Search And Rescue Transponder (Sart) - Furuno TZT9 Operator's Manual

6. RADAR
Shadow sector
Funnels, stacks, masts, or derricks in the path of the antenna stop the radar beam. If
the angle opposite the antenna is more than one or two degrees, a sector or shadow
sector appears on the screen. Targets are not displayed within the sector.
Radar position
Large ship
Sidelobe echoes
Each time the radar transmits, some radiation leaves on each side of the beam. This
lost energy has the name "sidelobes." If a target is detected by both the sidelobes and
the mainlobe, the sidelobe echoes can appear at both sides of the true echo at the
same range. Sidelobes normally show only on short ranges and from strong targets.
You can reduce sidelobes if you reduce the gain, or correctly adjust the sea clutter.
6.23.2

Search and rescue transponder (SART)

A ship in distress uses a radar-SART to show a series of dots on the radar display of
nearby ships to indicate distress. A SART transmits when it receives a radar pulse
from any X-Band (3 cm) radar within a range of approximately 8 nm. When a SART
receives a radar pulse, the SART transmits a response across all the radar frequency
band. The SART first sweeps quickly (0.4 μs) through the band. The SART then slowly
sweeps (7.5 μs) through the band back to the starting frequency. Each sweep is re-
peated 12 times. At some point in each sweep, the SART frequency matches that of
the radar and is within the baseband of the radar receiver. If the SART is within range,
the frequency match during each of the 12 slow sweeps creates a line of 12 dots, 0.64
nautical miles between dots.
When the range to the SART is reduced to approximately 1 nm, the radar display
shows the 12 responses created during the fast sweeps. The dots from the fast
sweeps, which are 0.64 nm from one another, appear with the 12 dots created from
the slow sweeps. The dots from the fast sweeps are weaker and smaller than the dots
from the slow sweeps.
6-18
Wharf and its echo
Shadow sector
occurs because
wharf is hidden
behind ship.
Mainlobe (beam)
Sidelobe Sidelobe
Antenna
Radar position
Size of blind sector depends on
size of obstruction and range.
False echoes by sidelobes
Wharf and its echo
Shadow sector occurs
because obstruction
(like mast) is in path of
radar beam.
True target