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Fundamentals of radar - how radar works




FUNDAMENTALS OF RADAR - HOW RADAR WORKS

The design of a radar begins with consideration of its intended use, that is, the function to be performed by the radar as a whole. The uses generally divide into three categories:

1.   Warning and surveillance of activity, including identification.

2.   Aids to the direction of weapons, that is, gunfire control and searchlight control.

3.   Observation of terrain echoes or beacons for navigation and con­trol of bombing.

There is nothing mysterious or complex about radiolocation. It rests on the foundations of ordinary radio theory, and is a technique based on the transmission, reception, and interpretation of radiofrequency pulses. Considered as a whole, it must be admitted that even the most elementary of radar equipment is difficult to visualize, but this is simply due to the fact that so many (normally) curious circuits and pieces of apparatus are gathered together under one roof. No particular circuit or detail, of the equipment is in itself especially difficult to understand, and once the elements are known the complete assembly is no longer mentally unmanageable.




The word 'radar' is derived from the phrase 'radio direction-find­ing and range', and it may be more expressive than the older 'radi­olocation', or it may not. Finding the position of an aircraft or a ship by means of radio covers a very wide field of electronic applica­tion, covers, in fact, the whole area of radio direction-finding (R. D. F.) from the elementary bearing-loop to the principle of the reflected pulse which represents the latest principle of the technique. In this article the term will be used to cover only those methods of detection which depend upon the reflected pulse, the characteristic (by popular opinion) which distinguishes radar from all other methods of position-finding in that no co-operation is required on the part of the target. We shall not dwell, therefore, upon the older and morefamiliar methods which depend upon the reception at two or more points of a signal transmitted by the body under location itself.

The actual equipments in use Which employ the reflected pulse principle are greatly varied from the point of view of physical appear­ance, but their basic principles are the same.

First, let us tabulate and briefly analyse the problem to be met. The aim of radar is to find the position of a target with respect to a fixed point on the ground — say the position of an aeroplane or a bar­rage balloon with respect to the radar equipment situated in a field a mile or so away. Three quantities must be measured in order to define the position of the aeroplane or the barrage balloon: first, the slant range, the length of the most direct line drawn from the radar site to the target; second, the angle of bearing, i.e. which point of the compass the target occupies; third, the angle of elevation. When the target is an aeroplane, these three quantities are continuously varying so that the problem of posi­tion-finding is somewhat complicated by the fact that the radar equipment has to 'follow' as well as find. In the case of barrage balloon, things are not quite so difficult, and the three important factors may be found at leisure.

SLANT RANGE

The measurement: of slant range is, in principle at least, a surprisingly simple procedure.

Post Office engineers lay claim to being the first people to recognize that aircraft reflections were detect­able, for they suffered 'interfer­ence' on their short-wave equipment when aircraft were flying in the vicinity. This was in 1933, but pulse measurements, equivalent to range-finding, had been carried out successfully on the upper ionized layers of the earth's atmosphere long before that time. The application of the latter to the problems of the former came later, in 1935.





Consider for a moment the familiar business of 'measuring' the distance of a thunderstorm. We watch for a flash of lightning, observe it at a particular moment, then we wait for the peal of thunder. We know that sound travels approximately 1100 feet per second through air at normal temperatures, or about 1 mile in 5 seconds, therefore the time interval between the flash of lightning and our knowledge of the thunder noise is a measure of the distance from where we are standing to the centre of the atmospheric disturbance. The method is correct, though crude, for the velocity of sound is liable to quite large variation and our estimate of 1 mile in five seconds is rough in the extreme . However, we have at least amethod,and a very simple method it is. So it is with the measurement, by radio, of slant range. .

Suppose a transmitter situated at the same point as a receiver sends forth a short, powerful pulse of high-frequency carrier wave. This wave will be radiated in all directions, travelling out from the transmitter site, like a rapidly expanding ball. At a certain point this wave will strike the target, say, the barrage balloon, and induce, in its conduct­ing parts a radiofrequency current which in turn will radiate its own carrier into space — that is, the balloon will re-radiate or reflect, part of the original transmitted pulse. Some of this re-radiated energy, also expanding in every direction from the balloon, will make its way back to the receiver where, by suitable means which need not concern us at the moment, it may be made to produce a response. If now we have also permitted the transmitted pulse to produce a response in the receiver at the precise instant that it left the transmitting aerials, then clearly the response caused by the reflected pulse will occur some finite time after that due to the pulse received directly, for the former has had to travel a much greater distance. The interval between these two responses therefore represents the time taken by the transmitted pulse to travel from the transmitter to the receiver by way of the tar­get. Since the velocity of propagation of electromagnetic waves is constant at 3.28 * 108 yards per second 2000 yards in 6.1 mic­roseconds), the distance between the receiver and the target is propor­tional to the time elapsing between responses. We have only, then, to devise a method of measuring the time interval and the problem of slant range is solved for us.

It must be admitted that the solution is not at first sight a very simple one to visualize, for the velocity of propagation is so great that even for targets' at distances up to 30 miles the time interval involved is of the order of microseconds, and the accurate measurement of such minute intervals may seem difficult, if not impossible, to achieve. However, the difficulty is capable of. solution, and methods have been devised to give range accuracies to within a few yards for targets situated many miles from the radar station.



The simplest way to measure extremely short time intervals is by means of a cathode-ray indicator tube, and nearly all radar receivers are equipped with one or more of these devices. Suppose a transmitter is sending out a series of radio pulses of some few microseconds dura­tion, a regular repetition of short bursts of radiofrequency carrier wave. The signals reflected from a target are detected by the receiver and are presented to a cathode-ray tube. In the most elementary, instances, a linear 'time-base'—a deflection of the electron beam that is pro­portional to time — is provided and this time-base is 'fired' in synchronism with the transmitted pulse; that is, the spot on the tube com­mences to sweep the screen every time a pulse leaves the transmitter aerial array. Then two vertical deflections appear along the time-base as shown in Fig. 7; the direct transmitter pulse causes the large de­flection on the extreme left of the time-base, while the reflected pulse appears a little farther along. The amount by which these two deflections are displaced depends upon the distance of the target, and by measuring the length of the time-base between them, provided the speed of the latter is known, the range of the target can be found. This simple system is, of course, not very accurate as it stands, for even with a large cathode-ray tube and a linear scale a measurement of range within, say 100 yards on a time-base whose length represents, perhaps, up to 20 miles is hardly to be expected. This elementary idea is, however, the basis of all range measurement in radar technique, and must not therefore be dismissed lightly as an

The determination of bearing is not quite so easy as impracticable scheme.

BEARING finding range, but it is, nevertheless, much simpler than the determination of eleva­tion. Target bearing is determined by the use of special aerial arrays.

Proud owners of portable broadcast receivers who regard their loop aerial as a 'special'' array, may find the bearing of the local B.B.C. stationbysuitableorientation of their equipment. Radar, requiring a somewhat more refined method, with corresponding accuracy, finds itself unable to rely on sim­ple loops and generally uses such a system as that of the Slowcock array. This system consists essentially of two collinear horizontal half-wave dipoles with centres one wavelength apart connected to the receiver as shown in Fig. 8. When the line of the aerials is perpendicular to the horizontal direction of arrival of the reflected pulse, zero signal is presented to the receiver, even if the wave is arriving obliquely from above. If now the receiver moves slightly off bearing, the direction of arrival, of the reflected signal departs from the zero-signal position and a small signal is presented to the receiving circuits, the phase of this signal reversing as the direc­tion of it passes through that of 'on bearing' or zero signal. Comparison oi the phase of this signal with a 'standard' signal received on another earial then makes it possible for the operator to determine to which side of the receiving array the target has moved and so enables him to return quickly to the zero signal position. In practice it is not convenient to work to a minimum, or null, point, especially when the signal is being observed on a cathode-ray tube, and so special methods of presentation are adopted which in the main consist of either a maxi­mum signal or the equality of two signals for the correct on-bearing position of the receiving array. There are many variations of this type of aerial array for special purposes, but most radar equipments depend on the above simple fundamental system.



ELEVATION

The determination of elevation is the most difficult to achieve and requires a knowledge of the wave reflected from the ground unless the receiving aerial is so directive in the vertical plane that no ground-reflected wave is received. The general system employed is not adapt­able to even a brief explanation, but the essential method relies on the comparison of signals received by. aerials situated at different heights above the ground. This system has its drawbacks, the chief one being that the results obtained depend for their accuracy on the nature of the country surrounding the radar s,ite.' Ideally the ground should be absolutely level, and also perfectly reflecting factors not achieved in practice. The only alternative terthis system is the employment of very high frequency apparatus, when a very directive aerial is employed, the angle of elevation of the target being determined by the tilt of the aerial giving maximum response. This method is not very accurate for frequencies below 200 megacycles per second.





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