GROUND CLUTTER
WEATHER CLUTTER
INTERFERENCE
RADAR HORIZON
SHADOWING
h
FEET
R
NAUTICAL MILES
H
FEET
ANTENNA
HEIGHT
POINT "H"
H = 0.672(R-1.22 h)
2
5000
4000
3000
2000
10,000
1000
500
200
100
50
0
250
200
150
100

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.

0
10 20 30 40 50 60 70 80 90 100
12
11
10
9
8
7
6
5
4
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
R
max
(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