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AIS Class B, as specified within IEC-62287-1, in fact operates slightly differently in the way transmissions are controlled it does not use SOTMA it uses CSDTMA .

Bob Lee, Technical Sales Consultant at Software Radio Technology plc – says

…”Class B vessels using CSTDMA (Carrier Sense Time Division Multiple Access) mainly send fixed length telegrams using time slots not used by Class A vessels. Class A vessels reserve a particular time slot (SOTDMA) and negotiate the use of that time slot with other Class A vessels within radio range.
Class B vessels use any time slot as and when it is available. If

…….Click AIS CSDTMA – what is it ? …. to continue reading

AIS Class B was specified as a much less expensive, limited range and limited feature sub-set of the original Class A.

Class B has a reporting rate less than a Class A (e.g. every 30 sec. when under 14 knots, as opposed to every 10 sec. for Class A)

So Class B:-

Does not transmit the vessel’s IMO number or call sign
Does not transmit ETA or destination
Does not transmit navigational status
Is only required to receive, not transmit, text safety messages
Is only required to receive, not transmit, application identifiers (binary messages)
Does not transmit rate of turn information
Does not transmit maximum present static draught

Andy Norris

…….Click AIS Class B – Whats the difference …. to continue reading

A commercial vessels AIS unit has two dedicated VHF receivers and one transmitter to which the International Telecommunications Union (ITU) has allocated two dedicated frequencies, these are 161.975 MHz and 162.025 MHz (VHF 87B & 88B), however they may not be available in every part of the world.

In the United States for example, VHF 87A & 87B were recently allocated for AIS communications instead.

In addition the transponder unit includes a GPS system (GLONASS or GPS) to determine the ship’s own position and for time synchronization, as well as the means for displaying the received information from other AIS equipped ships.

…….Click AIS – which VHF band ? …. to continue reading

Most commercial vessels will use radar and very soon every one over 300 gross GRT will have AIS too (July 2007).

Why would AIS be a good adjunct to the use of ARPA?

Typically the ARPA will track the most reflective part of the vessel, on a VLCC this would be the superstructure. Before a change of heading of such a ship is detected on the ARPA, the VLCC will probably have turned 30-40 degrees, equivalent to 4 or 5 minutes. Another 1 to 2 minutes are needed to obtain a steady vector of the target, for smaller vessels this is somewhat

…….Click ARPA v AIS …. to continue reading

AIS introduction schedule:

Mandatory carriage requirements are set for all ships of 300 GRT and over engaged on international voyages, and those of 500 GRT and over not on international voyages as well as all passenger vessels irrespective of size.

All ships constructed on or after July 1st, 2002 must be fitted with AIS. Existing ships engaged in international voyages must be fitted with AIS as follows:

all passenger ships not later than July 1st, 2003
all tankers not later than the first safety equipment survey after July 1st, 2003
other vessels of 50,000 GRT and over not later than July 1st, 2004
other vessels of

…….Click Who should have AIS? …. to continue reading