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2
.
16-
A
Buoy
B
Own ship
Target on colli-
sion course
Buoy
A
B
Own ship
Target on collision course
(a) True vectors in head-up mode (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 extension passes through own ship is on the collision course. (Dotted lines in the
figure are for explanation only.)
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.
1. Press the PLOT MENU key to display the ARPA 1 menu.
2. Press the [1] key to select the VECTOR TIME option.
3. Further press the [1] key to select (or highlight) a desired vector time.
4. Press the ENTER key to conclude your selection.
5. Press the PLOT MENU key to close the menu.
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 target.
R/N/W version: ARPA vector time can be programmed onto the function key. (e.g. pressing
the function key can change the vector time.)
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 number 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.