Furuno FR-1710 manual Sart Search and Rescue Transponder, Virtual image, Shadow sectors

Target A

Target B

(Spurious) Target B

(True)

Figure 2-3 Sidelobe echoes

Virtual image

A relatively large target close to your ship may be represented at two positions on the screen. One of them is the true echo directly reflected by the target and the other is a false echo which is caused by the mirror effect of a large object on or close to your ship as shown in the figure below. If your ship comes close to a large metal bridge, for example, such a false echo may temporarily be seen on the screen.

Target ship

Own ship

True echo

Mirror image	False
of target ship	echo

Figure 2-4 Virtual image

Shadow sectors

Funnels, stacks, masts, or derricks in the path of the scanner 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.

Radar antenna

Radar mast

Shadow sector

Figure 2-5 Shadow sectors

2.3SART (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 nm. 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.

2-3