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AP3302 Pt3 Contents

AP3302 Pt3 Section 1 Contents

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AP 3302 Pt. 3

Section 1

CHAPTER 4

Some examples of the uses of pulsed radar

All modern air defence radars have arrangements to reduce clutter due to ground reflections and precipitation (rain, snow and sleet). Moving target indication (MTI) systems and circular polarisation of the radiated energy are among the arrangements in use (see later). In the air defence radar system steps must also be taken to reduce the effects of interference from other radars and the effects of deliberate jamming by the enemy (see p 437).

Whichever air defence system is in use, sufficient information on range, bearing and height of the target is extracted to enable the controller to direct the fighter to the target.

Airborne Interception (AI) Radar

When the fighter is within the target area its own radar equipment is used for the final stages of interception. The airborne equipment does not need a large maximum range (up to 20 nm may be sufficient) but, as we have seen, the 'blind' area surrounding the radar must be as small as possible to enable the radar to be effective down to very short ranges (less than 100 metres). Thus, the p.r.f. can be high (1,000 p.p.s. or higher) and the pulse duration short (1 jis or less). The short pulse duration also gives improved range discrimination so that the fighter can tell whether there is more than one enemy aircraft on the same relative bearing.

The radar must also be capable of showing the relative bearing of the target to port or star-board of the fighter's heading, and of indicating its angle of elevation above or below the fighter. The aerial assembly (or scanner) used for this is a parabolic reflector mounted in the nose of the aircraft (the radome). It produces a narrow pencil beam which can be moved rapidly for scanning (Fig 7).

While searching for the enemy aircraft the aerial beam has to scan a wide area ahead of the fighter. A spiral scan may be used for this (Fig 8a). When the selected target is picked up, the aerial may then be switched to conical scan (Fig Bb). The target echoes received during various portions of the conical scan are then used to operate circuits which give automatic 'tracking' of the enemy aircraft.

A typical Al indicator uses one c.r.t. in a Type B display to show range and relative bearing


 

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