AIS (Automatic Identification System) is mandatory in all commercial ships over 300 GT and the larger fishing vessels. AIS disseminates data on own ship's identity and movements via a transmitter / receiver in the VHF band (156,025 MHz - 162,025 MHz). Displays, plotters or radar and ECDIS systems will show those data dynamically as ship symbols. With the activation of the individual AIS ship symbols with a 'curser', all information about each ship will be shown as a 'pop - up' window on the display. With an AIS transponder, the information about own ship and its movements, destination etc. will be transmitted to other vessels.
FURUNO markets Class A and B transponders (FURUNO FA-150 and FA-50) and pure AIS receivers which are unclassified (FA-30).
What is Class B versus Class A?
AIS transponders are divided into Class A and Class B units. Roughly speaking, the devices operate in the same way: Both devices send and receive AIS information including static data about the ship and the dynamic ship data including ship motions. While a Class A unit sends its information with a specific interval, the Class B unit will also send information, but not with the same content and in the same sequence.
The Class A unit transmit the information with 12.5 Watt adjustable to 2 Watt , while the Class B transponder transmit with 2 Watt power.
In addition, the transmission methods differ, thus classifing the devices differently. The core of an AIS unit is the timing of the signals controlled by the internal GPS receiver. The transmission is a VHF signal being exchanged between vessels using a VHF antenna. There are two different principles for signal transmission and reception:
Class A units utilize the SOTDMA (Self Organized Time Division Multiple Access) principle. This can be summarized as the unit automatically 'book a particular' time interval = a 'slot in which the information is located' and in cooperation with the other transponders in the area, the different 'slot’s find its own place with information, which then will be distributed - fully automatic.
Class B units works according to the CSTDMA (Carrier Sensing Time Division Multiple Access) principle. This means that the unit 'wait' until a free 'slot' appear in the VHF transmission and thus the Class B unit can burst the information. Are there no vacant 'slots', the Class B unit waits to send the information, until there is a ‘free’ slot.
Please be aware : a Class B unit cannot send the information unless there is a class A unit in the vicinity that can organize the 'slots'. This, however, is not expected to pose a specific problem, since the Class A units are mandatory in all cargo ships over 300 GT and in larger fishing vessels, thus it will be rare for a Class B unit not being in active transmission 'mode'.
In the table below we have indicated differences in Class B and Class A units.
|Vessel type||Solas Vessel||Non SOLAS vessel|
|Signalmethod||SOTDMA(Self organized Time Division Multiple Access) Reserves a time slot and negotiate' the use of this slot with other Class A units.||CSTDMA(Carrier Sensing Time Division Multiple Access) Utilize a vacant timeslot. If no 'slot's are avaiable the class B unit will try to get contact later after a predefined space of time. |
|Frequency||156.025-162.025 Mhz||156.025-162.025 Mhz|
|Update interval ||Variable with vessels course and speed||2 level defined by vessels speed.|
|Ship at ancher and not above 3 knots. |
Ship at ancher and over 3 knots
Ships speed 0- 14 Knots
Ships speed 0- 14 Knots and course change.
Ships speed 14-23 Knots
Ships speed 14-23 Knob and course change.
Ships speed above 23 Knots
Ships speed above 23 Knots and course change.
|Ships speed less than 2 Knots|
Ships speed over 2 Knots
New rules for the implementation of AIS on fishing vessels:
Vessels with an overall length exceeding 15 meters must install AIS equipment according to the following schedule:
- Vessels with an overall length of 24 meters and above but under 45 meters, not later
on 31 May 2012
- Vessels with an overall length of 18 meters and above but less than 24 meters, not later
on 31 May 2013
- Vessels with an overall length exceeding 15 meters but less than 18 meters, not later than 31, May 2014.
Newbuilded fishing vessels with a lenght above 15 meter must install AIS latest November, 30th 2010.
The above vessels must install the Class A version.