Autopilot and GPS heading don't match

[cardude01]

I had the radar repositioned recently and I can't seem to get things matched up for some reason.

I did the swing compass thing and thought I had it, but I guess not.

Another strange thing that happens is when I push auto pilot I get a "no rudder reference" error, then it catches. The autopilot swings around wildly.

And ideas?

 

[GregBrannon]

You're asking about 3 heading displays which are not likely to be using the same source. Expecting all three to match, especially at rest, is unreasonable. The GPS heading should be 'calculated' based on the boat's movement over the earth taking into account all of the other forces it can sense - the key is that movement is required. The wet compass heading is what it is, certainly influenced by magnetic variation (declination) at your location and may be further corrupted by nearby electronics or metals. And the autopilot would typically use a compass sensor (fluxgate?) located somewhere on the boat and wired into the autopilot's network. A compass swing is often done to 'calibrate' the compass sensor or to understand if it has problems. You can search the internet for most of these terms and concepts. It's also important to understand the difference between True and Magnetic headings, how they're related, and how that relationship changes depending on the local magnetic variation. Your autopilot manual might be another good read on the topic, especially to understand the system's compass sensor.

Good luck, and come back with specific questions in small bits (if possible).

 

[cardude01]

Ahh Thanks. I just thought it was strange that the display on the plotter was so far off. Didn't realize I had to be to be moving.

My main problem right now is that the Raymarine AP (st6002) is not recognizing the rudder sensor for some reason. I swapped out another rudder sensor and got the same thing-- I get a "no rudref" error.
Had a marine electronics guy out and he was stumped for two hours.
This all started after I had a vendor put the radar on my new mast. They also added some windvanes and wind speed inputs on the mast, and they tied all that into the nmea2k network. Anyway, maybe it's a coincidence, but the AP isn't working now.

 

[GregBrannon]

Boat ownership has convinced me that there are no coincidences. There's a reason the AP is not seeing the rudder position sensor. Trace the lines from the sensor to the AP rudder reference input and verify that it's wired in correctly. (Hope your hired help did the basics.) Adding new components to the network may be causing a problem, but the rudder reference may be wired directly to the AP logic module. I think mine is, but it's a much older system. Raymarine has a forum and I've heard that the technical help at their toll free numbers is good. Check the forum and then make a phone call if you need to. Good luck and keep us posted.

 

[cardude01]

Cool. Thanks.

 

[twistedtree]

How does the rudder reference connect to the AP? Is it a direct connection, or is it via NMEA 2000, or SeaTalk, or something else? I agree with Greg that a coincidence here is unlikely. If you know where the connections are, try disconnecting the new equipment to see if the problem goes away.

On your headings, I assume it's the Garmin that is incorrect? What's really begin displayed if Course Over Ground, or COGs, and really isn't heading. "GPS Heading" is really a misnomer, but that's Garmin making their marine nav equipment look like car nav systems. Again, Greg is correct that GPS course over ground will only be valid when the boat is moving, and it should be very close to the heading, with the difference being caused by wind and current (set and drift in nautical terms).

Are you in an area with small variation? That would be the difference between the AP's reported true heading, and the compass heading. Where I am variation is 15 degrees, but you are only showing a couple of degrees. You cam probably change the AP display to magnetic if that makes it easier to cross check.

As for the AP turning wildly, it suggests that there is a big discrepancy between what the AP thinks and what the Garmin chart plotter thinks. Check the "GPS heading" while under way and see how it compares to the AP and magnetic compass. They should all be really close, accounting for variation as necessary.

 

[TG]

The Declination for Victoria, TX is +3° 20' and your AP is reading True and of Course you Binnacle is reading Magnetic so 191° and 194° on the compass are correct.

If your GPS doesn't get a heading signal from your heading sensor it will likely default to a heading calculated by the GPS using a coordinate difference calculation. If you are stationary when the picture is taken then the GPS heading would not be accurate because it depends on movement.

 

[FlyWright]

The Declination for Victoria, TX is +3° 20' and your AP is reading True and of Course you Binnacle is reading Magnetic so 191° and 194° on the compass are correct.

If your GPS doesn't get a heading signal from your heading sensor it will likely default to a heading calculated by the GPS using a coordinate difference calculation. If you are stationary when the picture is taken then the GPS heading would not be accurate because it depends on movement.

I normally run in the Course (or Track) Up orientation so what I see looking out the windshield is always at the top of my nav displays. When I anchor, I try to remember to change it to North Up for this reason. Without movement, the GPS has no reference for heading. The visual outside rarely lines up, so it can be confusing. In North Up, I know what I'm looking at while stationary.

It's really a point-to-point system. It knows where it is and where it was. It uses that info to compute where it's going at this moment in time only. Also, knowing where it is and where your programmed waypoints are, it knows how to get you there. It relies on the database to display obstacles and passages between you and your destination.

Your success depends on your system's accuracy, the database's accuracy and the accuracy of your ability to navigate, track and decipher the difference between this and the real world.

 

[tpbrady]

The rudder reference on the ST6002 connects direct from the sensor to the course computer. It isn't part of any network. The ST6002 is actually an autopilot control head that connects to the course computer via SeaTalk. I had the same setup until last year and when one of the two Seatalk ports on the course computer failed. Check the wire color coding on the rudder reference connection to the course computer.

Tom

 

[cardude01]

The rudder reference on the ST6002 connects direct from the sensor to the course computer. It isn't part of any network. The ST6002 is actually an autopilot control head that connects to the course computer via SeaTalk. I had the same setup until last year and when one of the two Seatalk ports on the course computer failed. Check the wire color coding on the rudder reference connection to the course computer.

Tom

I hooked a second known good rudder sensor to the Smartpilot computer, and still got the no rusted error code.

 

[Bacchus]

In addition to the new equip on the mast consider other changes in the area around your flux gate compass for the AP.
I have had similar significant AP errors that resulted from placing a tool box under steps to my galley (flux gate in bilge below).

 

[sunchaser]

Last year my AP 20 started showing rudder error. A new rudder sensor did not solve the problem. Adding an additional power supply to the Raymarine course computer helped considerably. But it was a patch.

A new Furuno AP was installed and works ever so much better. It is set up to follow either the NN3 or Trident when in Nav mode, working well in this respect.

Thus far, the 10 year past history with Raymarine AP and sporadic faults a fleeting memory. In fairness though, installers do strange and damaging things.

 

[cardude01]

Finally found it. The rudder sensor was bad, but also the old Raymarine ST60 AP control head was bad. The display worked but it would not pick up the new rudder sensor for some reason.

I should have suspected the head unit because about 6 months ago I lost the rudder indicator bar on the display, but everything else worked so I just lived with it. Bad idea-- that was a sign it was failing I guess.


New spiffy head unit