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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

crossed lines in the Type F display, the fighter is then pointing directly at the target. The range meter previously described indicates when the enemy is within range of the fighter's weapons and the hostile target can then be attacked even if the fighter pilot still cannot see it. In modern equipments the radar circuits are pre-set such that when the target has been positioned dead ahead at a given range (say, 800 metres), the fighter's weapons are fired automatically.

Air Traffic Control (ATC) Radars

We have seen briefly some of the functions of ground radars in air defence. Primary ground radar is also required in air traffic control (ATC) to enable aircraft to operate safely, with tactical freedom, in all weather conditions; to prevent collisions between aircraft in the air; and to ensure an orderly flow of air traffic. With large numbers of aircraft operating in congested air spaces, the use of primary radar in this role is vital.

The important parameters for ATC radars are: data rate, maximum range, horizontal beamwidth, and wavelength. For a high data rate, the aerial rotation should be high; but it must not be so high that insufficient pulses per beamwidth strike the target. The maximum range required depends upon the role of the ATC radar (see later); for long ranges we require a relatively low p.r.f. but, at the same time, we require a high p.r.f. for good MTI performance; range may have to be sacrificed to obtain this. A narrow horizontal beamwidth is required for good azimuth discrimination; however, if it is too narrow we have insufficient pulses per target during the rotation of the aerial. To get good MTI performance and to reduce the effects of weather phenomena, many ATC radars operate in the 50 cm (P) band; however, for more accurate measurements at short ranges (particularly for precision approach systems) shorter wavelength radars in the 10 cm (S) band and the 3 cm (X) band are also used.

The various parameters discussed above are selected according to the role of the air traffic control radar. Let us now consider these roles.

En-route surveillance.

This is used for aircraft flying along airways before they arrive at terminal control areas. The radar requirement is for long-range cover down to low angles of elevation-typically 180 nm for an altitude of 20,000 feet. Suitable parameters for a radar operating in this role are:

 

Wavelength

50 cm

(P band)

Frequency

600 MHz

 

Peak power

500 kW

 

Pulse duration

3 us

 

PRF

370 p.p.s.

 

The aerial used is a large structure (67 feet (20.5 m) across and 13 feet (4 m) high). It is rotated at some 7.5 r.p.m., and the shape of the aerial is such that it gives a half-power horizontal beamwidth of 1.7o and a gain of 32 db. The aerial may be pre-set to a tilt angle between -1o and + l0o from the horizontal. A typical radar coverage diagram is illustrated in Fig. 11.

Terminal area control radars. The radars used in this role may be similar to that described for the en-route surveillance radar in terms of transmitter/receiver and aerial. However, the range requirement is now less (up to about 120 nm) so that the p.r.f. may be increased; 500 p.p.s. is a typical figure. Also, because of the increased traffic density, the data rate must be increased; the aerial now rotates at 10 revs per minute.

Approach control radars. Many approach control radars in use operate in the 50 cm band - similar to those described above for the terminal area control, but with increased p.r.f. and


 

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