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Jim Hebert

AIS Base Stations

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I noticed that US Coast Guard RESCUE21 installations often identify themselves as AIS base stations, sending a Type-4 AIS message every ten seconds. I also see this same behavior from Canadian radio sites. However, not every RESCUE21 site seems to be transmitting an AIS base station message. In checking four RESCUE21 sites along the coast of Lake Huron, it seems like only every other site is transmitting as an AIS Base Station.

I am curious about this pattern. If anyone is familiar with the operation of the RESCUE21 sites as AIS base stations, please comment on your observations or knowledge of their practices.

It would be nice if all RESCUE21 sites identified as AIS base stations--in that way you could easily get their locations.

By the way, I have visited a couple of RESCUE21 sites and posted a few pictures of the antennas, tower, and buildings at

10 Replies

  • Following up on my own inquiry, I have learned more about the transmission of AIS Type-4 (base station) messages from RESCUE21 sites. The transmissions are not part of RESCUE21 but are part of the USA's Nationwide-AIS (NAIS) system. NAIS is utilizing some existing RESCUE21 sites for their base stations.

    The NAIS base stations are intended to be able to receive AIS transmission from a range of 50-nautical miles, and to be able to transmit to a range of 24-nautical miles.

    The Coast Guard plans to publish a list of their operational NAIS base stations and their locations in the 2014 Edition of the USCG Light LIst.

    Regarding Type-4 AIS messages, I found that not all implementations of AIS decoders will handle AIS Type-4 messages. Whether or not an AIS base station position will be plotted on your chart plotter will depend on how your AIS decoder and chart plotter handle these messages. My interest in AIS base stations messages grew out of the observation that my AIS receiver was getting a lot of messages that were not being decoded or plotted on my chart plotter. On a closer look, these turned out to be AIS base station messages.

  • The USCG has published the 2104 Light List, but it does not contain any listings of AIS Base Stations, as promised. The USCG told me that they will issue Notices to Mariners at some later data that will include information on AIS Base Stations.

  • Not really the same thing, but I noticed that the VTS in SF Bay is showing up - at the top of Mt. Tam, which must be where their comm radios are, not down on Yerba Buena where their office is.

  • The Coast Guard of the USA has announced that in a few days, on October 21, 2104, its National Automatic Identification System (NAIS) base stations across the USA will be transmitting AIS messages as part of a test. If you have an AIS receiver and have it connected to a chart plotter, it may be interesting to receive these AIS base station test messages and observe the behavior of your receiver and chart plotter during the test period. The test period is from 1100 to 2215 UTC, beginning on the hour and lasting for up to 15-minutes.

    AIS messages are described in

    During the testing period, messages 8, 25, and 26 will be sent. The USCG advices any mariners who can decode these messages they should ignore them. If there is any negative effect from this test on your navigation equipment you should contact the USCG Navigation Center at 1-703-313--5900 or at via email to

    The AIS Base Station Messages are typically not particularly well known to many existing recreational grade navigation displays or software that decodes AIS messages. Presently most USCG NAIS stations are sending just AIS Message ID 4, "base station report." Even this basic message is often not decoded or interpreted by navigation hardware and software at the consumer level. A basic test of your AIS hardware and software regarding AIS base stations can be made by checking to see if the position of the base station is plotted on your electronic chart.

  • Are they actually transmitting the location of the base station, or rather locations that are ideal for a mariner to navigate by, like the location of a navigational aid or light house?

  • The AIS Message #4 from an AIS base station typically sends the position of the base station.

    An AIS Aid to Navigation (AtoN) sends its own position, unless it is a synthetic AIS AtoN or a virtual AIS AtoN.

    I refer you to

    for more about AIS AtoN. The position of a synthetic AIS AtoN is usually sent in AIS Message 21 from an AIS base station. The position of a virtual AIS AtoN is usually sent in AIS Message 21 from an AIS base station.

    This test coming up is not a test of AIS AtoNs but rather of AIS base stations. It is not sending a test of message 21. (See above.)

  • As luck would have it, I was away from home on October21, 2104, and I could not make any observations of my own on the NAIS test conducted by the Coast Guard of the USA. If anyone did observe these tests, perhaps they can comment here about the reaction of their chart plotter and AIS receiver to the data that was sent.

  • UPDATE: As Winter comes to the Great Lakes, the USCG has been busy pulling out many floating aids to navigation and replacing them with "winter buoys" which are often smaller. They have also been rolling out Synthetic AIS AtoN markers to augment many floating aids. There are now dozens of AIS AtoN signals in the Great Lakes

    I recently was able to receive more than twenty AIS AtoN signals and display them on my chart plotter. I describe this is more detail in an illustrated article at

  • I have published a new article that looks at several RESCUE 21 installations near me. I have also discovered the location of all RESCUE 21 sites in the upper Great Lakes. The new article and photographs appear at

    The article also gives some details about the antennas and networking equipment in use at typical Rescue 21 installations.

  • Very interesting articles, thanks for the legwork