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AP3302 Pt3 Contents

AP3302 Pt3 Section 1 Contents

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AP 3302 Pt. 3

Section 1


Basic Requirements Of A Pulse-Modulated Radar System

    b. Modulator. This produces rectangular d.c. pulses of known pulse duration which switch the oscillator on and off.

    c. Oscillator. This produces the very high frequency, high power output in pulses of short duration. The p.r.f. is determined by the master timing unit and the pulse duration by the modulator.

    d. T-R switch. This automatically connects the transmitter to the aerial for the duration of each output pulse, and connects the receiver to the aerial for the intervals between pulses.

    e. Aerial. This radiates the transmitter output in a narrow beam and picks up the reflected echoes for application to the receiver The aerial may be moved for scanning, the movements being conveyed by synchros or servomechanisms to the indicator.

    f. Receiver. This amplifies the very weak echoes and presents them in a suitable form for display on the indicator c.r.t.

    g. Indicator timebase generator. This produces the range trace on the c.r.t. screen. The sync pulses from the master timing unit ensure synchronization of indicator and transmitter operations.

    h. Indicator display. This presents the required target information in a suitable form.

Secondary Radar Systems

So far we have considered pulse-modulated primary radar systems which rely on the reflection of the incident energy by the target. Instead of relying on reflections we could use the pulses received from the primary radar to 'trigger' a transmitter carried by the target. Such systems are known as secondary radar systems and can only be used when the target is friendly. When used in this manner the primary radar transmitter is known as the interrogator and the secondary installation in the target is called the transponder (transmitter-responder).

One of the earliest applications of secondary radar was the identification equipment carried by all friendly ships and aircraft. In this system the primary radar on the ground is the interrogator and the ship or aircraft carries the transponder. When the transponder receives a signal of the correct frequency and pulse duration from the interrogator it automatically transmits coded pulses in reply and this response identifies the target as friendly on the ground radar p.p.i.

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