JRC JMA-7710-6 Instruction Manual page 37

Relative Vector and True Vector ·······························································
When collision avoidance is considered from the view point of the two aspects (prediction and
avoidance), the relative vector is necessary for the prediction and the true vector is necessary for
the avoidance to find aspects of other ships.
Fig. 1.10 shows the relations between the relative vector and the true vector.
The meanings of the both vectors are as follows:
The relative vector, first of all, enables us to easily understand the outlines of the CPA and the
TCPA. The merit of this vector is that the user can understand, at a glance, the degree of danger
that all ships shown on the radar scope will collide against each other.
The true vector enables us to easily know speeds and courses of other ships and to find all the
aspects at a glance. In the other words, we can find the traffic situations (such as transverse, out
sailing, parallel running, reverse running, etc.) stipulated in the Provision of Collision at Sea Act.
So, when finding a ship which is going to collide against the own ship, the user can find what rule
is applied and how to maneuver the own ship.
True vector
Fig. 1.10 True Vector and Relative Vector
Radar and Collision Avoidance ································································
Radar is still playing an important role for prevention of collision as well as for measuring positions.
A plotter is used to improve functions of radar. This plotter is used to check movements of other
ships by plotting their positions at intervals of 3 or 6 minutes. The plotting enables the user to
know traces of these ships in relation with the own ship and the possibility of collision, or CPA
and TCPA. This method is considered effective but should be executed by person, and the number
of target ships is limited and the execution takes some minutes.
V
θ
V
V
θ
θ
θ
Relative vector
1 – 16
1.6
: Own ship speed
O
: Own ship course
O
: Target ship true speed
T
: Target ship relative speed
R
: Target ship true course
T
: Target ship relative course
R
: Aspect
A
1