AIS (Automatic Identification System)
Navigation
AIS lets vessels swap name, position, course and speed automatically by VHF radio. A receiver shows AIS-equipped ships on your plotter; a transponder also shows you to them. It's a big help near shipping lanes and in fog, but plenty of boats don't carry it.
How it works
Transponders broadcast short digital reports on two dedicated VHF channels and listen for everyone else's, each report carrying the sender's MMSI. Class A sets, required on many commercial vessels, send at 12.5 watts every 2-10 seconds underway. Class B sets for smaller boats send less often and at lower power: Class B SO at 5 watts, Class B CS at 2 watts and roughly every 30 seconds. Receive-only units show others but send nothing.
Using it
- Show AIS targets on your plotter or radar and set a closest-point-of-approach (CPA) alarm and time (TCPA) that suit your waters, so you're warned early about ships converging with you.
- Use the name shown on AIS to call a ship directly on channel 13 or 16 instead of saying 'ship off my port bow'.
- Remember what AIS can't show: boats without AIS (most small craft and many sailboats), floating debris, and land. The Navigation Rules (Rule 7) expect you to use all available means, including radar and a proper lookout.
- A new target may appear without a name for several minutes; ships send their position every few seconds but their name and details only every 6 minutes.
- AIS is an open radio system and can be wrong or even faked. If a target doesn't match what you see or what radar shows, believe your eyes and radar, and report ghost targets to the Coast Guard NAVCEN.
- Check your own broadcast with the Coast Guard's online Vessel Information Verification Service (VIVS), which also flags errors in your MMSI, name or size.
- Don't rely on AIS text messages or AIS man-overboard signals to call for help; the Coast Guard doesn't treat them as distress alerts. Use DSC and voice MAYDAY.
Rules & requirements
- Carriage (USCG, 33 CFR 164.46): Class A AIS is required on self-propelled commercial vessels 65 ft and over and on certain towing, passenger and dredging vessels. Recreational boats aren't required to carry AIS.
- If you fit AIS voluntarily, the Coast Guard says you must still keep it properly installed and working, broadcast your officially assigned MMSI, and keep its data accurate (33 CFR 164.46(d); NAVCEN AIS FAQ 6).
- Voluntary users may fit Class A or Class B, but the unit must be FCC-certified for US use (NAVCEN AIS FAQ 4).
- Class B units sold in the US aren't user-configurable (47 CFR 80.231, per NAVCEN); the installer, maker or retailer enters the MMSI and boat details.
- Your AIS uses the same MMSI as your DSC radio; licensed-by-rule status covers any type of AIS on a US-only recreational boat (47 CFR 80.13(c)).
- Only one AIS may be transmitting aboard at a time, and AIS Class A or B broadcasts from land are prohibited (33 CFR 164.46(f), (i)).
Looking after it
- Before each trip, check that your AIS shows a GPS position and the right MMSI and boat name; NAVCEN asks users to check their data before each voyage, especially after a unit has been switched off for a while.
- Inspect the AIS antenna, coax and any splitter shared with the VHF radio. An antenna maker notes that AIS transmit needs a dedicated antenna, or a splitter if the boat has only one VHF antenna.
- Treat software updates with care: NAVCEN says AIS has no built-in virus protection, so use trusted update files and media.
- LED lighting near the antennas can degrade AIS reception as well as VHF (USCG Safety Alert 13-18).
Troubleshooting
Other boats (or VIVS) can't see you
- Confirm the unit is a transponder and that transmit isn't switched off.
- Check it has a valid GPS position; units without a position fix can't send valid position reports.
- Check an MMSI is programmed; US Class B sets need the dealer or maker to enter it.
- Inspect the AIS antenna, coax and splitter.
- Look yourself up on NAVCEN's VIVS to see whether the Coast Guard's AIS network has heard you in the last 30 days.
No AIS targets on the plotter
- Make sure the AIS layer or overlay is switched on in the plotter.
- Check the data link: an NMEA 0183 AIS feed normally runs at 38,400 baud, not the usual 4,800, so the plotter port must be set to match.
- On NMEA 2000, check the AIS appears in the plotter's device list.
- Check the antenna and splitter, and rule out LED interference.
A target jumps around or two boats show the same name or number
- This usually means two vessels share an MMSI, often a default or made-up number. NAVCEN asks users seeing this to tell the other vessel its AIS is broadcasting bad data.
Buying a boat that has one
- Find out what's fitted: Class A, Class B CS, Class B SO or receive-only. A receive-only unit won't make you visible to ships.
- Check which MMSI and boat name it broadcasts. On a US Class B unit a wrong MMSI needs the dealer or maker to fix.
- Some old, non-type-approved AIS units stopped producing valid positions after a 32nd GPS satellite came into use in 2008, according to NAVCEN; check an older unit actually reports position.
- Check how the antenna is arranged: its own antenna, or shared with the VHF through a proper splitter.
Sources (11)
navcen.uscg.gov · navcen.uscg.gov · navcen.uscg.gov · navcen.uscg.gov · navcen.uscg.gov · navcen.uscg.gov · navcen.uscg.gov · ecfr.gov · fcc.gov · shakespeare-marine.com · shipmodul.com
General guidance for recreational boaters, researched from the sources listed and written in our own words. Your equipment manuals, local knowledge, the Navigation Rules and a qualified instructor come first.