Class A vs Class B AIS

There is a persistent belief that going offshore means you need Class A AIS. It does not, and understanding why clarifies the decision that actually matters.

#The four options

#Why Class A is not the offshore answer

Class A exists to satisfy the carriage requirements placed on regulated commercial and SOLAS shipping. It reports more often, at higher power, with more voyage data, and it is specified and priced accordingly.

AMSA identifies Class B for non-SOLAS vessels including pleasure craft. A private cruising yacht crossing an ocean is still a non-SOLAS pleasure craft. Fitting Class A does not make it safer in proportion to its cost, and it is not what the recreational ecosystem is built around.

If you operate commercially, carry passengers for hire, or are subject to specific survey requirements, that is a different question — confirm it with AMSA rather than inferring it from a buying guide, including this one.

#The decision that matters: SOTDMA or CSTDMA

Both are Class B. The difference is how the unit gets airtime.

CSTDMA listens for a free slot and transmits when it finds one. Simple, cheaper, and entirely serviceable — in quiet coastal water it works fine.

SOTDMA organises and reserves slots. Higher-performance modern Class B SOTDMA units can bring:

• improved slot management, so you are less likely to be squeezed out when the channel is busy
• higher transmission power on many units
• more frequent reporting in relevant situations

The circumstances where that matters are exactly the offshore ones: busy shipping approaches, distance from the receiving station, and closing situations where an out-of-date position is worse than none.

As the AIS guide concludes, for a new offshore cruising system in 2026, SOTDMA is generally the preferable direction where budget permits.

#What no class of AIS does

None of them shows a vessel that is not transmitting.

That is not a limitation to be engineered around; it is the definition of the system. A kayak, an unlit tender, a small timber fishing boat, a shipping container or a whale are invisible to every class of AIS ever made. This is why the collision avoidance guide treats AIS and radar as answering different questions, and why offshore boats carry both.

#The antenna decision is separate

Whichever class you fit, AIS needs an antenna, and the choice is really about failure separation rather than hardware count. A dedicated antenna gives physical separation and independent hardware; an approved active splitter reduces antenna count and can give AIS the benefit of masthead height.

The same logic applies to integrated VHF/AIS units such as Garmin's Signal VHF 400, launched May 2026: fewer boxes and a cleaner helm, traded against one hardware failure removing two functions.

#MMSI, once, correctly

A transmitting AIS needs a correctly allocated MMSI. AMSA manages Australian MMSI allocation.

Two disciplines, from the VHF and DSC guide:

• Never copy an old MMSI into new equipment without confirming it remains correct for the vessel
• Enter the static data properly — name, callsign, vessel type, dimensions and antenna position. A target with no name and wrong dimensions is less useful to the bridge watching you.

#Recommendations

#Bottom line

Transmit. That is the main thing. Then choose SOTDMA if you go offshore and the budget allows, get the MMSI and static data right, think about the antenna as a failure-domain question, and keep the radar — because none of it shows the boat that is not transmitting.