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AIS CSDTMA – what is it ?

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 no slot is available and a Class B vessel wants to transmit it simply misses out that ‘go’ and tries again after a pre-set time.

Its ability to provide a service therefore degrades gracefully and the user is informed if three potential transmission slots are missed by a warning signal. As soon as time slots become available the Class B unit picks up the reporting pace and goes back into the standard reporting timing.

So the environment is dynamic and depends on such issues as:
– How many Class A and Class B vessels are within radio range of each other
– What the Class A vessels are doing (moored, under way, etc)
– What the mix is of Class A and Class B vessels at that location …”


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AIS Class B – Whats the difference

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 of the University of Nottingham, writes in the Journal Of Navigation…

“The CSTDMA Class B system has been designed to prevent overloading of the AIS VHF data link. This is briefly reviewed but there are a number of other aspects that need to be considered. These include: the increased garbling of Class B messages compared to those of Class A; the problems accruing from the low update rate of Class B information; the increase in display information that will need to be managed; and the possible increase in inappropriate manoeuvres of leisure craft caused by misplaced reliance on AIS. As a result of the investigation the paper highlights the fact that Class B users must not assume that their own presence, in the form of Class B transmissions, will be particularly visible on the bridge of many SOLAS vessels. This will continue to be the case for many years into the future, until such vessels are mandated to carry radar with AIS target overlay capability….”


AIS – which VHF band ?

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.



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 less, however an average of five to six minutes between wheel-over and recognition of this fact on ARPA is common.

In contrast, the data of an AIS-ship, sailing at a speed greater than 14 knots and turning, will be updated every 2 seconds, showing the course alteration virtually from the moment that the wheel was put over.


Who should have AIS?

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 10,000 GRT to 50,000 GRT not later than July 1st, 2005
  • other vessels of 3,000 GRT to 10,000 GRT not later than July 1st, 2006
  • other vessels of 300 GRT to 3000 GRT not later than July 1st, 2007

Existing ships not engaged on international voyages, constructed before July 1st, 2002, must be fitted not later than July 1st, 2008.