Radar System Engineering

184 THE GATHERING AND PRESENTATION OF RADAR DATA [S~C.6.11
tion of a complicated air situation, and also is beneficial in reducing the
effects of ground clutter. Ship radar for displaying surface targets will
show greater range, other things being equal, the shorter the wavelength
used. This is a result of interference between the direct beam and the
beam reflected from the water surface (Sec. 2.12). The greater azimuth
resolution provided by a sharper beam is also useful in resolving complicated
target. situations. The only drawback of microwave radar for
ship search is the resultant narrowing of the beam in elevation; in order to
keep the rolling of the ship from tilting the beam away from the surface
of the sea, either the antenna mount must be mechanically stabilized
(Chap. 9), or the vertical dimensions of the antenna must be reduced to
fan the beam sufficiently in elevation to take account of roll. In airborne
PPI radar, the attainment of sufficiently good display detail to
permit navigation over land, away from distinctive shorelines, demands
the use of microwave frequencies. In fact, a considerable premium is
placed on attainment of the narrowest azimuth beamwidth possible.
Antenna stabilization is also desirable in aircraft, to correct for the effects
of changes in attitude; but it is so costly in weight and complication that
it has been seldom used.
Typical examples of PPI radar systems intended for ground and for
airborne use are analyzed in detail in Chap. 15.
6.11. Height-finding Involving Ground Reflection.-The height of
an aircraft target is usually determined by finding separately its range
and its angle of elevation, and then solving the equation H = R sin e.
In the wartime use of radar, height was such an important datum
that considerable ingenuity was expended on the problem of its
measurement.
The resulting methods can be roughly separated into two classes
depending on whether the reflection of energy from the level surface
surrounding the radar antenna is important to the scheme of height-finding
used. In this section are considered methods which do involve
ground (or sea) reflection; in the next, free-space beam methods which
do not.
If the antenna of a radar of medium wavelength, for example, about
one meter, is less than about fifty wavelengths above the ground, the
reflected energy from the part of the beam that strikes the ground will
produce maxima and minima in the elevation pattern. These are shown
for a representative case in Fig. 6.19.
Null Readings. -In a pattern such as that shown in Fig. 6.19, the
range at which an aircraft first appears on the screen depends upon the
height of the aircraft. If the antenna pattern is known, height may be
estimated from range of first appearance. If the course of the aircraft
I By E. C. Pollard and L. N. Ridenour.