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Old 27-07-2010, 06:23   #1
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Can Someone Please Explain to Me . . .

Can someone please explain to me how the heading sensor data is used in our radars and autopilots.

It just doesn't make any sense to me. I always took it for granted because it all worked. Now I have an intermittent heading sensor (sympton: boat icon on the chart plotter points off in random directions) which I can't get fixed (Cowes Week -- all the specialists are booked up for weeks) and I'm about to make a night crossing of the English Channel.

It seems to me that you don't need heading data for the radar display if you set it to "head up" mode. That's because the scanner is fixed to the mast and knows which way it's pointed in relation to the heading of the boat.

But if you try to use "north up" you will get nonsense, if the system doesn't have heading data.

But what about "course up"? Doesn't the system use the COG data from the GPS? So "course up" should be functional without heading data, right?

MARPA does NOT work properly when the heading sensor is wacky, but I expected that.

Oddly, our autopilot works perfectly whether or not we have good heading data in the system. I do NOT understand how the f**** that can be so. How is it possible?

I really hope someone can give me some insight into these maddening questions.
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Old 27-07-2010, 06:37   #2
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Now I have an intermittent heading sensor (sympton: boat icon on the chart plotter points off in random directions) which I can't get fixed .
Is this whilst underway or stationary? seen lots of GPS Icons wandering around whilst the boat is stationary, but as soon as you start moving the GPS know which direction you are moving in.

We always run North up, baffles my 17 year old gadget loving son especially when we are heading South The old man has to keep some tricks up his sleeve.

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Old 27-07-2010, 06:43   #3
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Is this whilst underway or stationary? seen lots of GPS Icons wandering around whilst the boat is stationary, but as soon as you start moving the GPS know which direction you are moving in.

We always run North up, baffles my 17 year old gadget loving son especially when we are heading South The old man has to keep some tricks up his sleeve.

Pete
Both. Definitely something wacky in the heading gyro gizmo and/or fluxgate compass.

The GPS does give me accurate COG data. I suppose that the autopilot must be using this data in order to function correctly, which it does.

I alway use the chart plotter North Up like you do, and recently had a hot debate with my father about it. He prefers Course Up. I told him that you lose orientation in Course Up mode. That this mode makes it easy to know where to expect the landmarks to appear, but that you lose touch with your chart and with your overall position.

I assume my radar will not give me a reliable North Up screen picture without reliable heading data, but I'm still hoping someone will explain the principles of how all this works.
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Old 27-07-2010, 07:35   #4
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Hmm, how good is your French? would be an interesting conversation trying to explain this to a French Radar Technician about an English Radar system (Raymarine?) built in Hungary by a company now owned by who knows.

However, we did have a Furuno radar repaired at short notice in Guernsey a few years ago aboard an RN fleet tender. We hit thick fog on the way back past Alderney, so £250 well spent.

We don't have radar, so don't worry about fog.

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Old 27-07-2010, 08:44   #5
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On a practical note, if it is the gyro or fluxgate and the fault is intermittent, it could be something as simple as a slightly loose connection. I had a similar fault, a friend advised me to check all the connections both at the radar unit and distally. Worked for me.

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Old 27-07-2010, 08:50   #6
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On a practical note, if it is the gyro or fluxgate and the fault is intermittent, it could be something as simple as a slightly loose connection. I had a similar fault, a friend advised me to check all the connections both at the radar unit and distally. Worked for me.

P.
Well, I'll check again. I've turned the boat upside down looking for a stray bit of metal which might be throwing it off. I'll check the connections.

Somebody over on YBW suggested just unplugging the gyro. I thought the fluxgate compass would be wired through the gyro -- not so? This sounds like an excellent idea in case it's the gyro messing things up.
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Old 27-07-2010, 09:16   #7
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I do hope you manage to get it sorted out. I'm normally happy going anywhere at any time but I do dislike crossing the Channel at night.

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Old 27-07-2010, 09:23   #8
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I do hope you manage to get it sorted out. I'm normally happy going anywhere at any time but I do dislike crossing the Channel at night.

P.
Thanks.

But I'm curious -- why do you dislike crossing at night? I thought it would be easier to see the ships at night.
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Old 27-07-2010, 09:26   #9
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I see that it's Dawn at 04.44 on the English side of the Channel, perhaps a delayed departure may make it less annoying if you can't find the fault.

Portsmouth, United Kingdom - Sunrise, sunset, dawn and dusk times for the whole year - Gaisma

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Old 27-07-2010, 09:33   #10
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I see that it's Dawn at 04.44 on the English side of the Channel, perhaps a delayed departure may make it less annoying if you can't find the fault.

Portsmouth, United Kingdom - Sunrise, sunset, dawn and dusk times for the whole year - Gaisma

P.
Thanks. We're actually thinking now to sail over to Poole on Sunday and anchor out in Studland Bay. From there it's 60 miles straight shot to Cherbourg. If we leave at 05:00 then even if we bob around a bit waiting for wind we'll be there before dark.

But I'm really not so worried about dark -- I like sailing at night. It's fog that terrifies me, and that's more likely in the morning than at night.
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Old 27-07-2010, 09:33   #11
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I agree that seeing them at night is normally OK but with the frequent fog patches and I've seen ships that don't have someone on the bridge, it's not my favorite place at night. I'll still do it but daylight is still a preference.
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Old 27-07-2010, 09:49   #12
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I agree that seeing them at night is normally OK but with the frequent fog patches and I've seen ships that don't have someone on the bridge, it's not my favorite place at night. I'll still do it but daylight is still a preference.
I find fog more frightening in the day time. It seems to me that at night you can see navigation lights through fog a little further off, than the shape of a ship in the day time.

I always assume, day and night, that no one is on the bridge and that it is my sole responsibility to keep out of the way of shipping.
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Old 27-07-2010, 10:15   #13
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Actually, I think you will find that most commercial shipping keeps a better look out when it's foggy. They will often give way BUT tend to leave it too late for comfort. That's the beauty of AIS - you can call them by name and ask their intentions.
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Old 27-07-2010, 10:30   #14
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Hi Ed,

I agree, most commercial shipping keeps a better look out. It's the maverick that worries me. I'm sure you remember the P&O Port of Bilbao disaster. Like Dockhead, I always assume that they don't know I'm in their vicinity. Agreed about AIS, I love it.

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Old 07-09-2010, 07:20   #15
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OK, I did get back and forth over the Channel with no problems. I ran the radar in Head Up mode and it was sort of useful, but we had good visibility both ways and so the radar was not critically important.

But I still haven't figured out the heading sensor problem. I checked and was unable to locate any loose connections.

Here's the puzzle:

The autopilot works perfectly well. It uses heading data, doesn't it, just like the radar/chart plotter?! And even depends on it, doesn't it??!!

Yet the chart plotter shows the boat spinning around on all kinds of false headings, which makes many functions of the radar unusable (no MARPA, no overlay of radar image over the chart, no view mode other than Head Up).

What the h*ll?! Is this a fault in the main electronics, rather than the heading sensor? Doesn't the correctly function autopilot prove that there is valid heading data in the system somewhere? Anyone have a clue?

To remind: we have a Raymarine fluxgate compass plus gyro for heading data. RL80CRC+ and RL70RC+ monitors/plotters, Pathfinder radar, all vintage 2003 or so.
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