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Old 22-07-2015, 19:56   #1
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OpenCPN - incorrect bearings from ship

My Globalsat BU-353S4 had been working properly but now has a problem in that it gives erroneous bearings to marks from my ship position. The distance to the mark is correct as is other data including the lat. & lon. of my ship, etc. This problem started after I tried to provide some settings that would enable the BU-353 to control my autopilot.

OpenCPN still works correctly with my Raymarine chartplotter GPS connection.

There is an approximately 30 to 32 degree difference in the incorrect bearings measured by the BU-353S4 and the Raymarine. For example the Raymarine measures correctly 134deg to a mark and the Bu-353S4 measures incorrectly 102deg to the same mark.

The magnetic variation is 16+deg where I am operating in case that may be an incorrect data input caused by a setting that might have been changed.

I am frustrated since after having spent many hours getting familiar with OpenCPN and finally getting it to work, I apparently have triggered something that causes this dangerous mis-information.

Any suggestions would be welcome.

Bob on s/v ACADIA
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Old 22-07-2015, 22:04   #2
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Re: OpenCPN - incorrect bearings from ship

Bob...

Sounds like a sign error on the variation data, somewhere, having the effect of doubling the 16 degree correction in the wrong direction.

We would like to see some NMEA data stream from the BU353, as captured by the VDR PlugIn.

Thanks
Dave
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Old 23-07-2015, 06:09   #3
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Re: OpenCPN - incorrect bearings from ship

If your position "Tiverton, RI" is more or less correct, the variation should be indeed West or Minus and not Plus. You could also install the wmm plugin to doublecheck the local variation.
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Old 23-07-2015, 06:53   #4
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Re: OpenCPN - incorrect bearings from ship

Dave:
Thanks for the quick reply. Unfortunately I don't know how to obtain the NMEA data stream with a VDR plug in. This will take some time for me to learn. Presently we are anchored off Richmond, Isl. before heading for Portland ME where we will be for a few days during which i may have enough time to educate myself.

I will give it a try and let you know.
Bob
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Old 23-07-2015, 07:08   #5
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Re: OpenCPN - incorrect bearings from ship

Rooiedirk:
Also I thank you for the quick reply. As you can read in my reply to Dave, we are presently on the Maine coast where the variation is 16deg. and 45mi. W as shown on NOAA charts.

The possibility that an erroneous setting for variation is the cause of this problem occurred to me but maybe it is just a coincidence and there is some other setting that is the cause. I hope that I have not added confusion to solving this problem by bringing us the variation question.

One thing that all of you could help me determine is whether the problem could be with the Globalsat GPS unit. Or is the fact that it gives an accurate position for the ship all that it is expected to do and that OpenCPN has an incorrect setting.
Thanks again.
Bob
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Old 23-07-2015, 07:21   #6
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Re: OpenCPN - incorrect bearings from ship

Quote:
Originally Posted by rschaedel View Post
Dave:
Thanks for the quick reply. Unfortunately I don't know how to obtain the NMEA data stream with a VDR plug in. This will take some time for me to learn. Presently we are anchored off Richmond, Isl. before heading for Portland ME where we will be for a few days during which i may have enough time to educate myself.

I will give it a try and let you know.
Bob
Just go to the OpenCPN.org download page --> Plug-ins and scroll down to the VDR.
Execute the installer.
You will have two additional icons in the toolbar. Self explaining.
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Old 23-07-2015, 08:55   #7
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Re: OpenCPN - incorrect bearings from ship

Dave:
I went to the OpenCPN downloads page but could not find an item for download plug ins.
Do you mean that I should download OpenCPN 4.0 again and find an option to click on plug ins there?
Bob
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Old 23-07-2015, 09:03   #8
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Re: OpenCPN - incorrect bearings from ship

Bob,

no need to re-install.
When you hover with the mouse over the Download tab at Opencpn.org the there is a fold down menu with the link to the plug-ins.

Hubert
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Old 23-07-2015, 09:28   #9
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Re: OpenCPN - incorrect bearings from ship

You could also take a look at the Settings-Options > Display > Units tab and see what you have set for magnetic bearings and heading and what you have set for assumed magnetic variation.

See this page Setting Options | Official OpenCPN Homepage down under
Bearings
Show Magnetic bearings and headings
: OpenCPN by default uses true courses and bearings etc. This box is for navigators who prefers to work with magnetic courses and bearings, or want to test this method. Note that OpenCPN knows nothing about deviation.
All courses and bearings will have a (M) post-fix, to show that they are magnetic.
and a little further down
...The following settings will be affected: Bearings and Courses in the Route Properties dialog, route leg rollovers, and Active Route console will be (M). COG displayed on the bottom status line will be COG(M). AIS target reports will show target COG(M).
Variation will be calculated at Own Ships position, if possible. AIS targets however, will use the targets position for variation, if possible.

How does OpenCPN know about the variation?
  1. If a GPS receiver that reports variation as part of RMC sentence, is available, this value will be used.
  2. Otherwise, if the WMM plugin is installed and activated, it will supply the variation.
  3. Lacking the above alternatives, the manually entered "Assumed Magnetic variation", will be used for all calculations. See below.
Perhaps you can figure out what how your system is working with this information?
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Old 24-07-2015, 16:38   #10
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Re: OpenCPN - incorrect bearings from ship

All:
Thanks for all your helpful suggestions.
It does appear that the error can be detected in the Options, Display, Units menu. The "assumed magnetic variation" was showing 17deg(W) which to my understanding is correct for our location at N41deg 43.622 and W070deg 12.181, yet the bearings shown were -34deg. incorrect When I manually change it to -17(E) the bearings shown at the ship's position are correct.

Do any of you know whether the Globalsat BU-353S4 GPS should correct for magnetic variation? If so maybe my unit is defective. However I thought that when I first used it, it was performing correctly. Should I reinstall the driver?

Or is the problem with OpenCPN and I will have to install the WMM plug in if I do not want to remember to manually adjust the variation as we sail down the coast of Maine? Also keep in mind that OpenCPN gives correct bearings when the GPS input is from my Raymarine chartplotter. Although I could do alright with this, my intention is to have redundancy with a totally separate system operated through the Globalsat GPS.

Any further suggestions that you can offer will be appreciated.
Bob
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Old 24-07-2015, 18:28   #11
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Re: OpenCPN - incorrect bearings from ship

bob...
1. We would need to see the NMEA output from the BU353 to be sure of the Variation polarity. If it is wrong, there is no amount of driver re-installation that will fix it. Anyone else using BU353 and can confirm? I'll try to get mine loaded tonight and check it.

2. If the Bu353 does not work right, then the next best idea is to use the WMM PlugIn, and tell OCPN to ignore any spurious variation information coming from the BU353. You do that using the message filters built into the connections tab. Essentially, we want to ignore RMC, and accept GLL and VTG.

Dave
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Old 25-07-2015, 09:04   #12
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Re: OpenCPN - incorrect bearings from ship

Dave:
I now have two buttons for the VDR plug in. One is labeled Record and the other is Play. When I click on either Record or Play it shows a list: Dashboard, Grib, VDR.
When I click on VDR it displays a small screen view titled Replay but nothing shows on Replay or Progress.
The Globalstat is working and shows ship's position, bearings, etc.

I did try to contact USGlobalstat by telephone yesterday during working hours but there was no connection with sales or tech help. Also I was unable to contact them by the email address they provided. Likewise I was unable to register with the users Forum.

If I use the WMM pug in and make changes indicated, I am concerned that it might disrupt the use of the GPS input from the Raymarine chartplotter which does work correctly.

I apologize for not responding sooner to your replies but I am limited to only periodic occasions while we have internet access.
Bob
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Old 25-07-2015, 09:28   #13
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Re: OpenCPN - incorrect bearings from ship

Bob,

have you enabled the plug-in in Options (the wrench) --> plug-ins?

When you click then the VDR button with the red dot ("record") a Window should open to define the file name where the record goes to. Define it, then "enter" and the recording is running.

Hubert
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Old 27-07-2015, 04:38   #14
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Re: OpenCPN - incorrect bearings from ship

I have BU-353 S4. I have no setting in mag var. (0).
I am using magnetic heading and have found no problem, however there is some small differences from my mag compass which was calib 5 years ago. The compass had 0 correction on all quadrants.

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Old 27-07-2015, 04:40   #15
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Re: OpenCPN - incorrect bearings from ship

Dont believe the problem is your compass. It is the settings.

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