AP 3302 Pt. 3
screen. This arrangement is illustrated in Fig 19. To ensure a bright, clear picture the c.r.t. screen has a long 'afterglow'. Thus when an echo is 'painted' it remains bright until the trace comes round again to re-paint it.
The display described is known as a plan position indicator (p.p.i.). The distances of the echoes from the centre of the screen indicate the ranges of targets, and the angular position of each echo indicates the bearing in plan or 'in azimuth' of the corresponding target. Such a display is in effect a 'radar map'. Fig 20 shows a typical picture on the p.p.i. of a ground search radar.
How the Height of a Target is Determined
So far we have seen how a target can be detected and how its range and azimuth bearing can be found. We are thus a long way towards pin-pointing the position of a target in space. The only factor missing, if the target is an aircraft or a missile, is the height of the target. A radar installation which indicates range, bearing and height of a target gives, in effect, a three-dimensional
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