Radar System Engineering

SEC.6.12] HEIGHT-FINDING WITH A FREE-SPACE BEAM 187
The angular range through which this height-finding scheme is useful
is shown in Fig. 6.21 by dotted lines. At low angles the variation of ratio
with angle is far too slow. At angles near 4° the variation is fast, but
the illumination of the plane by the lower beam may be so weak that no
echo is obtained, and therefore no height can be inferred. If both the
antennas are nearer the ground, the effective angle is increased, but the
beams are both tipped up so much that coverage on aircraft at low and
medium altitude is sacrificed. Even in the case we have considered, an
aircraft 10,000 ft high and 50 miles away, or one 5000 ft high at a range
of 25 miles, would fall in the unmeasurable class.
To overcome these limitations, a variety of overlapping elevation
lobe patterns are made available by providing a variety of antennas with
phase and antiphase feeding. This additional complexity increases the
time required to find height, since if the operator cannot guess which
combination of antennas is suitable, a wrong choice makes it necessary
for him to wait for a full 360° rotation of the antenna mount before he
can try a better pair.
In addition to the elaboration necessary in equipment for heightfinding
by this method, siting is also a problem. A suitable site should
be level within a few feet for a half-mile radius around the radar. Even
sea reflection can be troublesome, because tides cause a profound change
in the calibration of the station as a height finder. For these reasons, a
free-space-beam height finder, not depending on ground reflection, is
usually preferred.
6.12. Height-6nding with a Free-space Beam.’ Searchlighting. —The
simplest form of free-space-beam height finder is one which measures
elevation angle of the desired target directly, by pointing an antenna
producing a narrow beam directly at the target and measuring the elevation
angle at the antenna mount. This technique is called “searchlighting.”
Even with centimeter wavelengths, beamwidths attainable
with antenna reflectors of practicable size (a few degrees) are large in
comparison with the accuracy desired in the measurement of elevation
angle (about a tenth of a degree) for height-finding purposes. Therefore,
target height is not usually determined by directing the beam at the
target simply by maximizing the echo.
Instead, arrangements are made for shifting the beam rapidly in elevation
angle by an amount which may be, typically, a third of a beamwidth,
and comparing the echo signal received in one position with that
in the other. When the two signals are equal, the bisector of the angle
between the two positions of the beam is pointed directly at the target.
The signals from a given target may be compared for the two positions
of the beam by presenting them side-by-side on a type K indicator or
1By E. C. Pollard,L. N. Ridenour,and D. C. Soper.