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Radar Echo Overlay · 371
Radar Echo Overlay
Introduction
This ECDIS can have an optional Radar Echo Overlay. This means that the radar echo image can be transferred
from the radar unit into the ECDIS chart display.
This ECDIS has many features to support exact match in scale and orientation of the chart and radar echo image.
Exact match of the radar echo image and chart is an essential security feature. If the radar echo image and the chart
display match, then the mariner can rely on what he sees and the mariner also gets a very good confirmation that his
navigation sensors such as gyro and position receivers operate properly and accurately. However if the mariner is
unable to achieve exact match then that is a very strong indication that something is wrong and he should not rely on
what he sees.
This ECDIS can use radar echo image in several user selectable modes. "ECDIS only" mode operates without radar
echo image. "ECDIS and radar" mode operates using rules for the type approved ECDIS to visualise radar echo
image. "Radar over chart" mode draws radar echo image in top of the chart. This mode should be used temporarily
because legally the equipment is not an ECDIS which can legally replace paper chart when operating with "Radar
over chart". Fourth mode available is "Radar video only" which again should be used only temporarily to check pure
radar echo presentation, because the equipment is not an ECDIS which can legally replace paper chart when
operating with "Radar video only".
The radar echo image is transferred directly from radar transceiver so the range used in a connected ARPA radar
display does not change scale in the ECDIS. This ECDIS is nearly independent of the ARPA radar display settings.
The only ARPA radar display setting which has effect for the radar echo image of this ECDIS is the used pulse
length of the radar transceiver. ARPA radar display controls the radar transmitted and thus selects the used pulse
length. This ECDIS listens the radar receiver without any possibility to control radar transmitter. All other controls
such as Gain, STC, FTC, interference reject, echo stretch, echo trails are independently controlled in this ECDIS.
Selected scale of displayed chart defines also scale of radar echo overlay. When you change scale of chart also scale
of radar echo overlay is changed automatically. The maximum range of radar echo overlay is 24 nautical miles.
Radar echo overlay can be displayed even if the own ship is not located in displayed chart area if the range is less
than 24 NM.
The thumb rule for range of displayed area:
· Chart scale 1:10 000 is about equal to traditional radar range of 0.75 NM
· Chart scale 1:20 000 is about equal to traditional radar range of 1.50 NM