Parallel indexing is an effective way of monitoring a vessels progress along a preselected track using a radar.
As a vessel moves on its chosen heading, fixed objects in its radar vicinity appear to be moving in a reciprocal direction to this motion. This technique provides the radar observer with a real time view of the ship’s lateral position relative to the planned track. The approach can be used in all conditions of visibility to monitor the vessels cross track movement.
The following must be confirmed before employing the radar in this way:
- The performance of the radar must be checked at regular intervals.
- Gyro errors must be checked
- Heading marker should be in alignment with the ship’s fore and aft line.
- The accuracy of range rings and the variable range marker (VRM) should be checked against a good fix.
- The fixed target that is to be tracked must be correctly identified.
Parallel indexing provides an active method of assessing the vessel’s progress continuously and immediately without the need for visual bearings that provide historic data due to the associated delay of assessment.
Parallel indexing is particularly useful in reduced visibility , areas of high traffic density, coastal and pilotage stages of a voyage.
The vessel is shown to be maintaining the required track, with the target (point of land) . The ship is following a ground track of 090°T.
The ship is shown to be setting away from the shore. The point of land is moving outside the PI Line. The ship is following a ground track of 100°T.
The ship is shown setting towards the shore. The point of land is moving inside the PI line. The ship is following a ground track of 080°T.