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2. RADAR OPERATION
2-48
2.26.3 SART (Search and Rescue Transponder)
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
Appearance of SART signal on the radar display