Raymarine E-Series, Справочное руководство

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Chapter 8: Using the Radar (Digital) 209
• Choppy sea state conditions exist yielding poor stability.
• Inadequate heading data exists.
Symptoms of such conditions are that acquisition is difficult and the MARPA vectors
are unstable; the symbol wanders away from the target, locks on to the wrong
target or changes to a lost symbol target.
If any of these conditions are present, acquisition and tracking may need to be re-
initiated or, in some cases be impossible to maintain. Improving the quality of the
heading data will reduce the effect of the other conditions.
To acquire a target
MARPA automatically tracks acquired targets, calculates target bearing and range,
speed and course, Closest Point of Approach (CPA), and Time to Closest Point of
Approach (TCPA).
Each target tracked (up to 25 MARPA targets using a digital Radar) can be displayed
with a CPA graphic which shows the target vessel, course and speed (as a vector)
and indicates the CPA. The calculated target data can also be shown on your screen.
Each target is continually assessed and an audible alarm is sounded if a target
becomes dangerous, or is lost.
Effective MARPA operation is dependant on accurate own-ship’s heading and
speed. Speed Over Ground (SOG) and Course Over Ground (COG) information are
required to show true target course and speed. The better the quality of the heading
and speed data, the better MARPA will perform. MARPA will function without SOG
and COG in relative mode. For the best heading data a Raymarine SMART heading
sensor or a gyro-stabilized autopilot is required.
Risk assessment
Each target is monitored to see if it will be within a certain distance from your boat
within a certain time. If so, the target is designated as dangerous and an audible
warning is sounded along with an on-screen warning being shown. The target
symbol changes to the dangerous target symbol and flashes to indicate that it is a
dangerous target. Pressing the appropriate soft key will silence the alarm and
remove the warning.
If a target is lost, either because the MARPA software has lost contact with it, or
because it has moved out of range, an audible alarm is sounded and an on-screen
warning appears. The on-screen symbol will change to the target lost symbol.
Pressing the appropriate soft key will silence the alarm and remove the on-screen
warning and the target lost symbol.
MARPA range
MARPA target acquisition is only available at radar range scales of up to 12nm,
although tracking continues at all ranges.
81244_4.book Page 209 Thursday, January 31, 2008 1:53 PM