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What is Class B+

What difference Class B and Class B+

The Class B+, often called to as “Class B SOTDMA” or “Class B 5W”, has been defined to bridge the gap between Class A and Class B transponders, offering some clear advantages for some types of vessels and applications.

Class B+ uses the same SOTDMA technology as Class A and therefore has the same priority when it comes to reserving a time slot, guaranteeing that it will always be able to transmit, even in busy AIS congested waters. For fast moving vessels this is important as a missed transmission can result in a vessel moving a long distance before it next manages to send a transmission.

Another feature that the new Class B+ technology it has taken from Class A, is the increased and automatic changing of transmission rates depending upon speed. Unlike Class A, the update rate is unaffected by whether the vessel is moving, but as the vessels speed increases, the number of transmissions increases so that other vessels get a clearer and more up to date view of where the boat is.

For slow moving vessels the increased update rates of Class B+ are not so important, but a fast power boat travelling at say 23 knots, will move 360 meters in 30 seconds, which is the update rate of a normal Class B transponder. On a Class B+ vessel travelling at 23 knots or more, the update rate is 5 seconds, so (using the above example) only 60 meters would be moved between updates.

Finally, Class B+ transponders have a higher power transmission 5 Watts instead of 2 Watts and this not only increases the range over which the  vessel’s transmission will be received, assuming good antenna height and performance, but it also significantly improves the AIS Satellite reception, enabling global tracking.

Comparison of AIS Classes

The following tables have been created to provide a “side by side” comparison of the three different classes of AIS.


Class A, B and B+ Functionality

Function Class A Class B+ Class B
Transmit Power 12.5W 5W 2W
Transmit Rate Up to every 2-3 secs Up to every 5 secs Every 30 secs
Minimum Keyboard + Display (MKD) YES NO NO
Guaranteed Time Slot Allocation YES YES NO
Voyage Data YES NO NO
External GPS Connection YES NO NO

As can be seen from the table above, in normal operation a Class A transponder transmits at a much higher power than a Class B. In “real-life” terms a well installed Class B transponder should be able to transmit up to 7-8NMs whilst a Class A transponder maybe seen as far as 20-25NMs away. With its 5W output, a Class B+ will be better than a Class B (2W), but not x2.5 better, typically 10-12NM should be seen.

As illustrated in the following table, Class B and B+ transmit the same data, a sub-set of the data transmitted by a Class A transponder.


Class A, B and B+ Transmitted Data

Data Transmitted Class A Class B and B+
MMSI + Vessel Name + Call Sign YES YES
Position + COG + SOG YES YES
True Heading YES YES
Rate Of Turn YES NO
Nav Status YES NO
Type of Vessel YES YES
Vessel Dimensions YES YES
ETA + Destination + Draught YES NO

Finally, the table below shows the different data transmit rates of the three systems. As can be seen, Class A transponders have several different transmit rates, based on speed, manoeuvring and Nav Status, whereas the Class B+ transmission rate is purely based on speed.

Comparing Class B+ to the original Class B, it can be seen that the simple two update rate (underway or stationary) of the original Class B has been expanded and increased in Class B+. For any boat that regularly travels at over 15 knots and particularly for boats capable of travelling at over 23 knots, the increased transmission rates offered by Class B+ are an important benefit.


Class A, B and B+ Transmit Rates

Ship’s Dynamic Conditions Class A Class B+ Class B
Ship at Anchor or Moored 3 mins 3 mins 3 mins
SOG 0-2 knots 10 secs 3 mins 3 mins
SOG 2-14 knots 10 secs 30 secs 30 secs
SOG 2-14 knots and changing course 3.3 secs 30 secs 30 secs
SOG 14-23 knots 6 secs 15 secs 30 secs
SOG 14-23 knots and changing course 2 secs 15 secs 30 secs
SOG > 23 knots 2 secs 5 secs 30 secs
Ship Static Information 6 mins 6 mins 6 mins



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