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Old 18-04-2021, 12:14   #1
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Autopilots and Compasses

I have a Simrad (nee Robertson) AC42 pilot computer. I have been using it with an Airmar H2183 rate compass for I guess 8 years by now.


The compass talks to the pilot over N2k. As far as I can tell from the manual, it only speaks three PGN's:


127250........Vessel Heading
127251........ Rate of Turn
127257........ Attitude

For the life of me, I can't figure out which PGN's the AC42 can use.

Now I've acquired a Furuno SCX20 satellite compass, which according to the specifications is 4x more accurate for heading and 6x more accurate for attitude, and with 50hz vs 20hz attitude update.

It speaks a plethora of PGNs giving a wealth of data. Unfortunately many of them appear to be proprietary:

065280. . . . . Heave
127250 . . . . . Vessel Heading
127251. . . . . . Rate of Turn
127257...... .... Attitude 127257 - Attitude Category: Steering This PGN provides a single transmission that describes the position of a vessel relative to both horizontal and vertical planes. This would typically be used for vessel stabilization, vessel control and onboard platform stabilization.
127252. . . . . . Heave (again)
130577. . . . . ..Direction Data (what is that?)
130578. . . . . . Vessel Speed Components (This PGN provides a single transmission that accurately describes the speed of a vessel by component vectors)

130842. . . . . . Six Degrees of Freedom Movement (wow)

The last thing has got to be supremely valuable for an autopilot which can use the data, but I guess the AC42 can't. Would also be supremely valuable for correcting induced wind at the mast head.

But now I don't know how to set any of this up. I guess I need to switch off the PGNs which are not being used by anything -- I have a lot of devices on my network.

I'm guessing that the pilot is only interested in 127250, 127251, and 127257, and I need to turn off everything else -- can anyone confirm that? Does anyone understand, or can point me to information about how the pilot uses this data?
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Old 18-04-2021, 14:24   #2
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Re: Autopilots and Compasses

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Old 18-04-2021, 15:09   #3
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Re: Autopilots and Compasses

I think an autopilot responds to more messages than just those 3. While I can't find specific documentation, Garmin lists PGN data used by 'some' of their equipment. Navigation information, and I've seen much of this displayed on the autopilot control head. The list includes:
129026 COG/SOG
129029 GNSS Position
129025 Position Rapid Update
129284 Navigation Data (?)
129285 Route
129283 XTE (Cross Track Error)
127250 Vessel Heading
127245 Rudder Angle

Why not allow all PGN's to the autopilot, at least as a first test? My Garmin autopilot is on the central NMEA2K network and therefore exposed to all data, without any issues.
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Old 18-04-2021, 18:12   #4
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Re: Autopilots and Compasses

It's a really good question. It sure would be nice to know what data various systems use, and how important they are. This clearly applied to pilots, but also radars and sonars.


Most devices list the PGNs (and sentences) that a device accepts, but I don't think I've ever seen a device that breaks down what's required for proper operation, what will enhance operation if it's available, and what not used functionally, but can be displayed for convenience.


Furuno generally does a good job telling you which PGNs/Sentences they prioritizes when using data. So for example, they will tell you the priority order of sentences from which they will extract GPS position. But they don't say what's minimum mandatory, what's optional, and what's for display convenience.


On thing I'd be cautious about is overloading devices. Some are better at handling lots of messages, even if they don't use any of them. They all still need to be looked at to decide. So, for example, I'd be careful about setting heading and ROT to 50hz. Even 20hz I think is likely overkill for a slow moving boat, and I'd be worried about overloading the AC42 with anything more. I don't think I ever tried more than 10hz when I had one.
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Old 18-04-2021, 20:06   #5
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Re: Autopilots and Compasses

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Originally Posted by tanglewood View Post


Furuno generally does a good job telling you which PGNs/Sentences they prioritizes when using data.

That's been my experience. Furuno documentation is pretty good, so I'd check and see what PGNs their AP uses and go from there.
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Old 18-04-2021, 22:05   #6
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Re: Autopilots and Compasses

Quote:
Originally Posted by pjShap View Post
I think an autopilot responds to more messages than just those 3. While I can't find specific documentation, Garmin lists PGN data used by 'some' of their equipment. Navigation information, and I've seen much of this displayed on the autopilot control head. The list includes:
129026 COG/SOG
129029 GNSS Position
129025 Position Rapid Update
129284 Navigation Data (?)
129285 Route
129283 XTE (Cross Track Error)
127250 Vessel Heading
127245 Rudder Angle

Why not allow all PGN's to the autopilot, at least as a first test? My Garmin autopilot is on the central NMEA2K network and therefore exposed to all data, without any issues.

Yes, of course all pilos use those too. I was just concentrating on the ones that would come from gyro compasses.
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Old 18-04-2021, 22:06   #7
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Re: Autopilots and Compasses

Quote:
Originally Posted by tanglewood View Post
It's a really good question. It sure would be nice to know what data various systems use, and how important they are. This clearly applied to pilots, but also radars and sonars.

Most devices list the PGNs (and sentences) that a device accepts, but I don't think I've ever seen a device that breaks down what's required for proper operation, what will enhance operation if it's available, and what not used functionally, but can be displayed for convenience.

Furuno generally does a good job telling you which PGNs/Sentences they prioritizes when using data. So for example, they will tell you the priority order of sentences from which they will extract GPS position. But they don't say what's minimum mandatory, what's optional, and what's for display convenience.

On thing I'd be cautious about is overloading devices. Some are better at handling lots of messages, even if they don't use any of them. They all still need to be looked at to decide. So, for example, I'd be careful about setting heading and ROT to 50hz. Even 20hz I think is likely overkill for a slow moving boat, and I'd be worried about overloading the AC42 with anything more. I don't think I ever tried more than 10hz when I had one.

I think I'm going to need qualified advice on this. I guess I need to go through the hell of hooking into Navico customer service.
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Old 18-04-2021, 22:07   #8
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Re: Autopilots and Compasses

Quote:
Originally Posted by jdazey View Post
That's been my experience. Furuno documentation is pretty good, so I'd check and see what PGNs their AP uses and go from there.

That's a good idea; thanks.
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Old 20-04-2021, 15:47   #9
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Re: Autopilots and Compasses

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I think I'm going to need qualified advice on this. I guess I need to go through the hell of hooking into Navico customer service.

And then you will need to decide if the answer is credible, or if the tech is just making stuff up. I've encountered quite a bit of the later.
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