Class B AIS Transponder

Class B transponders are designed to be used on marine vessels smaller than ships. Each unit has a VHF >Transmitter, two VHF receivers, one of which is multiplexed with the VHF Digital Selective Calling (DSC) Receiver, and a GPS active antenna

Static and dynamic vessel data

There are two categories of information transmitted by an AIS transceiver: >Static and dynamic data.

The vessel’s dynamic data, which includes location, >Speed over Ground (SOG) and Course overground (COG), is calculated automatically using the installed AIS Antenna.

Static data is information about the vessel which must be programmed into the AIS transceiver. This includes:

• Maritime Mobile Service Identity (MMSI)
• Vessel name
• Vessel call sign (if available)
• Vessel type
• Vessel dimensions

In most countries the operation of an AIS transceiver is included under the vessel’s marine VHF licence provisions. The vessel on to which the AIS unit is to be installed must therefore possess a Current VHF radiotelephone licence which lists the AIS system, vessel Call Sign and MMSI number.

Class B transceivers transmit at a lowerpower and at a lower reporting rate than Class A transceivers. Class B Vessel-mounted AIS transceiver (transmit and receive) which operates using either carrier-sense time-division multiple-access (CSTDMA)or SOTDMA; there arenow 2 separate IMO specifications for Class B. Aimed at lighter commercial and leisuremarkets. CSTDMA transceivers listen to the slot map immediately prior to transmittingand seek a slot where the 'noise' in the slot is the same or similar to background Noise, thereby indicating that the slot is not being used by another AIS device. Class >BS transmit at 2 W and are not required to have an integrated display: Class >BS can be connected to most display systems where the received messages will be displayed in lists or overlayed on charts. Default transmit rate is normally every 30 seconds, but this can be varied according to vessel speed or instructions from base stations. The Class B type standard requires integrated GPS and certain LED indicators. Class B equipment receives all types of AIS messages.