Furuno 1824C manual Sart Search and Rescue Transponder, Shadow sector

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. RADAR OPERATION

Radar antenna

Radar mast

2.27.2 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 ∝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
		Lines of 12
		dots are
Echo from SART Radar antenna		displayed in
		concentric
beamwidth		Echo from
24 NM	1.5 NM
		SART

Position of

SART

Own ship
position		Own ship
	SART mark	position	Position of
			SART
9500 MHz	length
		Radar receiver
9200 MHz	Sweep time	bandwidth
	7.5 ∝s
	95 ∝s

Low speed sweep signal

Sweep start

High speed sweep signal

Appearance of SART signal on the radar display

2-35