2. RADAR OBSERVATION
2-5
2.3 SART (Search and Rescue Transponder)
2.3.1 SART description
A Search and Rescue Transponder (SART) may be triggered by any X-band (3 cm wavelength)
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 interro-
gated, 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 dis-
play, thus a line of 12 dots equally spaced by about 0.64 nautical miles will be shown.
When the radar to the SART is reduced to about 1 nm, the radar display my show also the 12
responses generated during the fast sweeps. These additional dot responses, which also are
equally spaced by 0.64 nautical miles, will be interspersed with the original line of 12 dots. They
will appear slightly weaker and smaller than the original dots.
Appearance of SART echo
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 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
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