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Solved problems · Electronics · Crew answer

Why isn't my AIS transmitting or showing other boats?

AIS problems usually come down to the antenna, the GPS fix, the MMSI programming or a silent mode switch. Online tracking sites aren't a reliable test. Here's what to check.

Symptom

Other boats can't see you on AIS, your boat doesn't appear or shows only a number, or the plotter shows few or no AIS targets.

Likely cause

A Class B transponder only transmits with a valid MMSI programmed, a GPS position and a working VHF antenna. Range is limited by design. Milltech puts typical Class B range at about 5 to 7 miles in good conditions, less with a poor antenna. A shared antenna without a proper splitter, a damaged coax or connector, an antenna too close to other antennas or metal, or silent mode left on all stop it working. Some receivers show a Class B boat as an MMSI number only, because the name is sent separately.

Stop now if

You're relying on AIS to avoid traffic in fog or at night and it fails. Slow down, keep a proper lookout, use radar if fitted, and call vessels on VHF. AIS is an aid, not a substitute for watchkeeping.

The fix, in short

  1. Check status and alarms. Look at the transponder's status lights or app for errors: no MMSI, no GPS, antenna fault (VSWR), or silent mode. Turn off silent mode if it's on.

  2. Confirm the MMSI and boat details. Make sure the MMSI and vessel details are programmed. In some regions only a dealer can program a Class B unit.

  3. Check the GPS source. Most transponders need their own GPS antenna. Check it has a clear view of the sky and a fix.

  4. Inspect the antenna and coax. Check the VHF or AIS antenna, cable and connectors for corrosion and water. If you share the VHF antenna, use a splitter designed for AIS. Milltech advises keeping a dedicated AIS antenna at least about 6 feet from other VHF antennas.

  5. Test with a nearby boat. Ask a nearby boat with AIS to confirm they see you, since online tracking sites depend on shore receivers in range.

  6. Check plotter settings. If the transponder is fine but targets don't show, check the plotter's AIS data source and display filters.

Written by Sailor Cruiser crew. General guidance, not an inspection of your boat.

Equipment involved

  • Navigation electronics: Class B AIS transponder or receiver, GPS antenna, VHF/AIS antenna, splitter and chartplotter

Related guides

Sources

Background reading we checked while writing this answer. The wording above is our own.

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