AIS overview

AIS (Automatic Identification System) transceivers use digital radio signals to exchange 'real-time' information between vessels, shore based stations, or aids to navigation (AToNs) on dedicated VHF frequencies. This information is used to identify and track vessels in the surrounding area and to provide fast, automatic and accurate collision avoidance data. AIS technology achieves this by continuously transmitting a vessel’s identity, position, speed and course, along with other relevant information to all other AIS-equipped vessels within range.

AISOverview

Although AIS technology augments your Radar app by operating in Radar blind spots and detecting smaller AIS-fitted vessels, it does not replace your Radar, as it relies on receiving transmitted AIS information, and therefore cannot detect objects such as landmasses, navigational beacons or vessels not equipped with AIS.

Note

NEVER assume that AIS is displaying information from all vessels in the area, because:

  • Not all vessels are fitted with AIS.

  • Although it is mandatory for larger commercial vessels to carry AIS equipment, it is not mandatory to use it.

  • Most Class B AIS transponders only have a transmission power output of 2 W, which restricts Class B transmissions to a maximum range of 8-10 nautical miles, and means that transmissions from vessels using traditional Class B AIS transponders are often not received by the AIS satellites that provide global vessel tracking. (Please note, however, the AIS700 uses SOTDMA technology with a 5 W power output, providing an increased transmission range of approximately 10-12 nautical miles.)

AIS should be used only to augment Radar information, not to substitute it.

CSTDMA vs SOTDMA

SOTDMACSTDMA

The AIS700 uses SOTDMA (Self-Organized Time-Division Multiple Access) communication technology, which has significant advantages over the traditional CSTDMA (Carrier-Sense Time-Division Multiple Access) technology used in many Class B AIS transponders:

  • With its 5 W transmission power output, SOTDMA provides an increased transmission range of approximately 10-12 nautical miles.

  • SOTDMA provides Guaranteed Time Slot Allocation in its AIS messaging scheme, which means that the transmissions from this device will take priority over standard Class B transponders, when multiple transponders are attempting to reserve the same transmission time slot.

  • SOTDMA provides an increased update speed of up to every 5 seconds, compared to every 30 seconds for a Class B transponder using CSTDMA technology. This means that, as your vessel’s speed increases, the number of AIS transmissions increases compared to a vessel using the older CSTDMA technology, giving other vessels a clearer and more up-to-date view of where your vessel is. This is particularly useful for fast-moving planing vessels. For example, a vessel travelling at 23 knots will move 360 metres in the 30 seconds it takes one of the older Class B CSTDMA transponders to transmit its next AIS signal (or 0.5 nautical miles at 30 knots). By comparison, on a boat equipped with a Class B SOTDMA transponder travelling at 23 knots or more, the update rate is just 5 seconds, meaning that your AIS position would be transmitted every 60 meters travelled.