AIS course and speed information conveys dynamic movement.
It uses two specific metrics: course over ground (COG) and speed over ground (SOG). COG is supplied when the connected position sensor calculates it, while SOG is supplied by the same connected position sensor. These values must not be equated with heading, course through the water, or speed through the water, as those quantities answer different navigational questions.
COG or SOG may be unavailable, and poorly configured or calibrated position, speed or heading sensors can cause incorrect information to be transmitted. The officer of the watch should periodically check own-ship position, speed over ground and sensor information; the built-in integrity test does not verify the accuracy of sensor data before broadcast.
MCA guidance distinguishes AIS target course and speed over ground from the course and speed through the water relevant to applying the COLREG, and says collision-avoidance decisions should be based primarily on systematic visual and/or radar observations.
