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Class B AIS FAQs

Q: What information do I need to provide with my Class B AIS transponder order?
A: FCC regulations require Class B AIS units to be programmed with vessel information before shipping to US addresses. Milltech Marine provides this service free of charge.
  • To make it more convenient for you to provide vessel information, we have prepared this quick and easy online form. Please complete and submit at time of order.  
  •  We are required to have proof of valid MMSI, so please also send a copy of the issuing document.
  • If you do not have a copy of your MMSI issuing document, contact the issuing authority or organization. To search the  FCC licensing database, click here. For BoatUS information, click here.
If you prefer to not use the online form, you can download the Vessel Static Information form and fax (425-484-6218) or email.

Q: What is "MMSI"?
A: MMSI is an acronym for "Marine Mobile Service Identity." From the USCG: "MMSIs are nine digit numbers used by maritime digital selective calling (DSC), automatic identification systems (AIS) and certain other equipment to uniquely identify a ship or a coast radio station. MMSIs are regulated and managed internationally by the International Telecommunications Union in Geneva, Switzerland, just as radio call signs are regulated. The MMSI format and use is documented in Article 19 of the ITU Radio Regulations and ITU-R Recommendation M.585-4, available from the ITU."

Q: Will Class B AIS work with my Chartplotter?

A: All of the Class B AIS Transponders offered by Milltech Marine will work with any AIS-enabled chartplotter with NMEA 0183 RS422 or serial RS232 input (nearly all current-model plotters). To be sure, check with the chartplotter manufacturer or refer to the manual (in the product specifications, look for NMEA 0183 RS422 or serial input and AIS capability). Many of our transponders also support NMEA 2000 and will work with a modern chartplotter by simply connecting a NMEA 2000 cable from the AIS unit to an open T connector on the NMEA 2000 backbone. 

Q: Can I use my existing GPS for my new Class B AIS transponder?
A: No. While this should be technically feasible, the AIS Class B specs - unlike the Class A specs - require the AIS transponder to use its own GPS system. Therefore you need to use the GPS antenna provided with the transponder. In some cases the GPS antenna is embedded in the transponder unit. These units can also use an external antenna if needed. Some transponders come with a GPS antenna. Others have a GPS antenna connector but the antenna is not included and therefore needs to be ordered separately. Be sure to check this when you order.

All our Class B AIS transponders can output both AIS and GPS data through the NMEA and serial ports. It may be possible to use the GPS data for one of your other systems (e.g. DSC VHF radio), eliminating the need for other GPS systems. Through software settings, GPS can be filtered from the outgoing data, if needed.

Q: Can I install the GPS antenna below the deck of a fiberglass boat? 
A: Yes, the GPS antenna can be installed inside the cabin of a fiberglass or wood boat. Just make sure it is level and no metal such as a winch is directly above the antenna on the deck. Use the manufacturer's AIS program to view the satellite strength while trying different locations, then fasten it down once you find a suitable location. 

Q: Should I use a dedicated antenna for Class B AIS or a Class B AIS/VHF radio antenna splitter?
A: Antenna splitters simplify installation and reduce cable runs. Signal loss is minimal (<3dB) and is far outweighed by the advantages of an antenna optimally located at the highest point available (sailboat masthead, power yacht cabin top or arch). Be sure to use a splitter specifically designed for Class B AIS. Splitters designed for use with AIS Receivers are not compatible with AIS Transponders.

Q: When using a Class B AIS transponder, can I turn transmission of my vessel information on and off?
A: Yes, all our transponders allow the installation of a manual switch for turning transmission on and off. The most of the units can also be put into silent mode either via software.

Q: How do I know if my Class B AIS is transmitting?
A: All our Class B transponders have very effective self-diagnostics. Check the manual that came with your product. For best results, ask a friend with an AIS receiver to see if they can see you. Also see our blog post here on this subject.
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Q: Even though my Class B AIS is transmitting, why do some vessels with AIS take a long time to see my vessel name or not see it at all?
A: AIS information is sent in different message types. The MMSI, position, course and speed are sent frequently so this information shows first and will show on all AIS receivers, Class B and Class A. The Class B Static Data Report, or "Type 24" message, such as name of the vessel, type of vessel, destination, etc., is sent less frequently. This information takes longer to show on AIS displays and will not show on older Class A devices.

If a Class A AIS-equipped vessel does not see your vessel's name after a reasonable period of time, is typically due to AIS hardware or software on the receiving vessel that is not current. Many SOLAS vessels were outfitted with Class A AIS before the Class B Static Data Report format was defined. This means that they may not see your name, but they will see your vessel as a target with MMSI number, position, course and speed.

Q: Can vessels with AIS devices filter out Class B AIS?
A: In some cases yes. Some vessels will "filter" the display so that Class B targets don't show up. Keep this in mind and don't always assume you will be seen by other vessels with AIS equipment.