Furuno Marine Radar - Manual (Page 15)

Interference Rejection: This control reduces or
eliminates interference received from another ship’s
Radar. Turn it off when no Radar interference exists.
Guard Alarm: The guard alarm creates a zone
about own ship, either complete 360 degree zone or
a specific area forward of own ship. If targets enter
or exit the zone an audible alarm sounds to alert the
operator.
Echo Averaging: The Radar’s internal circuitry
processes echo data to obtain a desired effect. The
result depends on the Radar model. For example,
some Radars may suppress brilliance of unstable
echoes (sea clutter, etc.), or emphasize an unstable
small echo.
Huer, Color, Background Color: These
settings change display color and background color
respectively to improve display visibility. Note that
marks and characters also change color when the keys
are pressed.
HM Off: The heading mark may sometimes hide a
small echo. To show that echo, press and hold down
the HM OFF key to temporarily erase the heading
mark . Release the key to display the heading mark.
Echo Stretch: This function ‘stretches’ small
echoes to make them easier to see. It stretches not
only small echoes but also returns from sea and
rain clutter and Radar interference. For this reason,
make sure clutter and interference are sufficiently
suppressed before using echo stretch.
NAV: This key shows/hides navigation data,
including position, range and bearing to waypoint,
ship’s speed and more. This feature usually requires a
navigation aid which can output data in NMEA0183
or NMEA2000 format. If a gyrocompass is connected
to the Radar, a dashed line connects navaid-selected
waypoint with own ship’s position.
Figure 9 shows a sample Radar display. Own ship’s position is at the display center. The Radar range is 12
nautical miles and the range ring interval is 2 nautical miles. The circled objects are ARPA targets and the
triangle objects are AIS targets. The large, continuous echoes are from land masses. Note that the actual shape
of a target cannot be displayed on the Radar - only the portions struck by the radio pulse appear on the display.
Other Aids To Navigation
The Radar display sometimes shows a series of
dots or dashed radial lines. They are navigational
marks transmitted by Radar beacons. A Radar
beacon transmits a coded signal when it receives a
radio pulse from a Radar, and some Radar beacons
transmit continuously. Its main purpose is to help the
navigator find his own position in terms of range and
bearing from the beacon. There are two main classes
of Radar beacons: RACON and RAMARK. Another
type of Radar beacon is called a Search And Rescue
Transponder, or SART. When activated by a crew
member on a ship in distress it transmits a distinctive
signal when its transmitter is triggered by a radio
pulse.
RACON
The RACON (RAdar beaCON) automatically
transmits a signal when it receives a radio pulse.
The signal transmitted by the RACON appears
intermittently on the display as a Morse character, a
dashed or dotted line radiating out from the beacon.
In the U.S., RACONs are used to mark lighthouses
and buoys, inconspicuous coastlines, navigable
spans under bridges, offshore structures such as oil
platforms, or environmentally sensitive areas such as
coral reefs.
RAMARK (Radar Marker)
The RAMARK is a Radar beacon which transmits
either continuously or at intervals. When a Radar
scanner faces a RAMARK it receives the RAMARK
signal. The RAMARK signal appears on the display
as a Morse character, a dashed or dotted radial line.
RAMARKs are not commonly used in the U.S.
SART (Search And Rescue Transponder)
When a Radar transponder is activated by a ship in
distress, its transmitter emits a signal when activated
by a radio pulse. The appearance of the signal on the
display depends on the distance between the Radar
and the transponder. The closest transponder signal to
own ship position is the approximate position of the
ship in distress.
In the GMDSS (Global Maritime Distress and Safety
System), certain classes of vessel must carry a SART.
You should continuously monitor the display for
Radar transponder signals. Be prepared to offer
assistance to a ship in distress if required.
False Echoes
Occasionally false echoes appear on the screen at
positions where there is no target. In some cases the
effects can be reduced or eliminated. The operator
should become familiar with the appearance of these
false echoes so as not to confuse them with echoes
from legitimate contacts.
Indirect echoes
Indirect echoes may be returned from either a passing
ship or from a reflecting surface on your own ship,
such as a stack. Figure 12 illustrates the effects of an
indirect echo. Indirect echoes may be recognized as
follows:
They usually occur in shadow sectors •
They appear on the bearing of the obstruction but •
at the range of the legitimate contact
When plotted, their movements are usually
•
abnormal
Their shapes may indicate that they are not direct
•
echoes
8 13
Figure 12 - Example of Indirect Echo