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Old 11-06-2024, 15:26   #1
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Auto pilot and compass

I have a sitex sp 80 ap and a garmin 1242xvs plotter. They both have individual flexgate compass there was a discussion on another site about the eliminating the compass on the autopilot cuz it may be confusing what input is coming in from the garmin. Then I got reading about satellite compasses and how much more accurate they are so I have two questions if anybody has experience with this has anyone removed the compass from the autopilot so the plotter would have the only input and has anybody change from a flexgate compass to a satellite compass and what has been the results. My stuff is working okay I'm just looking to make the auto-pilot a little more accurate. Thank you
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Old 11-06-2024, 22:34   #2
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Re: Auto pilot and compass

I doubt you have 2 compasses. the garmin will likly be pulling compass data from the pilot's compass.

some pilots can use a sat compass and some pilots can only use the compass that came with it. you'd have to find that out.
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Old 12-06-2024, 00:21   #3
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Re: Auto pilot and compass

What smax999 said.

It's not actually a satellite compass; your GPS position is determined by satellite and this is used to determine your heading on your chartplotter. Chartplotter GPS headings are derived from time-series data, so they are not as useful as the compass/direction finder of your autopilot for making relatively fast changes. In fact, the delay inherent to GPS heading data can be great enough to be hazardous if you're relying upon that in real time.
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Old 12-06-2024, 02:45   #4
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Re: Auto pilot and compass

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Originally Posted by Ded reckoner View Post
What smax999 said.

It's not actually a satellite compass; your GPS position is determined by satellite and this is used to determine your heading CMG on your chartplotter. Chartplotter GPS headings are derived from time-series data, so they are not as useful as the compass/direction finder of your autopilot for making relatively fast changes. In fact, the delay inherent to GPS heading data can be great enough to be hazardous if you're relying upon that in real time.
FWIW: The GPS can only determine your 'Course Made Good' [direction of travel]; not 'heading' [direction your pointig].
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Old 12-06-2024, 03:59   #5
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Re: Auto pilot and compass

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Originally Posted by smac999 View Post
I doubt you have 2 compasses. the garmin will likly be pulling compass data from the pilot's compass.

some pilots can use a sat compass and some pilots can only use the compass that came with it. you'd have to find that out.
I do have two compasses as I have installed them both. The autopilot came with its own compass and then after I installed the radar on my Garmin I realized I need a compass for that too
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Old 12-06-2024, 04:01   #6
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Re: Auto pilot and compass

So if I'm understanding this, you guys are saying there will be no improvement or no advantage to one of the new satellite compass
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Old 12-06-2024, 05:14   #7
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Re: Auto pilot and compass

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Originally Posted by GordMay View Post
FWIW: The GPS can only determine your 'Course Made Good' [direction of travel]; not 'heading' [direction your pointig].
This is true for a GPS, but a satellite compass is a very different beast.

Try looking this upů https://www.advancednavigation.com/i...s/gnss-compass

These are not new, and are awesome tools. The cost seems to be the limiting factor for more widespread use.
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Old 12-06-2024, 05:22   #8
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Re: Auto pilot and compass

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Originally Posted by SailingHarmonie View Post
This is true for a GPS, but a satellite compass is a very different beast.

Try looking this upů https://www.advancednavigation.com/i...s/gnss-compass

These are not new, and are awesome tools. The cost seems to be the limiting factor for more widespread use.
Most of the comments here are not at all about satellite compasses. But standard gps, which is nearly unless as an autopilot heading input.

Satellite compasses are still a bit esoteric, and not widely known. They have much better specifications than a flux gate compass. But so what? They will not really improve the performance of your autopilot in any noticeable or practicable way.

If you are having trouble with your autopilot holding a course, it is not very likely that the accuracy of the compass is the issue. You will have spent a lot of money, and still have whatever issue you have now.

Of course you might just want the latest toy, which can be fun.
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Old 12-06-2024, 06:16   #9
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Re: Auto pilot and compass

You'll probably find that it is not possible for your relatively old autopilot to obtain heading data from another source. It has its own fluxgate compass, directly connected to the autopilot processor and it is probably using sin/cos analog signals.

You've probably also connected the autopilot processor to your Garmin chartplotter using NMEA 0183 so that it can follow a route or steer to a waypoint. The autopilot NMEA input probably just accepts the minimum number of NMEA 0183 sentences for following a route or steering to a waypoint such as RMC, APB, XTE, etc.

I'm not sure whether the Garmin chartplotter/radar can obtain heading data from another source, other than the second compass that you installed. It's possible that your autopilot processor has a NMEA 0183 heading output for the purpose of providing heading information to radars for overlays. But seeing as though you've already purchased and installed the second compass, whether the autopilot has a compass heading output is a moot point.

Nonetheless, while satellite compasses such as the Furuno SCX20 which provide both GNSS and compass data are very accurate (and expensive), I don't think it is of any benefit to you as I very much doubt that your autopilot processor can accept heading data from any source other than its own analog fluxgate compass.
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Old 12-06-2024, 06:24   #10
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Re: Auto pilot and compass

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Originally Posted by SailingHarmonie View Post
Most of the comments here are not at all about satellite compasses. But standard gps, which is nearly unless as an autopilot heading input.

Satellite compasses are still a bit esoteric, and not widely known. They have much better specifications than a flux gate compass. But so what? They will not really improve the performance of your autopilot in any noticeable or practicable way.

If you are having trouble with your autopilot holding a course, it is not very likely that the accuracy of the compass is the issue. You will have spent a lot of money, and still have whatever issue you have now.

Of course you might just want the latest toy, which can be fun.
Satellite compasses are already not esoteric at all -- in widespread use.

Furuno has sold them for years, but $$$. Used a lot on larger boats.

Navico have been selling the HS 60 and HS 75 for a while, also $$$.

Recently Furuno has come up with a better and cheaper one -- the SCX-20, which has sub-degree accuracy at a very high refresh right, with extremely accurate heel, pitch and heave data to boot, all for a bit over a grand.

I have one of these and it's brilliant. It's only a bit more expensive than a good gyro compensated magnetic compass like the Airmar H2183.

Good heading data is crucial for the autopilot, for MARPA, and for radar overlay. It's worth investing in one of these. I disagree that poor autopilot performance is unlikely to be related to bad compass data. On the contrary, bad compass data is a very common cause of autopilot problems.
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Old 12-06-2024, 06:37   #11
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Re: Auto pilot and compass

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Originally Posted by stevead View Post
You'll probably find that it is not possible for your relatively old autopilot to obtain heading data from another source. It has its own fluxgate compass, directly connected to the autopilot processor and it is probably using sin/cos analog signals.

You've probably also connected the autopilot processor to your Garmin chartplotter using NMEA 0183 so that it can follow a route or steer to a waypoint. The autopilot NMEA input probably just accepts the minimum number of NMEA 0183 sentences for following a route or steering to a waypoint such as RMC, APB, XTE, etc.

I'm not sure whether the Garmin chartplotter/radar can obtain heading data from another source, other than the second compass that you installed. It's possible that your autopilot processor has a NMEA 0183 heading output for the purpose of providing heading information to radars for overlays. But seeing as though you've already purchased and installed the second compass, whether the autopilot has a compass heading output is a moot point.

Nonetheless, while satellite compasses such as the Furuno SCX20 which provide both GNSS and compass data are very accurate (and expensive), I don't think it is of any benefit to you as I very much doubt that your autopilot processor can accept heading data from any source other than its own analog fluxgate compass.

It's correct that he can't use a different compass with his control head.


But he could change the control head and computer to something more modern, if he were so inclined.
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Old 12-06-2024, 06:40   #12
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Re: Auto pilot and compass

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Satellite compasses are already not esoteric at all -- in widespread use.

Furuno has sold them for years, but $$$. Used a lot on larger boats.

Navico have been selling the HS 60 and HS 75 for a while, also $$$.

Recently Furuno has come up with a better and cheaper one -- the SCX-20, which has sub-degree accuracy at a very high refresh right, with extremely accurate heel, pitch and heave data to boot, all for a bit over a grand.

I have one of these and it's brilliant. It's only a bit more expensive than a good gyro compensated magnetic compass like the Airmar H2183.

Good heading data is crucial for the autopilot, for MARPA, and for radar overlay. It's worth investing in one of these. I disagree that poor autopilot performance is unlikely to be related to bad compass data. On the contrary, bad compass data is a very common cause of autopilot problems.
Esoteric is relative. Obviously the first commenters in this thread do not know about themů

And of course bad data from a compass will make an autopilot misbehave. My point should have been that changing from a working fluxgate with proper gyro rate sensors to a sat compass is not likely to change anything noticeable on the average size cruising sailboat. Now if you have a 30 year old fluxgate alone and it updates at 1Hz then changing to any modern electronic compass will fix the issues.

It is important to understand that an autopilot needs repeatability and stability far more than it needs precision. A modern fluxgate combined with rate gyros does the job just fine.

When I ask my AP to steer to a course of 010, it holds a very steady course, it does not wander. And I for one really do not care if it is actually steeping 010.5

All that said, if I need to change my Ap compass, Iĺd absolutely go with a sat compass, for a lot of reasons. But none of them have anything to do with the ability of my AP to function as I need it to.
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Old 28-06-2024, 11:46   #13
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Re: Auto pilot and compass

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Originally Posted by GordMay View Post
FWIW: The GPS can only determine your 'Course Made Good' [direction of travel]; not 'heading' [direction your pointig].
I just want to point out that there are such things as GPS based compasses that do a fine job of telling you heading, not just CMG. it works with two antennae separated - perhaps one at the stern and one at the bow. The differential position between those antennae tell you your heading.

Admittedly, this is probably NOT the system that is being discussed and is probably a CMG.
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Old 28-06-2024, 12:19   #14
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Re: Auto pilot and compass

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Originally Posted by CyKlop View Post
I just want to point out that there are such things as GPS based compasses that do a fine job of telling you heading, not just CMG. it works with two antennae separated - perhaps one at the stern and one at the bow. The differential position between those antennae tell you your heading.

Admittedly, this is probably NOT the system that is being discussed and is probably a CMG.

Not "differential position" - phase. See:



https://www.google.com/url?sa=t&sour...CwZV6SKLKsaVZS
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Old 28-06-2024, 16:35   #15
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Re: Auto pilot and compass

I must say it makes me laugh nowadays when people complain about not getting accurate enough location data. I am old enough to remember when LORAN came out for sailors and was considered very Star Trek!!


Anyway if you want really accurate location data I have the unit for you. The Furuno SC 130 GNSS Compass. This little gem uses three satellite systems to hone in on your location, GPS (US), Galileo (EU) and GLONASS (Russian) It can tell you anything you want to know including heading and CMG, giving you three axis speed, bow, stern and longitudinal.



Now this unit is a bit pricey but the majority of the price is in the antenna which is about $8k as I recall. I happen to have one of these left over from a project, brand new in the box so if anyone wants to buy the display head for a few thou I will sell this antenna for $1500.

the Furuno link is below

https://www.furuno.com/en/products/compass/SC-130
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