Radar System Engineering

SEC.6,5] REPRESENTATION OF THE HORIZONTAL PLANE 169
In spite of the usefulness of a true map display such as the PPI,
occasions arise in which it is not ideal. These situations usually involve
the need for providing high resolution or dispersion in some particular
coordinate without restricting the field of view too severely in other
dimensions. A number of types of deformed displays have been devised
for such purposes, and are particularly useful in dealing with point
targets.
Radial Deformation.—A polar plot such as the PPI can be deformed
radially by shifting the range origin, by using a nonlinear range scale, or
both. The first means has been widely used.
On a normal PPI it is difficult to determine accurately the direction
to a target whose range is a small fraction of that covered by the display.
This difficulty is often partly overcome by expanding the zero-range
origin of the PPI into a circle so that
the radius vector to the echo from a
Elevation Amle
nearby target is greatly increased.
Such an arrangement is known as an
open-center PPI. Range and bearing
retain their identities and their linear
scales. For the degree of center-opening
ordinarily used, the deformation
introduced into the sizes, shapes, and
relative positions of the targets is
serious for only a fractional part of
the range portrayed. An open-center
PPI has been widely employed for
station-keeping and for homing on ~
such objects as ships, airplanes, runways,
and the like. The open center
FIG. 6.6.—Stretched PPI. Tw&mile
grid is shown by dashedlines.
is switched in only a fraction of the time; for example, during the closing
stages of a homing operation.
It is sometimes desirable to provide an expanded range scale over an
interval at a distance from the radar site without sacrificing an all-round
view as an off-center PPI would do. This may be accomplished by
delaying and expanding the range sweep in an otherwise normal PPI so
that a ring-shaped area is collapsed into a solid circle. Because of the
technique used in producing it, this display is called a delayed PPI. As
in the open-center PPI, range and bearing retain their identities and their
linearity. The deformation is very great except at the extreme edge of
the display. The delayed PPI is used to increase the resolution on closely
spaced targets and on coded beacon signals, to facilitate accurate range
measurements, and to increase the signal-to-noise or signal-to-clutter
discernibility under some circumstances.