AIS Class B coverage limits

I’m very interested in the use of AIS on yachts, and not just as an aid to navigation, but also as a way of tracking boats. There is now a proliferation of websites which “aggregate” AIS transmission from boats and plot their position on a map. Probably the most popular of these is, which I have written about earlier. At first glance, that would make a terrific way of tracking cruising yachts along our coast. But is it?

Last weekend’s Sydney to Gold Coast race provided an opportunity to see the difference between class A (commercial shipping) and class B (leisure vessels, work boats) versions of AIS as a couple of the boats in the race carried AIS transponders. Here is what I found….

Around mid morning, Joy and I took Te Moana to near the race’s rhumb line, around 8 Miles off Barrenjoey. We enjoyed a quiet lunch on the glassy seas and the winter sun. Meanwhile, as participating yachts struggled to get out of Sydney Harbour in the light conditions, I was delighted to see two of the competitors, Scarlet Runner and Imagination using class B AIS responders. I monitored their progress on and was pleased that it did a good job putting them “on the map”. Certainly, it provided a much more timely and accurate position for them, than the CYCA race tracker (which was no surprise).

SGCmtIMaginationI was less impressed with the effective range of their AIS transponders (mind you, I don’t know the make or installation details of the units used by either boat). Here is the plot created by Imagination, showing it was out of range (the uninterrupted straight line) between Palm Beach and Port Stephens. Significantly, the repeater at Newcastle did not pick up either Scarlet Runner or Imagination, while ships with class A transponders were clearly present and visible in the same area. 15NM appears to be the limit for Class B AIS. Vessels carrying class A will have a larger range because of a better positioned, dedicated antenna and the higher power output of Class A devices.

Coverage of class B boats on sites like Marine traffic can be improved by having more shore based repeaters, but it is unlikely that boat signals will be reliably picked up when more than 15 mile offshore, no matter how many repeater stations are dotted along the coast.

The moral of the story then, is that AIS is a terrific on-board navigation resource, but it cannot provide the kind of yacht tracking which many of us would welcome, automatically keeping track of cruising boats along the East Coast of Australia for family and friends. And that’s even before we look at spoofing and spamming, about which I’ll write another time.

One thought on “AIS Class B coverage limits”

  1. I can understand the dissapointment, but, that was not what AIS designed for. It is and remains primary a navigation aid for ships mutually to receive information abouth other vessels in their proximity. And for port authorities to see and help shipping coming in or going out of ports.
    Tracking ships is possible, and it is widely used, but this is NOT what AIS was intended and designed for.
    Their is a differene in the tracking by some comercial AIS tracking firms and the free marinetrafic.
    It could well be that is you used a comercial AIS tracking firm you would be able to follos the shipping better and further, but you would have to pay for this service.
    But even then, in area’s where no commercial shipping is likely to go, you would not get much information, as the receiving stations of those firms will be concentrated around commercial shipping lanes.

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