Radar Horizon and Shadowing Figure 2.. Earth Curvature Nomograph RADAR HORIZON / LINE OF SIGHT There are limits to the reach of radar signals.. As also shown in Figure 1, an aircraft may
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WEATHER CLUTTER INTERFERENCE
RADAR HORIZON
SHADOWING
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ANTENNA
H = 0.672(R-1.22 h)2
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R NM ' 1.23 h radar with h in ft
R NM ' 1.23 h radar % h target with h in ft
2-9.1
Figure 1 Radar Horizon and Shadowing
Figure 2 Earth Curvature Nomograph
RADAR HORIZON / LINE OF SIGHT
There are limits to the reach of radar
signals At the frequencies normally used for
radar, radio waves usually travel in a straight
line The waves may be obstructed by
weather or shadowing, and interference may
come from other aircraft or from reflections
from ground objects (Figure 1)
As also shown in Figure 1, an
aircraft may not be detected because it is
below the radar line which is tangent to the
earths surface
Some rules of thumb are:
Range (to horizon):
Range (beyond horizon / over earth
curvature):
In obtaining the radar horizon equations, it is common practice to assume a value for the Earth's radius that is 4/3 times the actual radius This is done to
account for the effect of the
atmosphere on radar propagation
For a true line of sight, such as used
for optical search and rescue, the
constant in the equations changes
from 1.23 to 1.06
A nomograph for
determining maximum target range
is depicted in Figure 2 Although an
aircraft is shown to the left, it could
just as well be a ship, with radars on
a mast of height "h" Any target of
height (or altitude) "H" is depicted
on the right side
See also Section 5-1 on
ducting and refraction, which may
increase range beyond these
distances
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10 k ft
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TARGET ALTITUDE (k feet)
RADAR AIRCRAFT ALTITUDE
0
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ANTENNA HEIGHT (feet)
R NM ' 1.23 h radar % h target
R Visual (NM) ' 1.05 Acft Alt in ft R ESM (NM) ' 1.5 Acft Alt in ft
Rmax (NM) ' 1.23 h r
2-9.2
Figure 3 Aircraft Radar vs Aircraft Target Maximum Range
Figure 4 Ships Radar Horizon with Target on the Surface
This data was expanded
in Figure 3 to consider the
maximum range one aircraft can
detect another aircraft using:
(with h in feet)
It can be used for surface
targets if Htarget = 0 It should be
noted that most aircraft radars are
limited in power output, and
would not detect small or surface
objects at the listed ranges
Other general rules of thumb for surface "targets/radars" are:
Figure 4 depicts
the maximum range that a
ship height antenna can
detect a zero height object
(i.e rowboat etc)
In this case "H" = 0, and
the general equation
becomes:
Where h is the height ofr
the radar in feet