What indicates a target may be on a collision course in ARPA?

In ARPA (Automatic Radar Plotting Aid), a key indicator that a target may be on a collision course is the information provided by the Closest Point of Approach (CPA) data. CPA is the point at which two vessels will come the closest to each other if their trajectories remain unchanged. When the CPA value is calculated and presented, it indicates not only how close the vessels will come to one another but also in what direction they are moving relative to each other. A CPA of zero means that the targets will intersect, suggesting a collision course.

Monitoring the CPA data is crucial for navigators, as it allows them to assess the risk of collision and decide whether evasive action is needed. This proactive approach helps ensure safe navigation in crowded waters or when encountering other vessels. Therefore, understanding the CPA data is fundamental in predicting potential collisions and taking timely measures to avoid them.