Radar System Engineering

SEC. 6.10] PPI RADAR FOR SEARCH, CONTROL, AND PILOTAGE 183
showed instantaneously only those targets located in a narrow azimuth
sector, made use of the persistent property of CRT screen phosphors to
preserve from one scan to another the target signals returned on the
most recent scan. The PPI thus presents a continuous map-like display
of all targets in the field of view of its radar, and permits attention to be
concentrated on particular targets without the penalty of losing sight of
the general situation.
It is interesting to notice that, before the advent of the PPI, users of
such equipment as the CXAIU and the SCR-270 frequently constructed
a plan display with pencil and paper, plotting in polar coordinates the
targets whose range and azimuth were called off by the radar operator.
Further, skilled operators of such sets developed an ability to form, by
watching the. A-scope, a sort of mental PPI picture of the radar targets,
remembering from scan to scan their ranges and bearing angles. When
the PPI became available, the operator was released from this necessity,
and could apply his skills to a more sophisticated mental appreciation
of the target situation; for example, a good PPI operator will often keep
in his mind the directions in which important targets are moving.
With the exception of special-purpose sets all radar equipment designed
in 1941 or later contained one or more PPI scopes. In many types of
equipment the use of the PPI is fundamental to the purpose of the radar.
PPI rticlar sets can be divided into three classes: land-based, shipborne,
or airborne.
Ground-based PPI sets are usually employed for the detection and
plotting of aircraft, either to give early warning of enemy air activity, or
to control friendly aircraft by radio instructions, or both. Mounted on
shore, such sets can also be used for plotting ship traffic to permit warning
and control, but this use is simply a two-dimensional case of the same
problem as that involved in plotting aircraft.
In wartime, radar sets ori shipboard must provide air-search and control
facilities, and in addition must give a display of surface targets, to
permit station-keeping in formation and pilotage in narrow waters under
conditions of poor visibility. In peacetime, usually only the surfacesearch
function is of importance. Because of the different beam-shape
requirements, separate radar systems are ordinarily used for air search
and for surface search,
Airborne radar equipments employing a PPI are used for pilotage in
the vicinity of shorelines or over land. This pilotage may be of a general
navigational character, as it always is in peacetime and usually is in war;
or it may be as precise as the limitations of the radar will permit, for the
purpose of blind bombing of a radar target.
The narrow beam provided by a microwave radar is important in all
these cases. In ground radar, a narrow beam permits the detailed resohl-