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AP3302 Pt3 Contents

AP3302 Pt3 Section 1 Contents

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AP 3302 Pt. 3

Section 1



b. Time interval of 8 microseconds.

Range (in metres) (300 x 106 x 8 x 106)/2 =1,200 metres.

In certain ground radar stations the equipment is calibrated in data miles (6,000 ft.) instead of in statute miles (5,280 ft.). Similarly, most airborne radar navigational equipments are calibrated in nautical miles (6,080 ft.) and knots instead of in statute miles and m.p.h. From the known equivalent time for one radar mile (l0.75 uS), the times for one radar data mile (l2.2 uS) and one nautical mile (l2.36 uS) may be calculated. 

How Range is indicated

We have seen how a stopwatch measures the time interval between the blast from a fog-horn and the reception of the echo from an object ahead of the ship. The process may be simplified by using a stop-watch which is marked not in seconds but directly in feet of range. The exact time interval corresponding to 1,000 feet range is calculated and the face of the watch is marked at that point as 1,000 feet. Similar marks are made to indicate ranges of 2,000 feet, 3,000 feet and so on (Fig. 8).

In radar measurement of range the time intervals to be measured are very short (a few microseconds). A stopwatch cannot measure such short time intervals and so we use a cathode ray tube (c.r.t.) to produce a spot of light which can be moved over the screen to build up a pattern or a picture representing the inputs to the deflecting system. Let us now see how these principles are applied to radar.

The horizontal trace on an electrostatic c.r.t. is obtained by applying a saw-tooth timebase waveform to the X-deflecting plates of the c.r.t. In radar, the timebase waveform must be 'synchronised' with the transmitter operation so that the spot starts to move across the screen at the instant the pulse of r.f. energy leaves the aerial (Fig. 9). The spot moves at an exactly known speed, e.g. it may move two inches in the time that the transmitted pulse moves twenty miles through space (see Fig. 9).

Suppose that after travelling 20 miles the pulse is reflected by an aircraft. The echo travels back 20 miles to the radar installation so that the pulse and the echo make a combined journey of 40 miles. In this time the spot will have moved four inches. If the received echo is converted instantly by the receiver into a voltage pulse which is applied to the Y-deflecting plates of the c.r.t. the spot is deflected upwards for the duration of the pulse, making a deflection 'paint' four inches from the starting point of the trace. This 'paint' indicates a target at a range of 20 miles.


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