AIS Antenna??

[twistedtree]

@rslifkin - I believe the difference between my B953 and your B951 is mine has an integral antenna splitter (back-story: I originally went with the Simrad VHF with integral AIS and separate Simrad splitter, but it never worked properly (not the same issue) so I returned it to Simrad and got a refund - then bought the EmTrak) . Much of my problem is the problem is intermittent; and I don't see the problem - only those to whom receive my broadcast see it. QUESTION: Any tips on how I can see what I'm broadcasting in real time?

@twistedtree - thanks so much for the tips (I owe you yet another adult beverage). I will run the traps you offer. I do have one novice question. I am apparently broadcasting my heading information correctly, but the boat-icon is rotated so I appear to be crabbing on other's screen. I don't really understand the whole PGN stuff (except that it's a config setting). Why would the boat's apparent heading be different than the actual heading?

The one item I do recall from EmTrak tech is they only use True, not Mag heading information. I wish I still had the email where I sent them the sys log information. I'll repeat the process when I return to the boat in a few weeks.

Peter

It's COG vs Heading. COG will come from the AIS's own GPS, but it has no way of knowing your heading. The transmitted AIS broadcast has space in the message for the AIS to report both, but it's depending on some external data source giving it heading. Otherwise the heading field in the AIS message should be blank, or perhaps a code saying it's not valid.

I suspect that any display software would show the icon based on the reported heading, if available, and based on COG if the heading isn't available.

What's most likely happening is that the AIS is receiving confusing/conflicting heading info, and sending intermittent and incorrect heading info. When there is no heading info, or the heading info is correct, the boat icon would display correctly. If there is heading info and it's incorrect, the boat icon will show the incorrect heading.

So what you ultimately need to sort out is what the offending heading data is on your network, and who's sending it. Then correct it, or shut it off.

 

[rslifkin]

That's great info, and I agree the next step is to start examining messages. I'm guessing this is an all N2K setup?

My setup is all N2K, I'll let mvweebles weigh in on his.

What's most likely happening is that the AIS is receiving confusing/conflicting heading info, and sending intermittent and incorrect heading info. When there is no heading info, or the heading info is correct, the boat icon would display correctly. If there is heading info and it's incorrect, the boat icon will show the incorrect heading.

Agreed. I did a little more testing with mine and I couldn't get mine to transmit heading at all, presumably because nothing on my network will output true heading. And the Emtrak B95x AIS units won't use mag heading + variation data to convert to true heading for AIS transmissions.

 

[twistedtree]

BTW, here are common sources for heading info. This may aid your search for the culprit.

- Sat compass - this is the one you want to be using anytime it's available
- Autopilot rate compass. If this is a stand-alone N2K device as it is on many modern Simrad AP systems, heading will come directly from the rate compass.
- AP controller. Even if the rate compass is directly on the N2K network, it's possible that the AP is repeating it. Also, if you have an older rate compass that only connects to the AP, then it will be the AP that puts the data on the N2K bus.
- Weather instruments. Most of the higher end Airmar and relabeled weather instruments have a rate compass built into them, and broadcast that info. I would consider these to be the least reliable data.
- A variety of other devices often think it's helpful to repeat data. This messes stuff up at least as often as it's helpful.

If you have Maretron's N2KAnalyzer, it would be really helpful. You will need to look at each device's Transmitted PGNs (data messages) looking to see who's sending PGN 127250. Then click on the message and it will show you the contents. Hopefully one will stand out as containing trash. Also explore whether it's possible to turn off that PGN from unimportant devices.

 

[mvweebles]

That's great info, and I agree the next step is to start examining messages. I'm guessing this is an all N2K setup?

Yes, 100% N2K.

As an aside, 25-years ago I built a hardwired 0183 "MFD" system using a laptop with Serial Port interfaced to a Comnav A/P via an A/B switch (GPS being the "B" to feed NAV on A/P). At the time, I was running Cap'n Navigator software running raster charts on the Laptop. I had a Furuno CRT that worked very well. Laptop would crash from time to time but otherwise worked really well.

Not sure technological advances equate to functional improvements......

Peter

 

[mvweebles]

BTW, here are common sources for heading info. This may aid your search for the culprit.

- Sat compass - this is the one you want to be using anytime it's available
- Autopilot rate compass. If this is a stand-alone N2K device as it is on many modern Simrad AP systems, heading will come directly from the rate compass.
- AP controller. Even if the rate compass is directly on the N2K network, it's possible that the AP is repeating it. Also, if you have an older rate compass that only connects to the AP, then it will be the AP that puts the data on the N2K bus.
- Weather instruments. Most of the higher end Airmar and relabeled weather instruments have a rate compass built into them, and broadcast that info. I would consider these to be the least reliable data.
- A variety of other devices often think it's helpful to repeat data. This messes stuff up at least as often as it's helpful.

If you have Maretron's N2KAnalyzer, it would be really helpful. You will need to look at each device's Transmitted PGNs (data messages) looking to see who's sending PGN 127250. Then click on the message and it will show you the contents. Hopefully one will stand out as containing trash. Also explore whether it's possible to turn off that PGN from unimportant devices.

The problem is intermittent so I wonder if there's some occasional cross-talk or conflict. I don't remember well, but I think there's a decending order of precedence for some fields (Look first XXX; then YYY, then ZZZ --- or something like that). Maybe HDG is one?? You'd think either Simrad or Emtrak Tech Support could diagnose that. Thinking about it, could be the Sys Logs I sent EmTrak were never taken when the issue was visible. Grr.....

Will be more than a month before I'm back on the water. Will update along the way.

As always, I cannot thank TF's Bigger Brain enough - you guys have been some of the best tech support on the planet for a variety of topics.

Peter

 

[rslifkin]

I don't remember my Emtrak AIS having any way to set a preference order for input from other devices (like heading). So if multiple devices are providing heading, it's quite possible it just uses the last received heading data (from any device) when it transmits. Meaning if one of multiple heading sources is spewing bad data, it will intermittently end up using the bad data.

 

[stevemitchell]

Did you mention what model and version of AIS transponder you have?

There are specific models, like the Vesper Cortex someone else mentioned, that have issues with their heading. Many can be calibrated using some tricks to fix the heading issue that others will see. Some require deeper investigation for the heading sources on your network, and changing priorities to try to get them to pick the right one. There is one that I've worked on that may not ever be fixable depending on what you have on your NMEA 2000 bus.

As others have mentioned, some sort of tool like N2Kview, Actisense's NMEA reader, or even Signal K with some plugins would be helpful for seeing your existing heading data and figuring out which one needs to be calibrated/changed.

 

[Iggy]

That was my thought too, of course. But when I called both Simrad and Em-Trak (953), sure seemed like they had little input or knowledge about what the issue might be. To their defense, it was about 1-1/2 years ago so I may try them again to see if their knowledge base has improved (and perhaps Simrad's customer service has improved).

Despite Navico (Simrad/Lowrance/B&G) being a major player, they didn't seem to have much familiarity with settings except to contact Navico.

Of course, DIY install runs this risk of finger-pointing.......

Peter

I had the same problem a few years ago after I bought my boat. Now every electronic device was only a few years old. In other words, I don't build this setup, the past owner did. I looked and looked at cabling and settings. Nothing! Than I looked at the data being sent on the network. I had two heading sensors. What I thought and looked like a GPS antenna was also a heading sensor. A Simrad GS25. Once I removed the GS25, problem solved.

 

[SteveK]

Peter, is the direction off straight ahead always the same deviation or random directions. Maybe even True north and magnetic?
I ask because I am tied to a dock and my chart plotter is randomly pointing in different directions. When moving always forward.

 

[mvweebles]

Peter, is the direction off straight ahead always the same deviation or random directions. Maybe even True north and magnetic?
I ask because I am tied to a dock and my chart plotter is randomly pointing in different directions. When moving always forward.

I found a picture of the offset. Redline is direction of travel. When it's off, it's always the same relative direction and approx 90-degrees.

Peter

 

[Antares]

After heading and cog having been inline since install of the system, one trip the boat symbol on our MFD was way off. I checked connections at the MFD and other junction points but all was well. When I checked the direction sensor, the problem was found. I had left open the lid on our Engel cooler and the top end of the lid was close to the sensor. Being metal it was affecting the heading. Closing the cooler lid corrected the problem.

 

[SeaGator]

We have a 5 year old Simrad AIS unit connected to a Shakespeare fiberglass 8' dedicated antenna. It sits across the flybridge from a matching 8' fiberglass VHF antenna. For best results, always use for purpose antennas and never splitters or combination models. We have never had issues with this setup. If separating the antennas by type as you plan doesn't work or improve the situation, you have another issue.

 

[twistedtree]

I don't remember my Emtrak AIS having any way to set a preference order for input from other devices (like heading). So if multiple devices are providing heading, it's quite possible it just uses the last received heading data (from any device) when it transmits. Meaning if one of multiple heading sources is spewing bad data, it will intermittently end up using the bad data.

I too think this is what’s most likely happening.

 

[tom campbell]

I agree it is probably not the antenna/splitter. You can take the splitter out of the equation by plugging existing antenna into AIS.