Heading sensors.

[Allan]

I'm working on a project to update a navigation system. Yesterday at the boat show I was told that if I fit a radar I need a heading sensor to enable it to work correctly. The boat has an autopilot and a GPS which both provide heading information at present I can't see why another sensor is required. One partial explanation seemed to say the autopilot information was only given every second via NMEA 0183 and a new sensor would give it 20 times a second for added accuracy. Is this really necessary? It seems over the top to me, my boat is all NMEA0183 and seems to cope well enough.
Allan

 

[bitbaltic]

Radars need fast heading data, faster than GPS NMEA outputs. Dunno why as I don't have one, but what you've been told is I think correct. Someone who knows about radar will be along in a minute I'm sure.

 

[pvb]

Depends what you want to do with your radar. If you just want an idea of what's around, your existing fluxgate compass will be OK. If you want to use MARPA, you certainly need the faster heading information to get sensible results. But with AIS data easily overlaid on the plotter, maybe MARPA isn't as important as it once was?

 

[Allan]

Depends what you want to do with your radar. If you just want an idea of what's around, your existing fluxgate compass will be OK. If you want to use MARPA, you certainly need the faster heading information to get sensible results. But with AIS data easily overlaid on the plotter, maybe MARPA isn't as important as it once was?

To add some more information, the skipper, who is footing the bill, has specified radar. I would not fit it if I were paying. Her rationale is that, in fog, the AIS would only pick up commercial vessels so the radar is there for smaller vessels and buoys etc. plus coastal features. Maybe when she hears it's another few hundred quid she will decide to try without to start with? Is there anyway to quantify the benefit?
Allan

 

[Tranona]

Radar and AIS are not direct substitutes. Whether you value radar depends on how you are going to sail and whether you can use and interpret radar. Its main differences from AIS is that it can show everything around you - other vessels and land. Downside is that it is power hungry so tends to be used selectively rather than running all the time.

 

[Allan]

Radar and AIS are not direct substitutes. Whether you value radar depends on how you are going to sail and whether you can use and interpret radar. Its main differences from AIS is that it can show everything around you - other vessels and land. Downside is that it is power hungry so tends to be used selectively rather than running all the time.

Yes, I agree, when I first learnt to radar (as part of my job), it was the only thing available. Now I have it overlayed on my plotter with AIS I find I never use marpa and the plotter shows the land etc. The argument I put forward earlier of small, non-commecial vessels seems to be the only one I can see for accurate marpa. At £1800+ I don't think it's justified.

 

[pvb]

AIS is no substitute for radar. Only radar will show you what's around you in poor visibility conditions. But you don't necessarily need MARPA for that. Why not fit the radar without the extra heading sensor and see what sort of results you get? You can always add a heading sensor afterwards if you're dissatisfied.

 

[laika]

You may be aware already but just in case....

Dedicated fast heading sensors which the manufacturer recommends for use with MARPA tend to be very expensive (£500+). The Navico GPS/Compass sensors for a hundred-and-something aren't approved for MARPA:
http://www.panbo.com/archives/2013/02/navico_new_gpsheading_sensor_vhfgps_handheld.html


Modern autopilots tend to have a usable fast (>=10Hz) heading output which I believe most people use, although my old raymarine unit predates that standard of output.

Whether DIY projects are any cop for this, either with cheap components and a lot of clever maths or one of the more expensive (>£100) heading sensors remains to be seen (by me anyway

 

[prv]

Before anything else - your GPS doesn't provide heading information, it provides track.

Strictly speaking, no source of heading (or track) data is required for radar, but without it you're stuck with a basic unstabilised head-up plot. I wouldn't be very happy with this.

On the other hand, I don't think fast heading data should be needed to give a stabilised north-up view, because unless modern radars are a lot cleverer than those of ten years ago, they won't be redrawing the plot quicker than the scanner revolves anyway.

So presumably the fast heading data is wanted for MARPA. AIS isn't a substitute for radar, but I think it does do a lot of the things that MARPA was good for, rendering MARPA less important.

Like pvb, I reckon you should try it without the dedicated sensor (but with heading data fed from your existing autopilot compass) and see how it goes. You can always add one later.

Pete

 

[davidej]

I was in the same situation as the OP and decided not to buy the hi-speed fluxgate.

On the very few occasions I have used MARPA, I find it takes a bit of time to settle down and give sensible answers (ie up to 30 secs) but it remains a useful tool.

I agree AIS isn't the same as Radar but every MARPA target I have ever tried to track has been on the AIS.

Marpa is fairly useless for fishing vessels as they are prone to make sudden and unexpected changes of course - often when they are very close to you.

 

[johnalison]

I replaced my old radar with a new one four years ago but didn't update the old compass. I have AIS, so little need for MARPA, but I have turned it on occasionally, in smooth water, when it seems to work fairly well, with the predicted data settling after a minute or so. I would suggest giving it a try with the old compass, so long as MARPA isn't a priority.

 

[Spirit of Glenans]

Radar and AIS are not direct substitutes. Whether you value radar depends on how you are going to sail and whether you can use and interpret radar. Its main differences from AIS is that it can show everything around you - other vessels and land. Downside is that it is power hungry so tends to be used selectively rather than running all the time.

AFAIK, there is a rule somewhere that says that if there is radar fitted and in working order, it is obligatory to use it.

 

[Talulah]

AFAIK, there is a rule somewhere that says that if there is radar fitted and in working order, it is obligatory to use it.

That has been knocked on the head a few times.

 

[Allan]

I've just been giving this a little more thought. On my boat I have a heading sensor which, I believe, is an older type (over tens years old). I find that no matter how carefully I calibrate the heading if I turn through say 90degrees, the plotter and radar are not lined up and I have to adjust the radar. I guess this is discrepancies in the sensor. It is however accurate enough for me.
I have only needed to use the radar twice in anger (in fog) in the 9 years I've owned the boat. I used to use it for MARPA when crossing the channel too but use AIS now. I should say I sail more than anyone I know and all year around.
Allan

 

[ip485]

I have HD radar, Raymarine Heading sensor and chart plotter and AIS. It is the "standard" heading sensor not super fast.

It is interesting to compare the radar image with the AIS image - they never overlay exactly, the difference is relatively small so the "smudge" of the radar image touches the AIS triangle. I was told that with a fast sensor they would accurately align, but decided not to bother as it is adequate for my purpose.

I see a lot using radar, that doesnt appear on AIS. I have more than a few passages in thick fog, and the AIS would certainly not prevent a collision with anything without AIS which is still a large number of "vessels".

In reality I find I still dont see everything on radar though - especially small yachts that have a habit of "showing up" very poorly.

 

[Allan]

It seems like the consensus on here is that I'm wrong about radar but it's worth at least trying without changing the heading sensor. As the skipper will make the final decision, and she wants radar, it seems that everyone will be happy.
I should add that I don't think AIS has replaced radar, I just think radar is not worth £1800, I also worry that you could be looking at the radar when something comes out of the murk.
Many thanks for all the help.
Allan

 

[Talulah]

It is interesting to compare the radar image with the AIS image - they never overlay exactly, the difference is relatively small so the "smudge" of the radar image touches the AIS triangle. I was told that with a fast sensor they would accurately align, but decided not to bother as it is adequate for my purpose

The smudge could be for other reasons. The AIS transmissions are not constant. You're always seeing the position it last transmitted at. Is the radar aligned exactly? Menu option to adjust rotation. Looking to see if land reflection matches chart.

 

[ip485]

Talulah

Yes the land refelction very accurately matches the chart.

There is always a radar smudge on the AIS target. I will check to see if there is any out of alignement and you are of course correct it may be because the IAS transmission are not constant, but I am surprised if there is that much lag?

I would guess if you take the centre point of the AIS the "error" is around 50 to 100 feet or of that order.

I find it interesting technically, but I have taken it any further because for all pratical concerns it is irrelevant.

 

[grumpy_o_g]

It seems like the consensus on here is that I'm wrong about radar but it's worth at least trying without changing the heading sensor. As the skipper will make the final decision, and she wants radar, it seems that everyone will be happy.
I should add that I don't think AIS has replaced radar, I just think radar is not worth £1800, I also worry that you could be looking at the radar when something comes out of the murk.
Many thanks for all the help.
Allan

I think you're being a little harsh on yourself by saying you're wrong - there's a lot of kit most people would put on a boat before getting to radar. It wouldn't be the last thing either though for me.

Up to a point you do need to make a decision go "onto instruments" and stick with it until conditions change. It's not really going heads down as you would in cloud in an a/c (when you just don't look out the window) but, if you're in amongst a lot of shipping and visibility is just a few yards say, you're more likely to avoid a collision by looking at the radar than peering into the murk so you may be doing the right thing. Just how often that happens is another matter altogether. One of the biggest problem with leisure radar (or rather the users) is that they simply don't get the practice. It takes time to read a display and the changes from the last picture are the important part. I very quickly lost the ability to glance at display and read it, comparing with previous displays.

Re: MARPA and headings sensors, the issue is the way the boat yaws in a sea rather than any misalignment os sensors. As a quartering sea passes under the boat it may yaw quite a lot and quite rapidly for example. The radar only gets a position on a target once every second or two (assuming 24 or 48 rpm) and, if the boat is pointing in a different direction each time the target is painted the MARPA software gets very confused about exactly where it is and where it's going. Putting a good fast heading sensor on the system allows the MARPA software to allow for the boat yawing, but obviously it needs to be good enough to pick up that rapid yaw. I have heard people say that a rapid heading sensor can drastically improve some autopilots as well but I suspect we're talking high end B&G or NKE kit here.

It may be that the reason the land is "behaving itself" is because you are looking at a long return such as a shoreline. Diferences due to yawing tend not to show up as much as all you get is a slight change in the distance of the target. You can check that by seeing if headlands, etc. show in the same manner as shipping (i.e changing position slightly with each scan).

Apologies if I'm teaching relatives to suck eggs...