Bearing Resolution; False Echoes - Furuno FR-8051 Operator's Manual

2.3 Bearing Resolution

Bearing resolution is a measure of the capability of
a radar to display as separate targets the echoes re-
ceived from two targets which are at the same range
and are close together.
The principal factor which affects bearing reso-
lution is horizontal beamwidth. Two targets at the
same range must be separated by more than one
beamwidth to appear as separate pips.
Target
Horizontal
beamwidth
Direction of
scanner rotation
Target
Horizontal
beamwidth
Direction of
scanner rotation
Radar cannot display targets as
separateechoes because they
are within the beamwidth.
Figure 2-3 Bearing resolution

2.4 False Echoes

Indirect echoes
Indirect echoes may be returned from either a
passing ship or returned from a reflecting sur-
face on your own ship, for example, a stack. In
both cases, the echo will return from a legitimate
contact to the antenna by the same indirect path.
The echo will appear on the same bearing of the
reflected surface, but at the same range as the di-
rect echo. Figure 2-4 illustrates the effect of an
indirect echo. Indirect echoes may be recognized
as follows: (1) they usually occur in a shadow
sector; (2) they appear on the bearing of the ob-
struction but at the range of the legitimate con-
tact; (3) when plotted, their movements are
usually abnormal, and (4) their shapes may indi-
cate they are not direct echoes.
2 – 2
Radar is able to display two
distinct target echoes.
Direct
path
Scanner
Target
Indirect
Direct
path
path
Own
ship
Bridge
Indirect
echo
Figure 2-4 Indirect echoes
Multiple echoes
Multiple echoes occur when a short range, strong
echo is received from a ship, bridge, or breakwa-
ter. A second, a third or more echoes may be ob-
served on the display at double, triple or other
multiples of the actual range of the target as shown
in Figure 2-5. Multiple echoes can be reduced
and often removed by decreasing the sensitivity
or properly adjusting the A/C SEA.
Target
Own ship
Figure 2-5 Multiple echoes
Heading
marker
Target
Indirect
path
Obstruction
(mast, funnel,
etc.)
Indirect
echo
Heading
marker
True
echo
Indirect
echo
True
echo
Multiple
echo
True
echo