Question About AIS

We know that information about antenna placement is part of static data on AIS even on our Class “B” sets. However, we cannot access this data at all. I assume that CPA calculations on all type of AIS sets are done as if both vessels were mathematical points located at their GNSS antennae – is this correct? And do you have access to antenna location data on your AIS sets?

I guess it’s not of practical importance so long as you are always crossing with everyone at a sufficient safe distance that the exact location of the antenna is not significant, but it’s still an interesting question.

It was hammered home to me last summer when I was sailing through an anchorage in the German Bight (waiting area for Weser/Bremerhaven) last summer. Weather was fine and calm, and I thought a cable would be enough from a very stationary anchored ship. Well, a cable from his antenna is NOT indeed a cable from the bow of a big tanker! Fortunately this became quite apparent in good time :slight_smile:

I have considered this same thing with a few of our navigational aids. I had considered the location of the radar and the CPA that was calculated. I looked at our manual but didn’t find much about it as far as the placement and input to calculation.

I ended up settling myself with the idea that as long as I navigate with prudence and keep sufficient CPAs this perceived issue will never materialize to an actual issue.

That seems reasonable. I had a seamanship instructor that went into great depth about the placement and distances of a ships radar relative to their overall length - when attempting to anchor in congested anchorages. I went through the motions on the exercise but it was only when anchoring in Singapore did I appreciate the larger concept.


As far as I know we can’t access antenna location. Better to assume +/- about 0.2 nm. Anyway sound like you were paying attention so no issue in that case.

In cases where I have a choice I prefer to pass astern of anchored ships. For one if I have to slow down or the set turns out to be more than I assumed it’s better to get set away then down towards on the anchored ships.

Also if you find your self too close, if you are astern you only have to let the wind or current push you away. On a large ship and you’re upwind/current the only way to get away is swing the stern towards the anchored ships.

Secondly not all “anchored ships” are anchored. I’ve had a ship we assumed was anchored one night start moving towards us with deck lights blazing and anchor lights on. Add in the fact you’re setting down on them and you might end up closer than you planned.


The most critical problem for incorrect antenna placement setting is when the ship is turning.

I’m traveling right now and can’t find the arcile I wrote about antenna offsets but here is a loosly related problem of interest:

Every class B and Class A I have installed/replaced has a menu to put the offsets into.

I’ve had to reset mine after a total power down in a shipyard. Own ship’s outline on the ECDIS was shown on land when starboard side along a berth. It can be done.