AIS Advice and Cautions (with references to longer articles

Michael Moradzadeh

Discussion: A sailor may find AIS an unbeatable investment in navigation and marine safety, whether it’s for identifying oncoming vessels, announcing ones own position, or locating a crew in the water.

Although not much more complex to install than a VHF radio, recent reports have highlighted a few areas where an AIS might fail. These can lead to a false sense of security*.

Of particular recent note is interference from LED and other electronic devices, discussed below.

Here are some key elements of your installation, with links to more in-depth discussions.  For a detailed and scholarly discussion of AIS overall, see this link to Frank Cassidy’s paper.

  • Choice of Equipment. You should find equipment sold by any recognized manufacturer to function properly. You will need to choose whether you will be transmitting (recommended) or receiving only, and whether you will use “Class A” like the big ships or “Class B,” which is for recreational vessels.
  • Antenna Location. As with your VHF, the higher the better. Masthead (with a proper splitter) is ideal, but you will want to assure that your antenna coaxial cable is in good condition, adequately-sized, and with properly-treated connectors. Stan Honey’s article covers this nicely.
  • Wires. In addition to the antenna cable, you will need to bring power to the unit, connect it to its GPS antenna (regulations require integral GPS to avoid false reporting of position), and data out to whatever you are using to learn where the other boats are. Plan your installation accordingly. Be sure to protect your cables against damage.*
  • INTERFERENCE! Reports have come from several quarters on this. Some devices transmit radio “noise” that will block some, but not all, incoming AIS signals. You’ll think it’s working, but it’s not. Even though made to high standards, and compliant with regulations, this can happen. Here are some examples:
    • Masthead LED Tricolor. Right next to antenna, the “chopper” voltage regulator creates noise. A sensitive AIS receiver, or amplified splitter, can pick this up and mask some boats.
    • LED Modules Elsewhere on Boat. Did you get some cheapies from China? These can do the same thing. Even expensive ones might. Test, as described below.
    • Other devices. Inverter, fridge, heavy power users. Heck, we even saw problems from a near-antique GPS on one boat. Again, test as set out below.
  • Testing. Detailed test regimens can be found from Coast Guard and Stan’s Article. Here is a summary:
    • Check your Cable. Even brand-new cable can have trouble. Stan’s suggestion of using an SWR meter to check your installation is a good one to get the most out of your AIS and VHF installations.                    
    • Are you transmitting? Turn on your system. Ask a nearby boat, or a more distant boat with AIS if they see you. No friends? Most harbors are covered by, and you should see your boat there within a few minutes.
    • Are you receiving? This is what can get messed up with interference. A few tests:
      • On and Off. Turn off everything except your AIS with a bunch of boats around. See how many boats you see. Now turn ON everything and wait ten minutes or so. Same number of boats? Good.  Some disappeared? Possible problem. You may need to run this test at night if your masthead LED is photo controlled.
      • VHF Test. Described in the USCG notice. Turn everything off. Tune the VHF to a quiet channel (VHF 28, 60-66, and 78-86 are closest to AIS) and turn the squelch way down. Now turn stuff on. Hear new noise on the VHF? You may have a problem.

*On my own boat, OAXACA, we had a double failure. 1000 miles offshore at 3 AM, a freighter passed a mere 500 feet to my stern. Only a VHF call had alerted the commercial vessel, not seen on our AIS, to our presence. A LunaSea masthead Tricolor appears to have been interfering with our AIS receiver via amplified splitter. At some other time, a spinnaker sheet appeared to have burned through the GPS antenna cable, killing our transmissions. We were effectively in stealth mode without even knowing it. Fortunately, the crew maintained a decent visual watch!