The Case of the Missing AIS Returns

AIS — Automatic Identifcation System — is one of the greatest safety inventions for the modern mariner. Even more than radar, it alerts you in detail about the approach of vessels that are on a collision or near-collision course with you.

But for it to be useful, it has to work. During the Baja Ha-Ha, Michael Britt of the Catana 47 Footloose discovered a serious problem with his and those of others that had something in common. We’ll let Michael’s letter to Lunasea Lighting explain the problem.

"Based on my experience and that of others, I think Lunasea has a problem with their masthead tricolor lights interfering with AIS returns.

"I did the recent Baja Ha-Ha with about 150 other boats, many of which have Class B active AIS systems. Because everyone was pretty much sailing a straight course, we would have as many as 40 AIS returns on our chartplotter at a given time. But after dark on the first night, I noticed that many of my AIS returns ‘disappeared’. Interestingly enough, the returns that disappeared weren’t the most distant ones, but were rather scattered throughout the fleet. I worried about the quality of my masthead installation.

"However, during a later onshore party I was discussing the problem with a friend — when several others who were listening piped in and said they’d had the same experience. What we all had in common was a Lunasea tricolor bulb. The others had a Lunasea masthead light with the tricolor, strobe and anchor light, while I had a Lunasea bulb in my masthead fixture.

"I later turned on my tricolor light and watched as about half — 20 of 40 — of the returns disappeared over the course of five minutes or so! Since class B AIS only transmits every few minutes, this would explain the gradual disappearance of the returns — the chartplotter holds the AIS return until after the next or several polling periods have passed. When I turned off the tricolor, the returns reappeared over the course of about five minutes.

"This is obviously a serious safety problem, and I’m wondering how Lunasea might help me address the issue. I have a pretty standard installation with my tricolor roughly in the center of my mast, and a Shakespeare VHF antenna mounted off to the side on a bracket attached to the side of the mast perhaps eight inches away. This is a very common installation since it provides optimal propagation of the VHF signal. My radio does not share its power source with the tricolor except, of course, the battery."

This is indeed a problem that others need to be aware of, and we hope that Lunasea — or whatever product is responsible — addresses the issue as soon as possible. At last word, a technical team was investigating.