Furuno 1942 MARK-2 Sart Search and Rescue Transponder, Showing Sart marks on the radar display

4.5SART (Search and Rescue Transponder)

A Search and Rescue Transponder (SART) may be triggered by any X-Band (3 cm) ra- dar within a range of approximately 8 n.miles. Each radar pulse received causes it to trans- mit 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 rela- tively 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 inter- rogating 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 smaler than the original dots.

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

Showing SART marks on the radar display

To show the SART marks only on the radar display, detune the radar receiver manualy. This erases or weakens all normal radar ech- oes, but, the SART marks are not erased be- cause 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, blur- ring a large part of the screen. Reduce the sensitivity and adjust the sea clutter control of the radar.

Summary to detect SART response

1.Use range scale of 6 or 12 nm as the spacing between the SART responses is about 0.6 nm (1125 m) to distinguish the SART.

2.Turn off the automatic clutter suppression.

3.Turn off the Interference Rejector.

General remarks on receiving SART

Radar range scale

When looking for a SART it is preferable to use either the 6 or 12 nautical mile range scale. This is because the total displayed length of the SART response of 12 (or 24) dots may extend approximately 9.5 nautical

Echo of SART	Radar antenna
	beamwidth
24 NM	1.5 NM

Position of

SART

Lines of 12 dots are displayed in concentric arcs.

Echo of

SART

miles beyond the position of the SART and it is necessary to see a number of response dots to distinguish the SART from other re- sponses.

SART range errors

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

Low speed sweep signal

Sweep start

High speed sweep signal

Figure 4-5 SART display

When responses from only the 12 low fre- quency sweeps are visible (when the SART is at a range greater than about 1 nm), the position at which the first dot is displayed may be as mush as 0.64 nm beyond the true po- sition of the SART. When the range closes so that the fast sweep responses are seen also, the first of these will be no more than 150 meters beyond the true position.

4-3