Furuno 1943C Sart Search and Rescue Transponder, Appearance of Sart signal on the radar display

2. RADAR OPERATION

2.27.3SART (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 ∝ s) through the band before beginning a relatively slow sweep (7.5 ∝ s) 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.

Screen A: When SART	Screen B: When SART
is distant	is close

Echo from SART

24 NM

Position of

SART

Own ship position

9500 MHz

9200 MHz

Radar antenna beamwidth

1.5 NM

SART mark length

Sweep time 7.5 ∝ s

95 ∝ s

Lines of 12 dots are displayed in concentric arcs.

Echo from

SART

Own ship

position Position of

SART

Radar receiver bandwidth

Low speed sweep signal Sweep start

High speed sweep signal

Appearance of SART signal on the radar display

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