/ 
The AIS Class B+ Reaches a New Milestone
Automatic ship identification has become standard equipment on recreational vessels that sail offshore or in high-traffic areas. While most systems still rely on AIS Class B transponders using CSTDMA technology, a new generation of equipment is emerging with AIS Class B+, also known as SOTDMA.
Distributed in France by Navicom, the new AMEC B650 series incorporates this technology, which is already used in professional Class A transponders. The goal is not to increase radio range, but to ensure better transmission reliability when maritime traffic becomes heavy.
Transmission Priority in Congested Areas
The main difference between a conventional Class B AIS and an SOTDMA model lies in the way transmission slots are allocated.
CSTDMA devices wait for a radio slot to become available before transmitting their position. When many ships are sailing in the same area, this wait can delay some transmissions.
SOTDMA technology works differently. It reserves its transmission slots in advance using the same principle as professional Class A transponders. This priority ensures that transmissions remain regular, even in busy ports, channels, or traffic separation schemes.
Another change is that the transmission power has increased from 2 W on a conventional Class B AIS to 5 W on this new generation.
Faster Updates for High-Speed Boats
Another advantage of the SOTDMA protocol relates to the transmission frequency. On a high-speed pleasure craft, the position changes very quickly. At 23 knots, a vessel travels about 360 meters in thirty seconds?which is the typical update interval for a conventional Class B AIS.
With an SOTDMA transponder, this interval is automatically reduced to as little as five seconds as speed increases. In the same scenario, the boat travels only about 60 meters between two transmissions of its position.
These more frequent updates improve the quality of the information displayed on the screens of other vessels. They make it easier to anticipate crossings and collision risks, particularly for high-speed patrol boats, powerful RIBs, or vessels operating in heavily trafficked areas.
Four versions to suit your needs on board
The AMEC B650 series is available in four versions that share the same technical platform. The B650 model is the standard version designed for existing installations. The B650S adds a built-in VHF antenna splitter, allowing a single antenna to be used for both VHF and AIS.
The B650W includes a Wi-Fi connection to stream AIS data to a tablet, smartphone, or computer equipped with navigation software. Finally, the B650WS combines Wi-Fi and an antenna splitter into a single unit.
The entire product line features a 72-channel GNSS receiver and NMEA 0183, NMEA 2000, and USB interfaces to integrate with most onboard electronic systems.
Technology derived from the merchant marine
The introduction of Class B+ AIS transponders reflects a trend that has been evident in marine electronics for several years. Technologies that were previously reserved for commercial vessels are gradually becoming available to recreational boaters.
For vessels that regularly travel along major shipping lanes, estuaries, or port approaches, the main benefit lies not solely in transmission power. It lies primarily in the system?s ability to maintain reliable transmissions as the number of vessels increases and the quality of AIS information becomes critical to navigational safety.
