Difference between True Motion & Relative Motion?

Relative Motion

 True Motion

1. All objects move relatively on the PPI. Stationary targets move on the screen with own ship’s reversed course & speed. Hence all objects produce relative trails on the screen, whether actually moving or not.

Objects move on the PPI on their actual courses and speeds. Stationary targets remain stationary on the screen. Hence moving targets create trails on the screen whereas stationary targets do not.

2. Course, speed, & aspect of any target only  available after plotting relative velocity triangle.

Course, speed, and aspect of any target directly obtained.

3. CPA range and time directly obtained

CPA range and time obtained only after plotting relative velocity triangle.

4. The lines of approach (LOA) of various targets immediately visible.

The line of approach of a target not readily visible.

5. Targets on collision course are easily identified and so further manual plotting is confined to only such targets.

(Automatic Radar Plotting Aid (ARPA) is a great help in overcoming the only drawback of TM – obtaining CPA range and time.) With ARPA, TM is very comfortable to use and more efficient than RM

6. Easier to use, for collision avoidance, in open sea as CPA range is directly available.

Easier to use in coastal navigation because the target’s alterations of course for navigational reasons can be anticipated & appreciated easily.