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'New' Class B+ AIS bridges the gap

For many boaters across country AIS is becoming a familiar tool. Although the technology---Automatic Identification System---isn’t mandatory for recreational boats, it adds a strong measure of protection by keeping you posted about any large vessel traffic in the vicinity. Class A AIS is tailored for commercial ships while Class B units are more appropriate for pleasure boats. There’s another type you should know about that somewhat bridges the gap between Class A and B—here’s a version of an article about the technology that appeared recently in NMEA Boater.

Class B SODTMA AIS isn’t brand spanking new—discussion about it has been around since at least 2015. But Class B S/O, as it’s often called, has been gaining ground because of the benefits it offers, particularly to fast vessels. UK-based Digital Yacht recently published a white paper about the new technology. The report is an excellent primer for anyone who hasn’t followed the development of Class B S/O.

AIS is now one of the most widely used and significant navigation safety technologies since the introduction of radar. The system was originally developed as a collision avoidance tool to enable commercial vessels to "see” each other more clearly in all conditions and improve the helmsman’s information about his surrounding environment.

AIS does this by continuously transmitting a vessel’s identity, position, speed and course along with other relevant information to all other AIS-equipped vessels within range. Combined with a shore station, this system also offers port authorities and maritime safety bodies the ability to manage maritime traffic and reduce the hazards of marine navigation. 

Due to the great safety benefits offered by AIS, the fitting of a Class A transponder was made compulsory throughout the world in 2002 for all vessels over 300 gross tons or that carried more than 12 passengers. For smaller vessels that fell outside of the mandate, a Class B transponder was defined, which allowed fishing and leisure vessels to fit a lower power/cost transponder that worked on the same AIS network and could receive and transmit signals to the Class A transponders fitted to commercial vessels.

AIS transponders are now commonly seen on many leisure vessels and with the approval of personal AIS SARTs for use as man overboard systems, in conjunction with search and rescue (SAR) vessels/helicopters now fitting SAR transponders, AIS is becoming an important part of the Global Maritime Distress and Safety System (GMDSS).

Another new AIS application is vessel tracking, with websites like Marine Traffic and AISLive that collect and display thousands of AIS targets from their shore-based AIS reception networks, and global satellite reception via companies such as Orbcomm, exactEarth and Spacequest. 

Many national marine authorities are installing special Aids to Navigation (AtoN) transponders that can replace traditional buoys and beacons and transmit local weather/tidal information to passing vessels, while some large and busy harbors or shipping areas use AIS as part of their Vessel Traffic Services (VTS) to manage and control shipping movements. 

It is this continuous expansion of the global AIS network that has led to the approval of a new Class B technology that sits half-way between the original Class B technology and the Class A technology found on commercial shipping. This new technology does not supersede or replace the original Class B transponders, but it does offer significant improvements for some types of vessels and applications. For the purposes of this white paper, we will refer to this new technology as Class B+. 

How AIS works

To fully appreciate the benefits of this new Class B+ technology, it is necessary to understand how AIS works. 

An AIS transponder consists of a GPS receiver and a VHF "data” radio. The transponder takes its GPS position and transmits this in digital form on two VHF channels dedicated to AIS (161.975MHz and 162.025MHz). 

In order that multiple AIS transponders can "play nicely together” and avoid all of the devices transmitting at the same time, causing interference and loss of data, AIS transponders use a system called Time Division Multiple Access (TDMA). This is a similar system to that used in mobile phones, where each AIS transponder claims a very short 26.6 millisecond "time slot” where it transmits its information. The claiming of Class A time slots uses "self organized” TDMA where multiple transponders know how to claim and reserve time slots and what to do if there is a dispute with another transponder trying to claim the same time slot. 

The system works well and allows up to 4500 ships to work within close proximity of one another, automatically giving priority based on distance apart, i.e. as the number of vessels increases, the ones furthest away do not get a time slot. 

When Class B transponders were introduced, they used a slightly different technology called "carrier sense” TDMA where the Class B transponder listens to the Class A transponders and as soon as it detects an empty time slot, grabs it and makes its transmission. Occasionally a Class A transponder will "steal” a time slot from a Class B transponder and the system is designed so that Class A transponders always take priority over Class B—the Class B transponder will have to delay its transmission and start listening again for another empty slot. 
The number of transmissions that a transponder makes and the type of data it sends varies, based on its Class (A or B), its speed, whether it is maneuvering and its navigation status. The Class A transponder of a fast-moving ferry may output its position every couple of seconds while a Class B-equipped pleasure vessel will only transmit every 30 seconds while underway. 

As previously mentioned, the AIS data is transmitted over two channels of the VHF frequency range and a Class A transponder transmits at 12.5 Watts, while an original Class B transponder only transmits at 2 Watts which, to put this in to perspective, is a third of the power of a hand-held VHF that transmits at 6 Watts. 
This 2 Watt transmit power restricts Class B transmissions to an absolute maximum range of about 8-10 nautical miles and also means that traditional Class B transmissions are often not received by the AIS satellites that provide global vessel tracking. 

New Class B+ technology 

The new Class B+, often referred 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. SOTDMA is short for Self Organized Time Division Multiple Access. 

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 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 maneuvering, but as the vessel’s 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. 
Comparing AIS classes

The following tables have been created to provide a side-by-side comparison of the three different classes of AIS. 
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 7-8 NM while a Class A transponder may be seen as far as 20-25 NM away. With its 5W output, a Class B+ will be better than a Class B (2W), but not 2.5 times better—typically 10-12 NM 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. 

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, maneuvering 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 rates (underway or stationary) of the original Class B have 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. 

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Comments | Leave a Comment
Page 1 of 3 ( 13 comments)

Jp:(8/11/2018 5:28:29 PM) "I have a 2018 Yamaha f40 la and Humminbird helix 7 di , I would like to leverage the nema 2000 capability of the helix 7 to display engine info, what do I need , Humminbird does have a gateway and lowrance makes a Yamaha nema cable, but I'm reading connectors are proprietary . How can I get what is needed?

Since these products are not NMEA 2000 certified there is little assurance that they will share data with each other.

1. Here is the link to see all NMEA 2000 certified products:

2. The NMEA 2000 cables and connectors are from many manufacturers: Here is the link for approved cables and connector manufacturers:

AC/DC grounding distance:(8/2/2018 1:29:35 AM) "What about the grounding points of AC /DC systems? can they be grounded at the same point?
If one system has both AC and DC can they both be grounded to a common buss-bar that has only one conection to the hull?

Here's what Ed Sherman, electric tech guru at the American Boat & Yacht Council, said:

The ultimate goal should always be to tie ac and dc grounds together on board at a single point. In ABYC Standard E-11, it is described as “the engine negative terminal or its buss.” It is most commonly done at a buss."
Hard-Over with Brushed APilot Pump:(12/18/2017 5:37:05 PM) "Jim.
What do you mean by ...."Garmin GHP 20 with SmartPump...Because it is a brushless system, it is fail-safe and won’t execute a hard-over turn the way a brushed pump can."

Thanks for the note. Since the description came from Garmin I contacted the company for an explanation. Here's what one of their engineers told me:

On brushed DC actuators, a single-point failure in the drive circuit (shorted wire or blown component inside the controller) could cause the motor to run full speed in one direction and take the rudder all the way to one rail. A brushless actuator relies on timing-controlled commutation, so a short or component fail would cause the actuator to stop moving rather than moving at full speed.

Hope this helps,

trawlerdeejay:(10/13/2017 3:46:51 PM) "Excellent article. I had no idea what the differences were between o183 and 2000, Thank you so much."
Darryl:(3/27/2017 10:17:15 PM) "Putting the MSRP with each unit reviewed would have been helpful. If each unit was actually tested, the reports on each unit would have been helpful too.

Thanks Darryl---we generally don't mention prices due to confusion over so many variations---MSRP (mfg. suggested retail price), MAP (min. advertised price), MRP (min. resale price) and then there are internet prices on some websites that go their own way. But your point is well taken--buyers need to know if something is in their price range. We'll work on it.
There is independent testing of some of these products on sites like but the information we receive from manufacturers rarely cites the results of any shootouts they may conduct against the competition's products. "
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