6. RADAR
6-27
6.30 ARPA Operation
The ARPA (Automatic Radar Plotting Aid) shows the movement of a maximum of 30
radar targets. The targets can be acquired manually or automatically. All 30 targets
can be acquired manually when the ARPA acquisition area is not active. If the ARPA
acquisition area is active, that total is equally divided between manual and auto acqui-
sition.
ARPA requires speed and heading data.
Note: The antenna unit DRS4DL does not have the ARPA function. With the DRS4D-
NXT, a maximum of 100 targets (Manual: 30, Automatic: 30, Doppler: 40) can be ac-
quired.
6.30.1 How to show or hide the ARPA display
1. Select [Targets] from the RotoKey menu on the chart plotter or radar display.
2. Select [ARPA] to show the ARPA display.
To hide the ARPA display, deselect [ARPA] at step 2.
WARNING
CAUTION
No one navigational aid should be relied
upon for the safety of vessel and crew.
The navigator has the responsibility to
check all aids available to confirm
position. Electronic aids are not a
substitute for basic navigational
principles and common sense.
· This auto plotter automatically tracks an
automatically or manually acquired radar
target and calculates its course and speed,
indicating them by a vector. Since the data
generated by the auto plotter are based on
what radar targets are selected, the radar
must always be optimally tuned for use
with the auto plotter, to ensure required
targets will not be lost or unwanted targets
such as sea returns and noise will not be
acquired and tracked.
· A target does not always mean a
landmass, reef, ships or other surface
vessels but can imply returns from sea
surface and clutter. As the level of clutter
changes with environment, the operator
should properly adjust the Sea, Rain and
Gain to be sure target echoes are not
eliminated from the radar screen.
The plotting accuracy and response of
this auto plotter meets IMO standards.
Tracking accuracy is affected by the
following:
· Tracking accuracy is affected by course
change. One to two minutes is required to
restore vectors to full accuracy after an
abrupt course change. (The actual amount
depends on gyrocompass specifications.)
· The amount of tracking delay is inversely
proportional to the relative speed of the
target. Delay is on the order of 15 - 30
seconds for high relative speed; 30 - 60
seconds for low relative speed.
Display accuracy is affected by the
following:
· Echo intensity
· Radar transmission pulsewidth
· Radar bearing error
· Gyrocompass error
· Course change (your ship or target)
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