What is the role of a Parallel Index (PI) in radar operations?

The Parallel Index (PI) is a navigational tool used in radar operations to enhance navigational safety, particularly during turns and when navigating in confined waters or near hazards. Its primary function is to provide a method for determining when to initiate a turn.

When operating a radar, a navigator can establish a PI line parallel to the intended course. This allows for continuous monitoring of the vessel's position relative to the PI. When the vessel reaches the PI, it indicates that the navigator should start the turn. This systematic approach minimizes the risk of overshooting the turn and helps ensure that the vessel remains on a safe course, especially in challenging navigational areas.

The other options focus on aspects that do not directly relate to the specific function of a Parallel Index. For instance, measuring ship speed pertains to speed calculations that do not involve PI. Similarly, calculating distance from the shore and plotting a course on a map rely on different navigational methods and tools, such as distance measuring instruments and chart plotting techniques, which are separate from the primary use and purpose of a Parallel Index in radar navigation.