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-3.4-
Shadow sectors (see above right)
Funnels, stacks, masts, or derricks in the path of the antenna block the radar beam. If the angle
subtended at the scanner is more than a few degrees, a non-detecting sector may be produced.
Within this sector targets can not be detected.
3.3 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 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.
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
Showing SART marks on the radar display
To show the SART marks only on the radar display, detune the radar receiver by turning the
TUNE control out of best tuning condition. This erases or weakens all normal radar echoes,
but, the SART marks are not erased because 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, blurring a large part of the screen. Reduce the sensitivity and
adjust the sea clutter control of the radar.