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2. RADAR OPERATION
2-38
Radar
antenna
Radar
mast
Shadow sector
Funnels, stacks, masts, or derricks in the path of the
antenna block the radar beam. If the angle subtended
at the antenna is more than a few degrees, a
non-detecting sector or blind spot may be produced.
Within this sector, targets can not be detected.
2.26.2 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
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
Appearance of SART signal on the radar display