Your ARPA has two guard zones. What is the purpose of the inner guard zone?

The inner guard zone in an ARPA (Automatic Radar Plotting Aid) system is primarily designed to provide early warnings about targets that are detected within a certain boundary. By identifying small targets that are initially detected closer than the outer guard zone, the inner guard zone serves to enhance situational awareness for watch officers. This allows for timely responses to potential threats that may not be immediately apparent from farther away.

When targets enter this inner zone, they are flagged for immediate attention, helping to prevent collision or other navigational incidents by giving the navigator crucial extra time to assess and react. By focusing on the targets that enter this area, the system ensures that the vessel is well-informed about all potential risks in its vicinity, particularly those that could be smaller or harder to detect until they are very close.

This also complements the outer guard zone, which serves a different purpose, typically related to monitoring targets as they approach the vessel but at a greater distance. Thus, the inner guard zone plays a vital role in the layered defense strategy of maritime navigation, ensuring the safety and efficiency of vessel operations.