MARPA ?

Re: Dependency...

[ QUOTE ]
Maybe I'm missing something here, so what is the point you're trying to make?

[/ QUOTE ]

Well some posts place doubt on AIS accuracy by eluding to the possibility that AIS transmissions may not contain correct data. The point I am making is that AIS collsion data is as accurate as the GPS source that provides the basic SOG and COG of the vessel but does not take current into account. Whereas MARPA data isn't necessarily that accurate either, substantially due to the rapid changes in velocity than can be exerienced by small craft in a seaway.
 
Re: Raymarine\'s view....

Hi boatmike. You and me alike. So many collisions have been assisted by lack of knowledge of how to use radar, ground based radars and mi-interpretation of MARPA / ARPA.

How about this accident report from the MAIB for a start. And it's just one of many examples of not knowing how to use radar when the pressure's on, and relying on MARPA especially when it's ground based!

An extract from the report on the collision between the Blue Sky and the Platina Reefer in the English Channel in 1995 states, "Mariners are cautioned that inputs providing speed over the ground are not to be used for collision avoidance decisions when using (M)ARPA since doing so may lead to a dangerous navigation situations and/or erroneous collision avoidance solutions.”

Colregs are quite clear. If radar is fitted, you must know how to use it. Rule 6 (b), 7 (b) (c), 8 (b) (d).

'Nuff said.
 
Not necessarily....

Not necessarily. I was just pointing out that Raymarine's view was different from your assertion that "MARPA does need input from compass and either log (best) or GPS". Some standalone radars won't necessarily have log or GPS feeds, and so their MARPA results will only give relative vectors, CPA and TCPA. Certainly not ideal, but many users might find the information much more useful than a chocolate teapot! At the end of the day, MARPA for leisure users is only an aid to navigation.
 
Re: Raymarine\'s view....

[ QUOTE ]
Colregs are quite clear. If radar is fitted, you must know how to use it. Rule 6 (b), 7 (b) (c), 8 (b) (d).

'Nuff said.

[/ QUOTE ]

Agree almost wholly with Piers. But and however, Rule 7b further requires 'Proper use shall be made of radar equipment if fitted and operational, including long-range scanning to obtain early warning of risk of collision and radar plotting or equivalent systematic observation of detected objects.' ( my emphasis )

That actually means that not only do we have to have the kit switched on all the time, at sea, but also that a proper radar systematic watch must be kept. How many on here not only don't do that 'systematically' but don't know how?

/forums/images/graemlins/shocked.gif

I recall giving 'Deep Thought' to these issues not many months ago, while on passage W-E along the Normandy Coast, and seeking to pass Dunkerque - specifically, to cross the buoyed approach channel 'Passe de l'Ouest' through the near-continuous two-way stream of ferries overtaking, changing speed and course every few minutes.

We had a good Raymarine radar/plotter/AIS, and excellent visibility, so it was a good opportunity to compare the info from each source ( 'Rule 7a - Every vessel shall use all available means....). We noted that the AIS info from several vessels, clearly visible to the eye and radar, was intermittent and did not accurately reflect some vessels' observed manoeuvres and speed changes. Further, several Very Large merchant vessels - clearly visible to the eye and radar following the NE-bound TSS some 5-6 miles away - did not seem to transmit any AIS data, although others close to them did.

Both of us were well-trained and experienced in the use of airborne radar, and it was commented that the civil aviation rules for the avoidance of collision seem simpler and contain significantly fewer 'ambiguities' than their marine equivalent.


/forums/images/graemlins/wink.gif
 
[ QUOTE ]
I am going to fit a new chart plotter this winter ( probably the new Garmin 4008 ) but wait till next lay up to buy the radar dome ( to spread the cost). am I right in thinking get the AIS with the chart plotter now and get the MARPA next year with the radar.

[/ QUOTE ]Those decisions depend on the conditions you are expecting. If you meet fog regularly and are accustomed to sailing in all weathers then radar is a must - it is due diligence. A GPS is due diligence but a chart plotter is a 'luxury'. MARPA works with all traffic (and solid objects! /forums/images/graemlins/wink.gif) whereas AIS only works with traffic fitted with it, switched on, and WORKING.

I don't have the rate gyro on my radar so the MARPA isn't as good as it could be but even then it is wonderful. A real treat. It becomes increasingly 'accurate' as you get closer.

I am guessing that you want to safely and regularly sail in fog or you wouldn't care much about any of this and, if you need to spread the budget, get the radar with MARPA first then the chartplotter with AIS if you fancy AIS but I wouldn't spend a lot on that function as big vessels usually paint well on the radar, and are easy to see or hear.
 
Re: Raymarine\'s view....

I have my radar on at night regardless of the weather conditions in the waters we sail in (W Europe) and most of the time during the day when passage-making with reasonable way on. Aside from the Colreg requirement, if the vessel is fitted with one and if the power is not a big issue, then not to turn the radar on is lack of due care.
 
Re: Not necessarily....

The full extract from your own Raymarine literature is as follows:

EffectiveMARPAoperation is dependent on accurate own ship’s heading,
plus SOG andCOG. The better the quality of the heading data, the better
MARPA will perform. (MARPA functions without SOG andCOGdata but
only relative vector, CPAand TCPAare shown; target course and speed
cannot be calculated).

Therefore to operate effectively Raymarine's MARPA needs compass and GPS inputs. It does NOT function effectively without and this paragraph DOES NOT conflict with anything either I or Piers have said. MARPA without Heading, SOG and COG will not give you anything you can't get more directly from the screen. Furthermore the fact that they want SOG and COG appears to indicate that even in full working mode it is ground stabilised and does not accept input from the log giving speed through the water. It therefore does not operate in the MCA/MIAB preferred mode even when working properly. All of which reinforces my view that ARPA itself should be used with caution and MARPA even more so.
 
Re: Not necessarily....

I think you need to have this argument with Raymarine - maybe they'll issue amendments to their literature. (But probably best to leave out your "chocolate teapot" comment!) /forums/images/graemlins/wink.gif
 
Re: Not necessarily....

I am not dissagreeing with Raymarine. Their literature is quite clear I feel it is your perception of the obvious that is at fault.
 
Re: Not necessarily....

Hi pvb. When I was researching the radar article for MBM, I discussed the water based issue with each of the main manufacturers in turn. The problems are manyfold.

First, an accurate heading sensor which is affordable to the leisure industry. The sensor needs to update rapidly, possibly at over 40Hz. (One well know manufacturer which also makes top end commercial radars said it was unlikely leisure boaters would spend £10k on such a gyro compass). You need such a compass the moment the sea starts to move and the boat starts to yaw. You cannot have the heading info lagging behind the scanner info. But running hand in glove with this you need really good electronics to be able to process the calculations fast enough which in turn means better components, which in turn adds costs. That's why manufacturers stay (in the main) with GPS COG which doesn't really care when the boat starts yawing. No kit needed, just a GPS COG signal.

Second, we come to processing info to determine which way a target is facing - its aspect. For this we need boat speed through the water. How accurate is the boat's log? Do you regularly keep it clean? Does it ever fail? And then comes the problem of where do you measure speed? The sea often runs faster at the surface in a wind, than 3 feet down. So are doppler logs better? One manufacturer is working on one of these to combat just this problem. That's why SOG is easier to use. No kit needed, just a GPS SOG signal.

In discussion with Raymarine, at least they were one step ahead of most by offering water based info on the radar screen on the set we tested on Calm Voyager, but crazily, the MARPA display list was ground based when showing CPA, TCPA etc, and therefore inaccurate. By the way, the late Robert Avis, the radar guru, was with us when we did this test.

To correct this would require not only the above kit, but a re-write of the algorythm which processes the data. The more open manufacturers stated quite openly that this would be too expensive for them to write given the calibre of electronics they had to use (cost-wise) in leisure radars.

Finally, on my boat, I had an expensive Simrad set installed, only to find out it was ground based. I have had the MARPA info display a target will pass behind me when in fact it passes in front. I only use MARPA as a rough guide, and I mean rough. My next radar will offer both water and ground based info! But I can't afford it, yet....
 
Re: Not necessarily....

It boils down to one factor. Cost. Making a heading sensor that can cope with a leisure boat yawing in anything but a calm takes a refresh rate of more than 40Hz according to one manufacturer which makes water based radars - expensive ones. Then adding sufficiently powerful electronics to cope with the enormous data processing that has to go on for MARPA (ever noticed how long MARPA takes to settle down after target aquisition and when changing heading?). And finally, creating a sufficiently capable program and algorythm to work within these constraints. All the while in a very price sensitive market.

And if you want a target's aspect, you need an accurate boat speed through the water sensor, and even this causes problems.

Far easier to use a GPS signal and hope no one notices....until the Wahkuna accident (see above) and others.

Meanwhile, have a go at this power point for ground versus sea stabilised.
 
Re: Not necessarily....

That's an excellent illustration of EXACTLY what the problem is. I confess to owning a Raymarine C70 colour set myself which is an excellent piece of kit for a small boat. I usually have it in "wake" mode when crossing the channel which gives me an excellent indication of anything I might bump into. Using EBL in conjunction with this gives me a fairly accurate assessment of danger. I believe that this, combined with a healthy paranoia that increases as the vis worsens is the best you can do to keep your vessel safe. I have tried it in MARPA mode but was not impressed by the accuracy at all. I had meant to look up how to interface my log with it this coming season to improve matters but it now seems I can't. The opinion I am left with is that large ship ARPA is a good early warning system that should alert the watchkeeper to track the other vessel directly by EBL etc when within close range, but even then it only works properly when sea stabilised, i.e. recieving and using course and speed through the water from the ships log which needs to be properly calibrated. Mini ARPA (MARPA) is a cheap and cheerful gimmick that should be used with caution on a small vessel. It can't be used in sea stabilised mode and can therefore lead to gross missjudgements of close quarters situations for exactly the reasons illustrated by your powerpoint presentation.
For those who think I am arguing unnecessarily, I think it is important that it's limitations are widely publicised and the MCA, MIAB, and people like your captain who posted the powerpoint presentation are listened to. It is also important to say that none of this conflicts with anything in the Raymarine literature.
 
Re: Not necessarily....

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I have tried it in MARPA mode but was not impressed by the accuracy at all.

[/ QUOTE ]

We added a Rate Gyro to our Raymarine GPS-SOG setup and it dramatically changed the performance of MARPA. The heading is now updated at 10Hz via NMEA where as the fluxgate compass via the Seatalk bus is only working at 2 Hz. In fact I think Raymarine say you MUST have a rate gyro for MARPA to work efficiently.

As a bonus the rate gyro also greatly improves the performance of the autopilot, so well worth the extra cost.
 
Re: Not necessarily....

[ QUOTE ]
Finally, on my boat, I had an expensive Simrad set installed, only to find out it was ground based. I have had the MARPA info display a target will pass behind me when in fact it passes in front.

[/ QUOTE ]

That makes no sense. The MARPA determines relative vector sans Course or Speed information - it takes successive ranges and bearings and connects the dots to give the relative leader. When you input your own course and speed information, it can then deduce the target's vector (course and speed). The only drawback to using ground-based 'ownship' data, is that the target's vector will also be ground-based, so it's visible heading and speed through the water will not match what the MARPA says. Regardless, it has no effect on the relative vector, so CPA and TCPA should be accurate. In your case, I suspect that you may have switched from 'Course up' to 'North up' or vice versa, or either you or the target vessel changed course and/or speed.
Honestly, I don't see the problem with using ground-based inputs - as with any equipment one chooses to use, one must be aware of the inherent limitations and account for same.

Kevin
 
Re: Not necessarily....

Extract from my original post

It is therefore only as accurate as your compass is which can be a problem with cheap fluxgate compasses. If you have a good compass and its on board deviation has been properly corrected it's good. A proper gyro-compass is better. Unless you have a very accurate compass treat MARPA with caution and use it as a warning of possible collision.

So I obviously agree with you but it is still ground stabilised which is a big problem (see piers previous post and the excellent powerpoint presentation for reasons why)
 
Re: Not necessarily....

PLEASE PLEASE PLEASE read the powerpoint presentation imbedded in Piers previous post!!!
Also read the two MIAB reports referred to where one of the main reasons for the accidents was clearly identified as the vessel using ground based rather than water based (or stabilised) ARPA. You obviously have not absorbed any of this information before posting and are in danger of misleading others.

Let me try to explain where you are wrong.
You say
The only drawback to using ground-based 'ownship' data, is that the target's vector will also be ground-based.
Thats wrong
You have no way of getting a ground based track of the other vessel unless as in AIS BOTH vessels GPS position is known and constantly updated. That's the advantage of AIS in fact.
Your Radar can only give you the RELATIVE track of the other vessel, and as both you and he are affected by wind and tide thats RELATIVE to YOUR track THROUGH THE WATER! which your Radar doesn't know because it doesn't have any direct input to tell it.

Honestly cruiser2b I know this is difficult to get your head around but it IS important. Go back and study the PowerPoint thing which explains it far better than I could with diagrams to show the danger of the wrong mindset.
 
Re: Not necessarily....

I always understood that it must be sea stabilised but find the Raymarine E series manual very hard work. How do I change inputs for the Radar?
 
Re: Not necessarily....

[ QUOTE ]
I always understood that it must be sea stabilised but find the Raymarine E series manual very hard work. How do I change inputs for the Radar?

[/ QUOTE ]

I hope Raymarine don't mind but the following is an extract from the RL70CRC manual:

"Effective MARPA operation is dependent on accurate own ship’s heading,
plus SOG and COG. The better the quality of the heading data, the better
MARPA will perform. (MARPA functions without SOG and COG data but
only relative vector, CPA and TCPA are shown; target course and speed cannot be calculated"

Having read all the posts that say MARPA is ground stabilsed, both from experience of use with the RL Series and the above extract I disagree. As long as the radar is operated in Relative Motion mode (RM) and not True Motion mode(TM), I beleive GPS generated SOG and COG inputs are used only for the calculation of target course and speed. Thus I believe the CPA, TCPA and vector are in sea stabilised mode. Radar Chart overlay mode must not be used as the radar will default to True Motion mode.
 
Brave man....

That was brave of you to raise your head above the parapet! You're quite right that basic MARPA will show relative vector, CPA and TCPA. It won't be as accurate as commercial systems, but as an aid to navigation for leisure sailors it's brilliant. And it's basically doing the same thing that old-fashioned radar plots used to do - but quicker and better.

None of the leisure radars offers sea-stabilised operation. I think there's a problem with people buying budget-priced leisure radars, expecting them to work in the same way as commercial radar, and then criticising them when they don't. Especially if they buy them without checking the specification first - comments such as "I had meant to look up how to interface my log with it this coming season to improve matters but it now seems I can't" are a real giveaway.

For most leisure sailors, affordable radar with MARPA is a valuable aid to navigation.
 
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