AIS target icons pointing wrong direction on chartplotter

[tommycook]

I recently installed a new Garmin 742xs on our sailboat. It is connected to our iCom IC-M506 VHF radio (with AIS receive) over a NMEA2000 network (also recently installed).

I was very excited to be able to display AIS targets on the Garmin, but the icons for other vessels are always showing an incorrect heading (sideways or completely backward). This causes the chartplotter to alert us to collision dangers when there is none, and doesn't warn us when there is actually a danger.

The VHF and Command Mic at the helm display the AIS target's headings correctly, but the Garmin is misinterpreting the data somehow.

Anyone run in to this issue, or have an idea about how to correct it? It has become quite annoying!

[donradcliffe]

Make sure that the Garmin software is the latest version. It it is, tell Garmin to fix their bug.

[continuouswave]

The behavior of a recreational grade chart plotter like the GARMIN product under discussion is not regulated by any standards or recommendations. To learn that a recreational chart plotter responds inappropriately to data sent to it from an AIS receiver that has received data about the location, course, heading, and speed of another vessel is not surprising. In some instances, users of GARMIN devices describe that there is no response at all to information sent to their GARMIN chart plotter from an AIS receiver.

The proper response to information from an AIS receiver about another vessel would be to display:
--the heading of the vessel,
--the course of the vessel, which could be different than its heading
--the speed of the vessel, as indicated by a projection of the vessel's course line of proportional distance ahead,
--the location of the AIS sensor on the other vessel relative to the length, beam, and distance from the bow of the sensor, and
--to drawn the vessel representation with a shape that reflects the vessel's length and beam dimensions oriented with respect to the location of the position sensor device on the other vessel.

For an example of such a rendering of an AIS target on a recreational grade chart plotter, see

http://continuouswave.com/whaler/ref...hipOutline.jpg

Any reasonably decent recreational grade chart plotter ought to exhibit that behavior when supplied with AIS information.

That the GARMIN device cannot even get the vessel heading or course drawn properly is quite a defect. You should report this defect to GARMIN for them to remedy.

[requiem]

Originally Posted by tommycook

I was very excited to be able to display AIS targets on the Garmin, but the icons for other vessels are always showing an incorrect heading (sideways or completely backward). This causes the chartplotter to alert us to collision dangers when there is none, and doesn't warn us when there is actually a danger.

Apart from the icons, what do the actual heading numbers say if you select one of them on the plotter, compared to the number displayed on the VHF?

Is the VHF displaying North-up, heading up, or course up? What about the plotter?

[tanglewood]

I think you have the plotter set to display relative motion vectors rather than true motion vector.

[Sailshabby]

Originally Posted by continuouswave

The behavior of a recreational grade chart plotter like the GARMIN product under discussion is not regulated by any standards or recommendations. To learn that a recreational chart plotter responds inappropriately to data sent to it from an AIS receiver that has received data about the location, course, heading, and speed of another vessel is not surprising. In some instances, users of GARMIN devices describe that there is no response at all to information sent to their GARMIN chart plotter from an AIS receiver.

The proper response to information from an AIS receiver about another vessel would be to display:
--the heading of the vessel,
--the course of the vessel, which could be different than its heading
--the speed of the vessel, as indicated by a projection of the vessel's course line of proportional distance ahead,
--the location of the AIS sensor on the other vessel relative to the length, beam, and distance from the bow of the sensor, and
--to drawn the vessel representation with a shape that reflects the vessel's length and beam dimensions oriented with respect to the location of the position sensor device on the other vessel.

For an example of such a rendering of an AIS target on a recreational grade chart plotter, see

http://continuouswave.com/whaler/ref...hipOutline.jpg

Any reasonably decent recreational grade chart plotter ought to exhibit that behavior when supplied with AIS information.

That the GARMIN device cannot even get the vessel heading or course drawn properly is quite a defect. You should report this defect to GARMIN for them to remedy.

What I found surprising was:
< The behavior of a recreational grade chart plotter like the GARMIN product under discussion is not regulated by any standards or recommendations. To learn that a recreational chart plotter responds inappropriately to data sent to it from an AIS receiver that has received data about the location, course, heading, and speed of another vessel is not surprising.>

[David Mark]

north hemisphere , south hemisphere set up could be the problem ,our seawind came from Australia and chart plotter did the same thing , was able to correct , check your manual .

[RichV2020]

Maybe related or unrelated:

I just brought my boat up to Massachusetts from Florida and had issues with headings on my Garmin 7612 chartplotter and AIS600.

The headings would sometimes be correct, 10 degrees starboard, 10 degrees port, or anywhere in between. My Garmin AIS600 would transmit the incorrect heading which was pretty dangerous in the middle of the night.

It became obvious when I used the heading ETA feature on the Garmin where I can set a heading line with hash marks in 10 minute intervals. The straight line set out at 30 minutes ahead made it hard to follow a course.

Apparently there is a "on the water" routine I need to perform with the chartplotter to calibrate the heading.

This is the opposite of your problem but they could be related.

[requiem]

Originally Posted by RichV2020

The headings would sometimes be correct, 10 degrees starboard, 10 degrees port, or anywhere in between. My Garmin AIS600 would transmit the incorrect heading which was pretty dangerous in the middle of the night.

It became obvious when I used the heading ETA feature on the Garmin where I can set a heading line with hash marks in 10 minute intervals. The straight line set out at 30 minutes ahead made it hard to follow a course.

Apparently there is a "on the water" routine I need to perform with the chartplotter to calibrate the heading.

While my guess is that it's likely more related to relative/true motion or north/course up, this is one reason to be cautious of AIS vectors for collision avoidance. If the unit takes an instantaneous heading measurement and transmits it, the natural yawing of your boat from side-to-side would easily result in such error.

Ideally measurements would be averaged over time to remove such variation. That's essentially what you get by doing a traditional radar plot, where you draw a line between positions of the other vessel taken every few minutes.

I don't know if the "on the water" routine is intended to calibrate that, or simply to calibrate the compass, as deviation error could cause similar results. In that latter case, you'd notice it as the compass would always read high on some headings, and low on others, instead of sometimes reading high or low regardless of heading.