Radar System Engineering

594 EXAMPLES OF RADAR SYSTEM DESIGN [SEC. 153
shows the general form of beam shape that gives the most desirable
type of coverage under these conditions. Its lower edge is determined
by the optical horizon, with corrections for atmospheric refraction and
interference due to surface reflection. At a distance d in land miles
from the radar set, the horizon height h in feet is given by h = ~dz for
normal atmospheric refraction (neglecting the interference effect, which
is small for microwaves; see Sec. 2.13). Reduction of the blind region
below this lower edge by locating the set on high terrain maybe advantageous
where the coverage is largely over water, but for overland surveillance
this recourse is severely limited because of permanent echoes,
which can mask out all return in regions where the radar beam is intercepted
by land surfaces.
4.5
2:
ẓ
30
z
z 25
.
1
g 20
~ 15
~ 10
‘5
o 0
0 25 50 75 100 125 150 175 200 225
Rangeon singlelargeawcraftin miles
FIG. 15.4.—Desirable coverage for long-range air-surveillance radar.
In setting a desirable limit for the upper contour for detection of a
given type of aircraft it was, necessary to balance the value of high
coverage against coverage in range, since adding to one subtracts from
the other unless the designer chooses to increase the complexity of the
equipment. For operations duting World War II, provision for detection
of a four-engine plane at all altitudes up to 35,000 ft was desirable, and
this value was taken to determine the upper section CD of Fig. 15.4.
Increasing the elevation angle of section AD of the contour does not add
greatly to the radar coverage requirements, since the region is small;
but practical difficulties of antenna design are encountered. Since
close-in regions have been in the past largely masked out by permanent
echoes, an angle of 30° was considered acceptable. With the upper and
lower limits of the contour determined, the problem was now to push
the maximum range out to the best possible value.
A detection range of 180 miles for a four-engine aircraft appeared to
be the best that could be expected, and the contour thus determined is