Radar target returns are offset relative to GPS

[Maggie Anne]

On returning to Birch Bay from Vancouver this morning we encountered a fog bank around Point Roberts. I started up the radar and noticed that the radar target returns were consistently offset roughly 5 degrees counterclockwise from the GPS locations of those same targets. I've attached a photo in my album of the Garmin screen showing three transponder green triangles for anchored ships and their corresponding radar images. Similar results for other targets such as buoys. To align these is there a digital solution or does one manually rotate the radar dome somehow?

 

[stwendl]

You can electronically adjust offset. See here
https://support.garmin.com/en-US/?faq=k ... a4hf1rs6xA

 

[Red Raven]

Be sure to check that your GPS heading is correct first. Changing the offset on the radar will just make them both wrong if your heading is not correct. To correct heading run the sea trial wizard on your Garmin autopilot dealer mode. Once that is calibrated then if the radar is still off change the radar offset.

Curt

 

[Maggie Anne]

I've also noted that the GPS track and the little boat icon are not aligned (depends on heading). I assumed it was a magnetic vs true issue and thought I'd get some help at the rendezvous to sort it out. Thanks to both for your suggestions. I'll run the wizard first.

 

[BB marine]

I have the same issue. Not all the time. I can't say that it is right or wrong because I have never been able to correct this issue. When I compare my compass to my auto pilot I have seen as much as 20 degrees variation. Not all the time, sometimes they are right on. When my compass, auto pilot, and COG all agree the radar targets agree with the Chart plotter when using the radar over lay. I have done the sea wizard and thought it corrected the issue but when I'm boating in different areas I have seen the issue again. If I am in limited visibility and using the radar overlay and my compass and COG agree which is 90% of the time and Autopilot does not. I use the radar targets as real time and use the offset chart imaging as references. Based on the photo you posted I would navigate with the radar targets you are seeing, are right and the chart references are slightly offset. Again I have never been able to correct this issue because it changes when I'm in different areas. I have learned to navigate comparing GPS COG and compass readings agreeing and autopilot showing offset of 0 to 20 degrees. Doesn't seem right but Ive never been able to correct it. ??? The result is exactly what your picture illustrates.

 

[Rocky]

I am no expert, but remember AIS is not always "Real Time." If the AIS Return was several seconds before the Radar return, the positions would be different - assuming the AIS boat was moving, of course.

I have also seen buoy radar returns substantially different than charted due to current.....

Just a thought.

Rocky