6. Radar
6-27
6.26.2 Search and Rescue Transponder (SART)
A vessel in distress uses a radar-SART to show a series of dots on the radar display
of nearby vessels to indicate distress. A SART transmits when it receives a radar
pulse from any X-Band (3 cm) radar within a range of approximately 8 nm. When a
SART receives a radar pulse, the SART transmits a response across all the radar fre-
quency band. The SART first sweeps quickly (0.4 μs) through the band. The SART
then slowly sweeps (7.5 μs) through the band back to the starting frequency. Each
sweep is repeated 12 times. At some point in each sweep, the SART frequency
matches that of the radar and is within the baseband of the radar receiver. If the SART
is within range, the frequency match during each of the 12 slow sweeps creates a line
of 12 dots, 0.64 nautical miles between dots.
When the range to the SART is reduced to approximately 1 nm, the radar display
shows the 12 responses created during the fast sweeps. The dots from the fast
sweeps, which are 0.64 nm from one another, appear with the 12 dots created from
the slow sweeps. The dots from the fast sweeps are weaker and smaller than the dots
from the slow sweeps.
How to detect the SART response
• Use the range scale of 6 or 12 nm. The distance between the SART responses is
approximately 0.6 nm (1125 m) to identify the SART.
• Turn off all the automatic clutter filters.
• Turn off the interference rejector.
9500 MHz
9200 MHz
Lines of 12
dots are
displayed in
concentric
arcs.
Echo from SART
Position of
SART
Own ship
position
Own ship
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 from
SART
Radar antenna
beamwidth
Screen A: When SART
is distant
Screen A: When SART
is close
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