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AP3302 Pt3 Contents

AP3302 Pt3 Section 1 Contents

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AP 3302 Pt. 3

Section 1

CHAPTER 2

Basic Requirements Of A Pulse-Modulated Radar System

 the 'grass' may hide the weak echoes from targets at long range. In a bright-up (intensity-modulated) type of display, such as a p.p.i., the signal and noise voltages are applied together to the c.r.t. grid (or cathode). Noise then appears as random and confusing flashes of light on the c.r.t. screen (Fig 9b), an effect sometimes called 'snow'.

We must therefore take precautions to keep the noise generated by the receiver to an absolute minimum. It is pointless having a high-gain receiver if it also has a high noise level; the noise is merely amplified further.

One further point may be noted about the receiver at this stage: to preserve the shape of the reflected pulse the receiver circuits must have a wide bandwidth. This follows from the fact that a square or rectangular pulse of voltage contains a very wide band of frequencies which must be accepted by the receiver if distortion is to be avoided. A distorted pulse on the c.r.t. display makes it difficult to determine range accurately.

The Indicator Unit

The indicator is used to show the operator the range, bearing and height of all aircraft within range of the radar. The relevant information is then passed on to the controller. Very often only the range of a target is required; a simple type A display is then sufficient. Where range and bearing are required a p.p.i. display is usually used. Where range and height are required a range-height indicator is used. Where all three co-ordinates are required it is usual to have two c.r.t.s-one a p.p.i. to show range and bearing and the other a range-height indicator.

We have seen in Chapter 1 how these displays are produced, and earlier in this chapter we showed how information concerning the attitude of the aerial is transmitted by synchro systems to the indicator. Fig 10 summarizes the main points.

Other Radar Displays

A display can be designed to present almost any required information about a target in terms of range, bearing, height and elevation, or any two combinations of these. Fig 11 illus-trates a few examples of additional displays in common use.

a. Type B. This shows bearing against range. The timebase voltage is applied to the Y-deflecting plates so that the range trace is vertical The position of the trace in the horizontal 


 

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