Nautical Institute Queensland Branch

Concentric Indexing - a straight forward approach to controlled turns by Captain Paul Chapman

Radar presentation		Use of stabilised electronic bearing line
Procedure for performing a planned turn		North up presentation
Planning considerations		Conclusion
Error sources and limitations		Resources
Adapting the technique to the radar available

Radar presentation
When a vessel follows the circular path of a turn, fixed targets displayed on a radar in the head up orientation will follow a predictable track concentric to the turn’s centre. (see figure 1)


Figure 1 - Concentric movement of fixed targets on a head up display during a turn


Notable features of the head up orientation during a turn are: A target at the turn centre will remain stationary. All other fixed targets will follow a circular path around the turn centre. The amount of movement of a target is proportional to its distance from the turn centre. The characteristics of head up orientation during a turn may be used for planning, executing and monitoring turns. This particularly applies to radars that allow the variable range marker (VRM) ring to be offset. Many radars have this ability coupled with an unstabilised display and the presentation of target wakes or trails.