3 sart (search and rescue transponder), Shadow sectors, Virtual image – Furuno FR-1710 User Manual
Page 53
2-3
Shadow sectors
Funnels, stacks, masts, or derricks in the
path of the scanner block the radar beam. If
the angle subtended at the scanner is more
than a few degrees, a non-detecting sector
may be produced. Within this sector targets
can not be detected.
Radar
antenna
Radar
mast
Shadow sector
Figure 2-5 Shadow sectors
2.3 SART (Search and Rescue
Transponder)
A Search and Rescue Transponder (SART)
may be triggered by any X-Band (3 cm) ra-
dar within a range of approximately 8 nm.
Each radar pulse received causes it to trans-
mit a response which is swept repetitively
across the complete radar frequency band.
When interrogated, it first sweeps rapidly (0.4
µ
s) through the band before beginning a rela-
tively slow sweep (7.5
µ
s) through the band
back to the starting frequency. This process
is repeated for a total of twelve complete
cycles. At some point in each sweep, the
SART frequency will match that of the inter-
rogating radar and be within the pass band
of the radar receiver. If the SART is within
range, the frequency match during each of
the 12 slow sweeps will produce a response
on the radar display, thus a line of 12 dots
equally spaced by about 0.64 nautical miles
will be shown.
Target B
(True)
Target B
(Spurious)
Target A
Figure 2-3 Sidelobe echoes
Virtual image
A relatively large target close to your ship may
be represented at two positions on the
screen. One of them is the true echo directly
reflected by the target and the other is a false
echo which is caused by the mirror effect of
a large object on or close to your ship as
shown in the figure below. If your ship comes
close to a large metal bridge, for example,
such a false echo may temporarily be seen
on the screen.
True
echo
False
echo
Own
ship
Target ship
Mirror image
of target ship
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Figure 2-4 Virtual image