Parallel Indexing (PI) is a useful method of monitoring cross track tendency in both and good visibility. It is good practice to inconspicuously marked the planned PI on the chart at the chart planning stage. Like any radar technique, it is advisable to practice using PIs extensively in good visibility before placing total reliance on them when thick weather makes visual navigation methods impossible.
This simple effective method of continuous monitoring the ships progress is carried out by observing the movement of the echo of the radar conspicuously navigation mark with respect to track lines previously prepared on a reflection plotter by using ARPA index lines. It is most effective when the radar is in the North Up relative motion mode ship centered.
A fixed radar target, such as a lighthouse or a headland, will apparently track the own ship, depicted as being at the center of the screen, on a line parallel and opposite to the ships ground track. Any cross track tendency, such as maybe caused by a tidal stream, will become apparent by the target moving off the parallel line.
The parallel index may also be used to monitor other events, like a wheel over position. In this case the range and bearing of the target at the wheel over point and the new track is marked on the PI. This also allows for a distance countdown to be made and the new track can be subsequently monitored.