AIS course and speed data may be unavailable or incorrect due to sensor errors, differs from water-relative values, and remains supplementary.
Course over ground (COG) and speed over ground (SOG) are dynamic elements transmitted by the Automatic Identification System. COG is supplied when the connected position sensor calculates it, and SOG is supplied by the connected position sensor. Consequently, poorly configured or calibrated position, speed or heading sensors can cause incorrect information to be transmitted. In some cases, COG or SOG may be unavailable.
It is critical to distinguish between over-ground values and those relative to the water. MCA guidance distinguishes AIS target course and speed over ground from the course and speed through the water relevant to applying the COLREG. These quantities answer different navigational questions.
On a suitable display, received AIS motion data can support target tracking and calculations such as closest point of approach and time to closest point of approach, while remaining supplementary information. MCA guidance says collision-avoidance decisions should be based primarily on systematic visual and/or radar observations.
