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Chapter 2 OPERATIONS
2.15 SETTING TT/AIS
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. 2.15-3.
Furthermore, the meanings of both vectors are described.
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.
Fig. 2.15-3 Relative Vector and True Vector
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 are
manually plotted, is limited and it takes several minutes to measure those.
2-122
N
V
T
T
A
R
V
R
Relative vector
Own ship true vector
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
: Aspect
A
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