trips.

You need to set up the system properly (there are alarms which you can set if something gets within a specific distance and unless you calm down you can get driven mad - CPA of 5m is a little too far but I was trying it out) and then it is fine.

My own view is that it gives a very clear indication of which ones are not a problem and you then use the system to closely monitor those that could be coming a bit close. The screen showing the predicted vector lines of other shipping is spot on. I'd go for it.

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[Miles]

Spot on. Even with the gyro stabiliser, I find that the CPA can fluctuate quite a bit. We tend to use it most when crossing shipping lanes and to correlate what one person on deck is seeing and what another is getting from the radar. So far we haven't had to use it in fog but I'd be most reluctant to do a channel crossing without it.


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[ParaHandy]

i read the MCA's AIS project (lurks somehere on their site) and there is a description of the mathematical algorithm used by ARPA which is that the target has to be seen 4 times in 9 revolutions of the radar antennae before ARPA will 'lock' onto a target. A large merchant ship is a relatively stable platform such that the heading of the vessel can be reasonably assured within the timespan required to revolve the antenna 9 times. That condition may not exist in a yacht where yaw and heading variation may well be significant. Demanding a gyro stabilised compass is I presume an attempt to mitigate the obvious defects of such a system when fitted to pleasure craft. And I wouldn't be surprised to find that the angle of yaw is used as well which might also mean that the height of the receiver from the roll centre has to be known.

It all sounds dodgy ....

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[qsiv]

The fact is that it can, and does, work on small boats. It works better with a faster compass.

The other consideration is that it is most useful in fog, and whilst fog can descend in rough wetaher, it is much much more prevalent in settled calm conditions, and as such both roll and yaw inputs are radically reduced.

In the fullness of time AIS may prove a better system than ARPA - but of course it isnt a mandatory fit for pleasure vessels, so you will only ever see the commercial traffic (but that is what concerns most of us in such scenarios).

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[pandroid]

We'll I've used it without, and subsequently with, the Gyro compass (actually the Smart Heading Sensor), and dont rate it much. Its a nice toy, but in practice the heading and distance vectors are just too unreliable to rely on. Its true that it'll probably get you out of real trouble, but its no substitute for a proper radar watch. We've used the radar in a shipping lane in real fog and find that the old Radar methods of EBLs are the best. On the upside, we've found that the Raymarine radars can even spot pot markers.

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[ParaHandy]

can you really say that it does work but then qualify that view by saying it works 'better' with a faster compass? There is an issue with speed in that fast (>25kn) and manoeuverable vessels are using ARPA at ranges less than 6nm but it is recognised that the rotational speed of the antennae is inadequate and requires increasing to ensure precision is maintained. If that is the prevailing wisdom for fast vessels it also applies to any other vessel that is in its way, irrespective of its speed?

I'm not saying you're wrong as you've not been in the habit of making such statements without good reason but I do think that (M)ARPA on a small yacht has some shortcomings and is not a panacea and I would further suggest that the sort of slop you get in the channel does appear at any time including periods of low visibility ........

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[tome]

The challenge with marpa is to manipulate large vectors which are own heading-dependent. This will result in significant errors if the compass isn't fast enough, so a minimum of 5Hz and preferably 10Hz is necessary.

Having used ARPA on large ships, it's a great aid but users should be aware of it's limitations. IMO standards on CPA accuracy are 0.7 miles accuracy in a crossing situation (3 minute trend) and 1.8 miles (1 minute trend). To achieve the higher accuracy, the system is required to maintain continuous target lock over 3 minutes which is quite a technical challenge on small boat radars.

In this context, a CPA of less than 2 miles might be interpreted as being reckless.



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