Doubts about AIS class B

Boaters keep on wondering what kind of data class A receivers are able to see.

Right now, most class A are only able to see mmsi and location of class B. IMO is likely to take a decision on how to solve this problem soon. They are most likely to decide that large boats should upgrade their class A equipment to make it compatible with class B in our opinion, during the next few years. It just doesn’t make sense having such a powerful navigation system, in which the two most important standards are just not compatible.

Some boaters wonder if large boats can decide whether to see class B or not. In some cases it may be possible via software to filter class B data, but why should someone decide to do so? Too much information? May be in extremely crowded waters?

We must take into consideration that there is no navigation system able to substitute the good criteria of the captain/skipper.


Nevertheless, in the future, it will be possible with the new standard for base stations to disconnect the transmission of class B when overinformation could affect navigation of large boats, or when overcapacity could affect the system. But this will only be possible in VHF coverage from the coast, never far away from it.

Capacity problems are practically impossible in class A, due to the SOTDMA (self-organizing time division multiple access) protocol that they use to organize transmissions. This protocol guarantees practically unlimited capacity to the system. Transponders communicate with each other to “decide” when to transmitt or not.

Class B uses CSTDMA (carrier sense time division multiple access) to organize them. Class B transponders “listen” during the first part of the slot, to know if somebody else is transmitting, and in negative case, transmit themselves during a therefore shorter slot (and thus smaller amount of data).

In order to have class B capacity problems, a large amount of boats should be transmitting frequently in the same area. If they are moored, transmission will be only once every 10 minutes, and thus overcapacity problems are very unlikely.

We believe that an actual problem in AIS in the future will be though, the asignation of mmsi numbers.

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