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18
3.3 SART
Note: The SART (Search and Rescue
Transponder) information below is excerpted
from IMO SN/Circ 197 Operation of Marine
Radar for SART Detection.
A Search and Rescue Transponder (SART) may
be triggered by any X-Band (3 cm) radar within
a range of approximately 8 nm. Each radar
pulse received causes it to transmit a response
which is swept repetitively across the complete
radar frequency band. When interrogated, it first
sweeps rapidly (0.4 µs) through the band before
beginning a relatively slow sweep (7.5 µs)
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
interrogating 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.
When the range to the SART is reduced to
about 1 nm, the radar display may show also
the 12 responses generated during the fast
sweeps. These additional dot responses, which
also are equally spaced by 0.64 nm, will be
interspersed with the original line of 12 dots.
They will appear slightly weaker and smaller
than the original dots.
9500 MHz
9200 MHz
Radar antenna
beamwidth
Screen A: When SART
is distant
Screen B: When SART
is close
Lines of 12 dots
are displayed in
concentric arcs.
Echo of SART
Position of
SART
Own ship's
position
Own ship's
position
SART mark
length
Radar receiver
bandwidth
Sweep time
7.5 µs
95 µs
Sweep start
High speed sweep signal
Low speed sweep signal
24 NM
1.5 NM
Position of
SART
Echo of
SART
Figure 3-6 Appearance of SART signal
on the radar display
General procedure for detecting SART
response
1. Use the range scale of 6 or 12 nm as the
spacing between the SART responses is
about 0.6 nm (1125 m) to distinguish the
SART.
2. Turn off the automatic clutter suppression.
3. Turn off the Interference Rejector.
General remarks on receiving SART
SART range errors
When responses from only the 12 low frequency
sweeps are visible (when the SART is at a
range greater than about 1 nm), the position at
which the first dot is displayed may be as much
as 0.64 nm beyond the true position of the
SART. When the range closes so that the fast
sweep responses are seen also, the first of
these will be no more than 150 meters beyond
the true position.
Radar bandwidth
This is normally matched to the radar
pulselength and is usually switched with the
range scale and the associated pulselength.
Narrow bandwidths of 3-5 MHz are used with
long pulses on long range and wide bandwidths
of 10-25 MHz with short pulses on short ranges.
Any radar bandwidth of less than 5 MHz will
attenuate the SART signal slightly, so it is
preferable to use a medium bandwidth to ensure
optimum detection of the SART.