AIS on Your Boat: Receiver vs. Class B, MMSI Programming, Antennas and Checking You're Seen

A Class B transponder needs a 9-digit MMSI number, the same number your DSC VHF radio uses. BoatUS can issue one for recreational boats under 65 feet that are not required to carry a radio and that don't visit foreign ports (Canada, the Bahamas, Mexico and the Caribbean count). It's free for members or $25 for others (BoatUS pricing in October 2026). Sea Tow also issues them. If you travel to other countries, you need an FCC Ship Station License, which comes with its own MMSI. Use one MMSI for the boat in both the radio and the AIS. Never make one up: the Coast Guard warns that fake or default numbers make boats show up wrong for everyone. A quick sanity check from the Coast Guard's AIS FAQ: a valid ship MMSI starts with a digit from 2 to 7, and a U.S.-assigned one starts with 338, 366, 367, 368 or 369.

This is where many owners get stuck. The Coast Guard says Class B units sold in the US are not user configurable under FCC rules; the MMSI and boat details are entered by the dealer, installer or maker. In practice:

• Many retailers program the unit before shipping if you send your MMSI, boat name, call sign (if any) and GPS antenna position. West Marine lists this information for its pre-programming service.
• Some units let the owner enter the MMSI once through the setup app. Garmin's AIS 800 instructions warn you can't change the MMSI after you assign it; a wrong number means sending the unit back to Garmin for a reset.
• Until the MMSI is in, most units work as receivers only.

Double-check every digit before anything is saved. If you sell the boat or buy a used one with AIS, contact the maker or dealer to reprogram it with the new owner's MMSI.

AIS uses the marine VHF band, so it needs a VHF antenna. You have two choices:

• A dedicated AIS antenna gives the best performance and doubles as a backup VHF antenna (West Marine's advice). Keep it apart from your main VHF antenna, as the manual describes.
• An AIS-rated antenna splitter lets the AIS and your VHF radio share one antenna. Some transponders, such as the Garmin AIS 800, have one built in. A splitter adds some signal loss; a cheap one can cut your radio range. Use only splitters approved for AIS.

Mount the antenna as high as practical, use good low-loss coax and seal every outdoor connector. A masthead install is a rigger job on most sailboats. A few points from BoatUS:

• Height beats power. A dedicated AIS antenna ideally sits at least about 10 feet above the water and a few feet away from the VHF antenna, shrouds, radar arches and other metal, so two antennas crammed together at the masthead don't work well.
• Every foot of coax and every connector loses signal. Use the heaviest low-loss coax that fits, as few joints as possible, and aim for the low total loss your installation manual or the NMEA installation standard calls for.
• With most splitters, talking on the VHF pauses the AIS for the length of the call. During a man-overboard search, use a handheld VHF (or a second radio with its own antenna) so the fixed set keeps listening for the beacon.

A transponder needs its own reliable position. Many units have a built-in GPS receiver that needs an external GPS antenna with a clear view of the sky; others have one built in. Mount it where the arch, radar or mast won't block it. Measure its position from the bow, stern, port and starboard sides: this goes into the programming so other boats see your outline correctly.

Mount the transponder in a dry place, away from heat and the compass, where you can reach the cables. Wire it to a fused 12-volt supply with the fuse size in the manual, using tinned marine wire and sealed crimp connectors (see our 12-volt wiring basics guide). Most units should be powered whenever the boat is in use; many owners put it on the electronics breaker.

Connect the AIS to your chartplotter with NMEA 2000 (one drop cable onto the backbone) or NMEA 0183 (a serial cable), following both manuals. Some units also have Wi-Fi to send targets to a tablet app. In the plotter's settings, turn on AIS targets and set a collision alarm: a CPA (closest point of approach) distance and a TCPA (time to closest point) that fit your waters. Too tight and the alarm is useless; too loose and it goes off constantly.

In the unit's app or your plotter, look at your own vessel data: MMSI, boat name, call sign (if any), vessel type (pleasure craft or sailing), length, beam and GPS position. The Coast Guard asks AIS users to check this data before trips, especially after the unit has been off for a while. Errors here are what other boats see.

With the boat in an area with other AIS traffic:

• Check the status lights or the app: there should be no error, and a transmit light should blink every few minutes when stopped (more often when moving).
• Ask a friend with AIS to look for you, or call a nearby boat on VHF.
• Check an online AIS map or app a little later. These rely on volunteer receivers and may not cover your area.
• Use the Coast Guard's free Vessel Information Verification Service (VIVS) at navcen.uscg.gov. It shows whether the Coast Guard's network received your broadcasts and flags errors in your data. It isn't real-time and can lag up to two business days.

The Coast Guard notes that approved units have a built-in self-test, so don't send "test" text messages over AIS.

"My friend sees a dot but no boat name": that is often normal at first. The Coast Guard explains that position reports go out every few seconds to every few minutes, while the name, size and type go out in a separate message only every few minutes. Give it time before you assume the programming is wrong; if the name never appears, check your static data (step 7).

The Coast Guard warns that some LED lights can interfere with AIS and VHF reception. If your AIS shows fewer targets than you'd expect, or your radio sounds noisier with the lights on, turn the LEDs off and compare. See our LED lighting guide for the squelch test.

Many Class B transponders have a silent mode that stops transmitting while still receiving targets. Some have a button or app setting; Digital Yacht units use a simple switch wired to the power/data cable, with a light to show when silent. Not every model has it, and on some sets turning off transmit also turns off receive, so read your manual. Digital Yacht suggests silencing in crowded harbors to cut clutter. Many owners also use it for privacy at a mooring. Remember to switch back to transmit before you head into traffic or poor visibility, and never leave it silent by mistake.

Personal AIS MOB beacons clip to a life jacket and send the wearer's position to every AIS plotter nearby, including your own, when activated. The Coast Guard identifies AIS MOB devices by an ID starting with 972 (AIS SARTs start with 970 and AIS-equipped EPIRBs with 974). Some newer beacons also send a DSC alert to your own radio. Know their limits:

• They help your crew and nearby boats find the person; they are not monitored by Coast Guard rescue centers. Pair them with a PLB or EPIRB for offshore trips (see our EPIRB and PLB guide).
• Each beacon has its own built-in ID; register or set it up exactly as the maker says, and test it only with the test function.
• Store them so they can't be set off by accident, and report any accidental activation to the Coast Guard.
• Expect short range: the beacon's antenna sits just above the water and waves block it, which is another reason your boat's AIS antenna should be high.

Record the MMSI, programming date, antenna type and splitter in BoatMend. Check your own data at the start of each season, check coax connections and antenna mounts for corrosion yearly, and confirm the unit still shows a transmit light after any electrical work.