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4-3
4.5 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 n.miles.
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
∝s) through the band before beginning a rela-
tively slow sweep (7.5 ∝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.
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
smaler 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
s
95
s
Sweep start
High speed sweep signal
Low speed sweep signal
24 NM
1.5 NM
Position of
SART
Echo of
SART
Figure 4-5 SART display
Showing SART marks on the radar
display
To show the SART marks only on the radar
display, detune the radar receiver manualy.
This erases or weakens all normal radar ech-
oes, but, the SART marks are not erased be-
cause the SART response signal scans over
all frequencies in the 9 GHz band. When the
radar approaches the SART in operation, the
SART marks will enlarge to large arcs, blur-
ring a large part of the screen. Reduce the
sensitivity and adjust the sea clutter control
of the radar.
Summary to detect SART response
1. Use 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
Radar range scale
When looking for a SART it is preferable to
use either the 6 or 12 nautical mile range
scale. This is because the total displayed
length of the SART response of 12 (or 24)
dots may extend approximately 9.5 nautical
miles beyond the position of the SART and it
is necessary to see a number of response
dots to distinguish the SART from other re-
sponses.
SART range errors
When responses from only the 12 low fre-
quency 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 mush as 0.64 nm beyond the true po-
sition 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.