Radar System Engineering

168 THE GATHERING AND pRESENTA TION OF RADAR DATA [SEC.6.5
of elevation angle). Except when viewing aircraft at very substantial
elevation angles, or when mapping the ground from a high-flying aircraft,
this error is imperceptible and can be neglected under practically all
circumstances.
The indicator used for the display just described is called the planposition
indicator or PPI.’ In the PPI, an outward range sweep is
rotated about the range origin in synchronism with the azimuthal
scanning of the antenna. Targets then appear in the proper directions
as well as at the proper relative distances from thk origin and the result
is a map.
The origin of the PPI may be at the center of the cathode-ray Lube,
giving an equal field of view in all directions. Frequently, however,
FIG.65.-Off-center PPI.
it is displaced, sometimes far off the tube face, in order to give a maximum
expansion to a given region; such a display is called an o~-center PPI
(Fig. 65). The expression sedtor display is often used when the displacement
is extreme.
The PPI is the most widely used and versatile of all displays. In
presenting the information with which it deals, its only fundamental
shortcoming is that, in common with all maps or charts, it cannot simultaneously
possess a highly expanded scale and a large field of view.
The vertical analogue of the PPI may be formed by substituting elevation
for azimuth angle. However, since only a restricted range of
elevation angle usually has interest, it is more customary to use distorted
displays for this purpose.
1See Sees. 13.15 to 13.18. The abbreviation “ PPI” is used interchangeablyto
representboth the display and the equipment (plan-positionindicator). Abbreviations
for other types of indicatorsareused in the sameway.