The following is drawn from MGN 379 with my amplification:
It is important to select the optimum stabilisation mode for the radar display. To assess risk of collision the relative motion of a target gives the clearest indication of CPA and may be monitored by observing either the direction of the target’s relative trail, or the CPA predicted by the relative vector.
By default, relative motion will display relative target trails and true motion will display true target trails.
Where true target trails is selected, a sea stabilised display will indicate all targets’ motion through the water. A ground stabilised display will indicate all targets’ motion over the ground.
IN THIS CASE TRUE VECTORS WILL NOT INDICATE THE TRUE ASPECT OF THE TARGET AND MAY LEAD TO A FALSE UNDERSTANDING OF THE SITUATION IN RESPECT OF IRCPS.
In coastal, estuarial and river waters where a significant set and drift may be experienced, a sea stabilised display will produce significant target trails from all fixed (stationary) objects possibly producing an unacceptably high level of clutter and masking. In such circumstances a ground stabilised display may reduce its effect and enable the observer to detect clearly the trails of moving targets, thus enhancing the observer’s situational awareness.
It should be noted that the observed and predicted relative motion of a target is unaffected by the choice of sea or ground stabilisation, allowing the same assessment of CPA and risk of collision. If switching between sea and ground stabilisation, the observer should be aware of the time required for the radar equipment to reprocess the stabilisation input data.