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Relative Vector and True Vector
From two points of view, collision prediction and avoidance, it is necessary to obtain the relative vector of other
ship for prediction and the true vector of other ship for collision avoidance in order to grasp other ship's aspect.
The relationship between the relative vector and true vector is shown in Fig. 5-3.
Both rough CPA and TCPA can be obtained easily from the relative speed vector of other ship. This method has
an advantage that the risks of collision with all other ships within the radar range can be seen at a glance. On
the other hand, the course and speed of other ship can easily be obtained from its true speed vector, enabling
other ship's aspect to be seen at a glance. Thus, the aspects of other ships (transverse, outsail, parallel run,
reverse run, etc. ) as described in the act of prevention of collision at sea can be readily grasped. If there is a
risk of collision with other ship, the operator can determine which rule to be applied and how to operate own
ship.
Target ship true vector
N
O
V
o
CPA
Radar and Collision Avoidance
Radar is still playing an important roll for collision prevention and positioning. A plotter is used to further
enhance the radar functionality. The plotter is capable of plotting other positions of other ships in 3 to 6 minute
intervals to monitor their movement. The plots of other ships represent their tracks relative to own ship, and it is
shown whether there is a risk of collision, namely CPA and TCPA can be obtained. This method using a plotter
is fairly effective, but the number of target ship, which is manually plotted, is limited and it takes several
minutes to measure those.
N
V
T
V
R
Own ship true vector
Fig. 5-3 Relative Vector and True vector
5 5
V
: Own ship's speed
O
: Own ship's course
O
V
: Other ship's true speed
T
: Target ship's true course
T
V
: Target ship's relative speed
R
: Target ship's relative course
R
T
: Aspect
A
A
R
Relative vector
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