Parallel index (PI)

Parallel indexing is a radar technique that monitors a vessel’s cross-track position by watching a fixed radar-conspicuous object track along a pre-drawn line, the index line, parallel to the planned ground track and offset by the intended passing distance. With the radar in a stabilized presentation, the echo of a headland, buoy, or beacon should run down the index line if the ship stays on track; any drift off the line shows set or steering error at a glance. Index lines, clearing ranges, and wheel-over points are drawn during passage planning under SOLAS Chapter V voyage-planning practice.