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2-11
2.10 Mode and Length of
Vectors
True or relative vector
Target vectors can be displayed relative
to own ship's heading (relative) or with
reference to the north (true).
Press the VECTOR TRUE/REL key to
select true or relative vectors. This fea-
ture is available in all presentation modes
(gyrocompass must be working correctly).
The current vector mode is indicated at
the upper-right corner of the screen.
True vector
With true vectors the radar display will look
like the one shown below in head-up
mode.
In the true motion mode, all fixed targets
such as land, navigational marks and
ships at anchor remain stationary on the
radar screen with vector length zero. But
in the presence of wind and/or current,
true vectors appear on fixed targets rep-
resenting the reciprocal of set and drift
affecting own ship unless set and drift
values are properly entered (see para-
graph 2.12).
A
Buoy
A
B
Own ship
Target on colli-
sion course
(a) True vectors in head-up mode
Buoy
B
Own ship
Target on colli-
sion course
(b) Relative vectors in head-up mode
Relative vector
With relative vectors the radar display will
look like (b).
Relative vectors on targets which are not
moving over the ground such as land,
navigational marks and ships at anchor
will represent the reciprocal of own ship’s
ground track. A target of which vector ex-
tension passes through own ship is on the
collision course. (Dotted lines in the fig-
ure are for explanation only.)
ORIGIN
MARK
VECTOR
TRUE/
REL
VECTOR
TIME
TARGET
DATA
TARGET
BASED
DATA
AUTO
PLOT
TRIAL
LOST
TARGET
HISTORY
Vector time
Vector time (or the length of vectors) can
be set to 30 seconds, 1, 2, 3, 6, 12, 15 or
30 minutes and the selected vector time
is indicated at the upper-right corner of
the screen.
Press the VECTOR TIME key to select
desired vector time. The vector tip shows
an estimated position of the target after
the selected vector time elapses. It can
be valuable to extend the vector length to
evaluate the risk of collision with any tar-
get.
2.11 Past Position Display
The ARPA displays equally time-spaced
dots marking the past positions of any
targets being tracked.
A new dot is added every minute (or at
preset time intervals) until the preset num-
ber is reached. If a target changes its
speed, the spacing will be uneven. If it
changes the course, its plotted course will
not be a straight line.