Vessels required to have an Automatic Radar Plotting Aid must have a device to indicate what?

The correct answer highlights the necessity for vessels equipped with an Automatic Radar Plotting Aid (ARPA) to indicate the speed of the vessel, either over the ground or through the water. This speed data is crucial for effective navigation and collision avoidance, as it enables mariners to assess their movement in relation to other vessels and navigational hazards. By knowing the vessel's speed, operators can make informed decisions about course changes, timing, and safe distances from other moving objects.

Understanding the speed allows for better prediction of future positions and aids in radar tracking, enhancing overall situational awareness in busy waterways. This capability is fundamental for the effective use of ARPA, as timely and accurate responses to changing conditions are vital for operational safety and efficiency.

The other answer options, while they may relate to navigation, do not encompass the fundamental requirement for an ARPA system. An ECDIS generated trackline, the vessel's position, and AIS information are all valuable navigational tools, yet the specific need for speed data directly aligns with the operational capabilities ARPA is designed to support.