What is the primary device required for vessels with an Automatic Radar Plotting Aid to indicate?

The primary device required for vessels equipped with an Automatic Radar Plotting Aid (ARPA) to indicate is the speed of the vessel over the ground or through the water. This is essential because ARPA systems depend on accurate vessel speed inputs to calculate the vessel's position, trajectory, and to provide the mariner with real-time information about other vessels in proximity. The system uses the vessel’s speed, along with radar data, to predict potential collisions and assist in navigation by tracking targets, providing vital situational awareness, and enhancing safety at sea.

Other options do not align with the primary functionalities of ARPA systems. For example, while knowledge of the distance to the next port and timing of navigational satellite passes are important for navigation, these are not directly indicated or calculated by an ARPA system. The focus of ARPA is primarily on vessel movement and navigation safety, established through metrics like speed.