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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

    2. Other radars are then brought into play to give an accurate determination of the height of the target and to provide 'tracking' facilities.

    3. The information so obtained is passed to fighters (already airborne) or to an alerted SAM station. The fighters will also show echoes on the ground display; but because they are carrying a secondary radar responder known as 1FF (identification, friend or foe), the echoes produced by the fighters are different from those produced by enemy aircraft and can be identified. The fighters may then be directed towards the target by instructions from the ground controller.

    4. When the fighters are within range of the target, their own airborne interception (Al) radar takes over to complete the interception.

Early-warning Survernance (Search) Radars

The most important function of a long-range search radar is the early detection of the target. To provide this, it is necessary for the radar to have very high peak-power transmitters, sensitive low-noise receivers, and a large aerial (to give the required high gain and narrow beamwidth for good angular discrimination). For long ranges, low values of p.r.f. are necessary and, for good discrimination in range, short duration pulses are needed. In addition, a quick rate of scan is called for, so that the information obtained during each scan can be quickly up-dated.

To get a high rotation rate for the aerial and good range performance, through high aerial gain and narrow beamwidth, implies the use of an aerial at the shorter wavelengths. However, as we shall see later, this carries with it the penalty of poorer performance than longer wavelength radars in regard to increased weather clutter and attenuation from rain, snow, and other weather phenomena. To get the best of both worlds, many static radar stations operate more than one surveillance radar, each operating in different bands. The bands in general use for this purpose are 23 cm (L band), 10 cm (S band) and 5.5 cm (C band). Let us consider the situation where we have two surveillance radars, one operating at 10 cm (S band) and the other at 23 cm (L band).

10 cm (S band) radar.

An S band radar operating in the long-range surveillance role may have parameters as follows:

 

Frequency

3 GHz

 

Peak power

2.25 MW

 

Pulse Duration

2.5 to 5us

Preset to order

PRF

300 to 600 pps

Preset to order

Slant range

up to 250 nm

 

 

In a radar with a given value of mean power, the values of p.r.f. and pulse duration can be adjusted, one against the other, to give the peak power output for which the equipment was designed (see p 28). The values of p.r.f. and pulse duration must be such that the pulse duty factor is kept constant at its design value (see p 29). The S band transmitter under discussion has a mean power of 3.3 kW, a peak power of 2~25 MW, and a pulse duty factor of 0.00l5. At a p.r.f. of 300 p.p.s., the peak power output is achieved with a pulse duration of 5 us. At a p.r.f. of 600 pps, the peak power output is achieved with a pulse duration of 2.5 us. Within these limits, the p.r.f. and pulse duration may be pre-set relative to each other, by the manufacturer, to provide the required conditions. A pulse duration of 2.5 us at a p.r.f. of 600 p.p.s. will give good definition and good discrimination in range at the cost of reduced range capability. A pulse duration of 5 us at a p.r.f. of 300 p.p.s. will provide maximum range capability but poorer discrimination in range. The pre-set values ordered by the user depend upon many factors - e.g. the site of the radar, the range required, the definition required.


 

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