User's Manual
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JMA-9100 Instruction Manual > 5.OPERATION OF TARGET TRACKING AND AIS > 5.1 PREPARATION
5.1.1.4 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.
Fig 5-3: Relative Vector and True vector
5.1.1.5 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
θ
T
θ
R
θ
A
θ
O
V
o
V
R
V
T
V
O
: Own ship's speed
θ
O
: Own ship's course
V
T
: Other ship's true speed
θ
T
: Target ship's true course
V
R
: Target ship's relative speed
θ
R
: Target ship's relative course
θ
A
: Aspect
Relative vector
Target ship true vector
N
CPA
Own ship true vector