Furuno FAR-1513 Operator's Manual page 122

3. TARGET TRACKING (TT)
Non-synchronous emissions
No effect.
Second trace echoes
When the radar beam is super refracted, strong echoes may be received at such long
ranges that they appear on a different timebase sweep than the transmitted pulse.
This gives an incorrect range indication. Second and third trace echoes can be tracked
if they are consistent enough to meet acquisition and tracking criteria but target course
and speed data will be in error.
Blind and shadow sectors
Radar shadow or blind areas caused by obstructions aboard ship, for example, fun-
nels and masts, in the path of the radar beam can result in reduction of radar beam
intensity in that particular direction. This may eliminate the detection of some targets.
The TT system will lose track of targets shortly after they are lost on the radar picture
and if they remain in a blind zone. These targets will however be acquired and tracked
when they pass out of the blind zone and again present normal radar echo. The an-
gular width and bearing of any shadow sector should be determined for their influence
on the radar. In certain cases false echoes in the shadow sector cause the TT system
to acquire, track, and vector them. Shadow sectors should be avoided.
Indirect echoes
A target at close range is usually picked up directly, but it can also be received as re-
flection from a large, flat surface. This will result in the radar presenting two or more
echoes on the display, each at a different range. The TT can acquire and track the
false echo if it is detected by five consecutive scans. Reduction in radar gain can elim-
inate the multiple echoing but care should be taken as range detection also will be re-
duced.
Radar interference
If interference is extreme due to another radar operating at close range, spiral "dotting"
and/or false targets may appear momentarily. The interference rejector can clear the
display.
To receive radar beacon or SART signals, turn on [SART] in the [ECHO] menu.
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