AIS Know-How: Data transfer (SOTDMA vs. CSTDMA)

Since the end of 2000, the Automatic Identification System (AIS) has become a binding standard to improve the navigation and safety at sea. Position, course, speed, and other ship data are transmitted alternately on the two maritime VHF channels 161.975 MHz and 162.025 MHz. Per minute there are 2,250 Slots available for the transmission of AIS data telegrams.

Two methods are available for transmitting the AIS data telegrams.
On the one hand, the CSTDMA (Carrier Sense Time-Division Multiple Access) method, which is used primarily for pleasure boating (Class B). The CSTDMA method uses carrier-sensing to defer transmissions until no other stations are transmitting. If a free slot is found, the transmission starts. Than the transmission is stopped for a 30 second time interval before the next one starts.
On the other hand, there is the SOTDMA (Self-Organized Time-Division Multiple Access) technology, which was used only by commercial vessels (Class A) but is now also authorized for pleasure boating at Class B level. With the SOTDMA method only the first free slot for the transmission has to be found
. The next five slots are always reserved automatically.

The following hierarchy results for the transmission of AIS data telegrams:

  1. Class A (commercial vessels) & SART (rescue transmitter)
  2. Class B SOTDMA (pleasure boating)
  3. Class B CSTDMA (pleasure boating)


The main advantages of the SOTDMA compared to the CSTDMA technology for pleasure boating (Class B) at a glace:

For more safety at sea - switch to SOTDMA Class B technology now!


Weatherdock awarded for Coastal Border Security

The German Weatherdock AG, leading manufacturer of emergency transmitter and electronic navigation devices all around “Tracking & Locating”, was awarded by a Caribbean Island to realize coastal border security by means of AIS technologies.

With this step the national authority gives a big and important component for national security. Maritime Domain Awareness, Coastal Surveillance and Offshore Border Control are only few items which are getting more and more important these days and can not be ignored any more.

“With this order we see confirmation in our constant efforts to stay in global leading position and to expand it. It is very important that the huge field of AIS capabilities will be brought into the decision makers minds”, say Alfred Kotuczek-Zeise, CEO and Speaker of the Board of Weatherdock.






Our easyRESCUE Rescue Transmitters have a basic approval and do not have to be registered anymore!

Each diver has certainly wondered what happens if he gets separated from his “Buddy” or dive group and in addition has no orientation where the dive boat is.

The easyRESCUE-DIVE is the solution to this problem.The small portable transmitter can be worn on the vest during the dive. Don’t panic, it is water-resistant to more than 100 meters!

If the case described above occurs, the easyRESCUE-DIVE is activated by the diver on water surface by the touch of a button. Via VHF radio the dive boat receives detailed information regarding the exact GPS position of the diver along with his/her Course Over Ground (COG) and Speed Over Ground (SOG) if he/she is in a current. This information is updated every minute. Just fix the unit at the carried along “ALERT BUOY” or reach you arm out of the water as high as possible.

  • Description: easyRESCUE-DIVE A040-D
  • Certification BSH: BSH / 4615 / 4361565 / 10 (for base unit easyRESCUE A040
  • Radiation Power: 2 Watt
  • Waterproof: 100% down to 100 meter diving depth
  • Operation temperature: -20°C – 65°C
  • Dimensions: Length: 125mm | Width: 68mm | Height: 30mm
  • Weight: 380 Gram
  • Battery lifetime: 10 Years (unit has to be disposed properly)
  • Radiated power: 2 Watt
  • Operation temperature: -20°C – 55°C
  • Dimensions: Length 128 mm
  • Width: 76mm Height: 32mm
  • Weight: 380 Gram
  • Battery lifetime: 10 years
  • Battery capacity in „Alert mode“: 45 hours
  • No registration
  • No VHF radio license
  • No additional costs

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