Radar - MARPA feature

[kandoma]

I have to replace my old radar set. I have narrowed it down to Raymarine and Furuno. I would prefer the CRT from Furuno, but the MARPA feature from Raymarine looks too good to ignore.

Question: is the MARPA (mini automatic radar plotting aid) from Raymarine working as advertised ?....a click and you get course, speed and a visible vector on the screen of a target on collison course? What additional hardware is required to get MARPA working ?


Peter

 

[fireball]

I believe a Fluxgate compass is needed to give meaningful results - GPS updates are too slow.

 

[john_morris_uk]

The radar needs some details of the boats heading, and for the display to be stable enough for accurate MARPA calculations a Raymarine electronic gyro fitted. If you have one of the higher spec' Raymarine autopilots fitted, the radar will get all the relevant information off the high speed bus to enable the MARPA function to work. (This is assuming you have the gyro option on the autopilot.)

I have used MARPA and commend it to you. It really does do what it says it will. Better than AIS in the sense that all targets are assessed. An unexpected bonus is that its easy to identify navigation marks on the screen as they are doing zero knots!

I am also a fan of Raymarine kit, so when we upgrade our radar there will be no choice.

 

[DavidJ]

I have MARPA without the extra fluxgate compass and it does 'fix' on a target easily but unless you are going in a dead straight line then it's predictions cannot be trusted. I really wouldn't use it going slow in the fog to make any decisions. However it's good fun for a while on a long journey.

 

[Oldhand]

You need a responsive electronic heading sensor with a 10Hz NMEA output. A "rate gyro" enhanced fluxgate comapss with a 10 heading reading/second output, not necassarily from Raymarine, is definitely required. Accurate boat speed through the water is also required and in my experience with MARPA, the normal boat speed instrument output once a second is the biggest cause of MARPA calculation variation when speed is low and variable due to sea state, assuming the required heading sensor is installed. Mobo's travelling with little percentage variation in their speed probably get more stable results than yachts sailing in waves.

 

[WayneS]

We have a Raymarine C80 and fluxgate compass. The Marpa feature is the Dogs doo-dads.

As as been mentioned, you really should be on a steady heading for it to work properly.

I would recommend the system to you.

W

 

[Piers]

If you read the Radar article on MARPA that I (with Robert Avis) wrote for MBM a few months ago, you will see the problems associated with MARPA if you do not have the correct inputs to your radar.

When using MARPA for collision avoidance your radar needs boat speed through the water, and boat heading. (This is called sea-stabilised radar). Unfortunately, most leisure radars are fed with GPS speed over the ground (SOG) and course over the ground (COG). (This is called ground-stabilised radar).

You will also see that in the article that we took a shot of Calm Voyager's radar screen which displayed a sea stabilised picture. Excellent. However, when the MARPA data box was called up it showed ground-stabilised information ! Completely confusing, and when under pressure to make decisions.... (This was the Raymarine radar on MBM's Broom)

Ground-stabilised MARPA info becomes more and more inaccurate the slower you or the target goes, and / or the faster the tide is running. Hence, ground-stabilised info should not be used for collision avoidance.

The mark one eyeball is best, and we all know that radar is an aid, only. But the time you need accurate info is when it's foggy and you might need to make decisions from radar info. For this, you need accurate info, which includes the vessel's aspect.

As an aside, my expensive Simrad isn't sea-stabilised. Coming accross the Channel the other day I was tracking a vessel and the MARPA stated it would pass in front of me by half a mile. With no change in hdg or spped on either my or the vessel's part, it passed behind me....that's the ground stabilised problem.

The best advice is that if you do not have sea-stabilised MARPA, don't use it. Use an EBL or trails instead.

How to check if you have a sea-stabilised radar? On a day when there is no wind,

Step 1 - Choose somewhere where the tide is running.
Step 2 - Target a fixed object, such as a buoy.
Step 3 - Stop the boat and drift with the water.
Step 4 - Locate the buoy on the radar, and choose a range scale to fit.
Step 5 - Target MARPA, and wait.

If MARPA shows the buoy is stationary (or virtually stationary given instrumentation errors), while the radar shows an apparent speed for your vessel, the radar is ground stabilised.

If MARPA shows that the buoy appears to be moving at the speed of the tide, but in the opposite direction to the tide, then you are sea stabilised, which is the correct setting to run MARPA for collision avoidance. The MARPA data box should detail this info as well.

There have been many incdents and accidents of radar assisted collisions when using ground stabilised radars. Try reading this. There are plenty more.

Finally, when we investigated current marine radars available at the time, virtually none was sea-stabilised. The reasons were common - getting accurate speed through the water and very quick updates of boat heading is very, very expensive.

 

[simon501]

Piers,

Thanks for the link to the report - excellent and sobering reading for everyone!

Simon

 

[Piers]

I should have added that this accident was the reason the CG asked the RYA to introduce the MARPA training to their radar course.

By the way, try entering ground stabilised radar into google and see what comes up.

 

[ShipsWoofy]

I agree with what you say, but,

While I am saving up for a fluxgate compass to feed my RADAR I have had excellent results using GPS fed marpa. Results which bore out with what I can see with mark one eye.

To use only marpa as collision avoidance would be dangerous. I think marpa is a handy heads up, it will tell me or my less experienced crew if a target is coming or going.

When a target starts to look suspicious then I will begin a manual plot or EBL plot.

If you follow "The Rules" it starts getting very expensive as you appear to have to pay this years overtime in the London Raymarine office (sorry Nigel).

 

[Piers]

As long as you know first, whether your's is sea- or ground-stabilised and therefore whether your MARPA data can be trusted, and second, the limitations of MARPA and that they get worse the slower you and/or target move or the faster the tide runs, that's fine.

But the concern remains that when under pressure it is easy to be misled and make mistakes when being presented with incorrent data, even if one knows its potential errors (as in the above report of yacht Whakuna. By the way, the yacht's radar was ground stabilised which contributed to the confusion of where the container ship was heading).

I should also add that some manufacturers are working on high speed fluxgate compasses (in excess of 40Hz refresh rate) and doppler logs for more accurate boat speed through the water. Watch this space.

 

[kandoma]

thanks for the good info. As I understand it now, it works well in the open sea with minimal currents; in tidal waters, it needs attention and may be off.

Peter

 

[Piers]

The concern is that when dealing with wind and tide on both vessels, and that the effects are amplified the slower you and the other vessel travel, it doesn't take much to make enough of a difference to MARPA to make it unreliable. (Vectors and all that.)

What can certainly be said is that the faster both vessels are travelling, and the less the tide and wind, the less <u>inaccurate</u> MARPA becomes when ground stabilised.

In all this, I suggest you first check to see which your radar is - ground or sea stabilised (method is above), and then play with targets in known conditions. And try it when off Cap de la Hague or in the Alderney race at Springs!

But, knowing your radar and its limitations is what counts. Have fun! /forums/images/graemlins/laugh.gif

 

[chrisb]

have been using the system for 3 years. initially without gyro compass but soon fitted one. excellent system. takes away alot of the stress especially when short handed or with inexperienced crew. fully reccommend.gyro compass essential

 

[kcrane]

Crossed Channel south to north on Saturday, flat calm, poor visibility and lots of traffic.

We used MARPA on our Raymarine equipment and cross checked it with what AIS on a PC plotter was telling us, at the same time as keeping as good a look out as we could in that vis.

There was a point when we had 11 targets, west-east, east-west, north-south and south-north, within 10 miles (I took a photo of the screen) and we were a bit overloaded with data, but, on balance, I'd rather have the data than not.

Crew preferred MARPA to AIS, not sure why. I suspect they felt "in control" of the MARPA.

We don't use MARPA or AIS to allow us to miss a boat by 250yds, we use it to make sure we miss by a country mile (much bigger than a nautical mile), which is a conservative approach, but it's what we feel comfortable with.

On whole a vote for MARPA.

 

[ShipsWoofy]

I meant to say close quarters.

 

[kengill]

I have Marpa on my RL70 system and NO COMPASS. It takes a couple of minutes to sort itself out from the GPS when given a new target but if a target was going to come from 12 NM away to collision inside two minutes then you would need Seawolf backed up with a Phalanx system anyway. and if you had those you'd be captaining a Type 23 frigate.
When I get round to fitting a compass It will doubtless improve things but at 10 or so scans per minute who really needs it.

 

[Piers]

Not quite sure what you are trying to say. If you use GPS for heading info (COG), I suspect your radar also uses SOG instead of boat speed through the water. If so, it's ground stabilised and the limitations of its MARPA accuracy for collision avoidance (detailed above) apply.