Radar and navigation
When using radar for position fixing and monitoring, the OOW should check:
· the overall performance of the radar;
· the identity of the fixed objects being observed;
· gyro error and accuracy of the heading line alignment;
· accuracy of the variable range markers (VRM), electronic bearing lines (EBL) and fixed range rings;
· that the parallel index lines are correctly set.
Parallel indexing
Parallel index techniques can be useful when monitoring the ship's progress in relation to the passage plan.
Parallel indexing does not fix the ship's position, but provides a method on the radar of verifying that the ship is maintaining a safe course to pass a fixed object, such as a headland, at the desired passing distance. Parallel indexing does not therefore replace the need to fix the ship's position on the chart at regular intervals.
The technique requires an index line to be drawn to pass through the radar echo of a fixed object, tangential to a VRM set to a range equal to the desired passing distance. The index line will line up parallel to the ground track that the ship will need to follow to maintain a safe passing distance.
Parallel indexing can be used on both relative motion and ground stabilised true motion modes of radar operation. With a relative motion display the echo of a fixed object will move ina direction and at a speed which is the reciprocal of own ship's ground track, and the echo should move along the index line. On a ground stabilised true motion display, the echo will remain stationary and the edge of the VRM should move along the index line as the ship passes the echo.
