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4 How to Interpret the PPI Screen
4.1 Height of and the Distance to the Target
The maximum distance to a target that can be observed with a radar depends not only on
the power of the radar's transmitter, beam width of the scanner unit, and the receiver's
sensitivity but also on height of a target, distance to a target and height of scanner unit line
etc. This is because the radio wave emitted by a radar runs straight, undergoing no
influence by the curvature of the earth surface.
[Distance and Target]
For example, when the scanner unit lies 3 meters above the sea level, the radar can
detect and display an island with a height of 10 meters at a distance 10 NM away from
the scanner unit position but cannot detect and display an island with a height of 5
meters at the same distance. This is theoretically true but does not always hold,
depending on weather conditions.
For a target located 10 NM away to be displayed on a radar, it theoretically needs to be
7.6 meters or higher. Any targets lower than 7.6 meters cannot be displayed on a radar.
The target may be unable to be observed when the height of a scanner unit or an target is low
.
a
1
=2.23√h
1
a
2
=2.23√h
2
a
1
+ a
2
=2.23 (
√
h
1
+
√
h
2
)
a
1
, a
2
：
Unit [Nautical miles]
h
1
, h
2
：Unit [Meters]
Island not displayed on the screen
10 NM
a1+a2(NM)
Distance
h2(meters)
Target height
h1(meters)
Scanner unit height
10 NM
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