RADAR is now such a well-advertised secret that no one, physicist or publicist, stops to tell the simple citizen the simple facts about it--how it works, what it does easily, what it can do only with difficulty if at all, how it may be better used, how it may be frustrated. Yet all this can be put in civilian English without coming anywhere near the limit of what is recorded in military Russian. Radar is at once a science and an art, at once a process, a device and a system. What it achieves depends only in part on physics. It depends very greatly on people, as was shown by those of whom I have spoken elsewhere, in describing radar as the secret that was kept by a thousand women. An unintelligent, uninformed, uninterested or unconscientious operator can effortlessly throw away half a million dollars' worth of radar performance. I have not the means of knowing whether the NATO nations are now giving as much earnest consideration to the man as to the machine. The radar man, or the radar woman, is, I suppose, assumed to be much less newsworthy than the electronic Hobab.

In my wholly prejudiced view, radar began only when there was recognition that the agelong military problem of "What is beyond the Hill" should take, in air defense, the form of an attempt to give a substantially continuous flow of information about the exact position, flying height, composition and character of every formation in a possibly considerable number of spaced formations of aircraft, along and beyond an invisible, intangible frontier of detection and location. The frontier had to be thrust so far towards possible enemy bases as to give time for the interception and destruction of each enemy aircraft before it came near an important target. The unstated demand covered independence of the normal cycles of light and dark, of cloud, rain and fog, and--dans la mesure du possible--of the character of the intervening terrain.

At the risk of talking too much about my personal contribution, I propose to use the first part of that contribution as an introduction to some of the possibilities and difficulties of radar, without using the austere and often obscure language of the specialist. Consulted by the British Air Ministry early in 1935 about the prospect of using a "death-ray" against enemy aircraft, I wrote disparagingly about death-rays, but encouragingly about radio detection rather than radio destruction; and I went on to the writing of a 2,000-word memorandum offering a brand-new project, unsupported by any ad hoc observation or experiment.


What this memorandum of February 27, 1935, said, in language addressed to the scientific but non-specialized reader, can be fairly translated into plain language, without benefit of "hindsight," as follows:

"You need not hope for any help from the enemy in your attempts to locate him by the light, heat, sound or ordinary radio which he sends out. Nor will you get any useful results by spraying him with beams of light, heat or sound. You must put up a short-wave radio frontier which he must penetrate. Simple arithmetic [not reproduced in the present 'translation'] shows that you will get lots of reflected energy back, because the wings and fuselage of his aircraft will act as a good horizontal receiving antenna, and the received currents in them, due to your short-wave transmissions, will be also transmitting currents in this same antenna sending back to you. If, further, you use radio pulses instead of continuous waves, you can use existing valves to much better advantage; they will give much more output power without 'blowing up.' And if conditions allow you to beam your energy, instead of sending it out widely broadcast in the radio equivalent of floodlighting, you can do still better. But you must remember that radio searchlights may miss targets that would be caught in radio floodlighting; so let's try floodlighting first.

"You might well be able to measure the span of the aircraft by varying the wave length you use until you get specially strong reflections. More important, however, is my reassurance that the aircraft is a very flatly tuned antenna, so that for locating it you need not match your wave length carefully to its span; you need not worry about quite a big mismatch. This fact will also help you to hold your target when he turns. The amount of reflected energy you will get back is quite amazingly big.

"Going back to my advice to use pulses (which was first given here because you could safely overrun your transmitter to get greater ranges of location), you will be able to measure the distance of the aircraft from you by measuring the time of travel of the outgoing and reflected pulses, using a cathode-ray tube as a clock ticking off millionths of a second. You can use methods familiar to me and my colleagues, but you'll have to improve the technique very greatly. I am sure we can make these necessary improvements. I am quite seriously visualizing your getting location at 200 miles away; and the cathode-ray method which I propose gives you a very clear picture in which the echo stands out well against the background of 'radio noise.'

"If you can do no measurement other than distance--if, that is, you are confined to radio range finding--you will have to do range-cutting from spaced stations. You will want automatic computers for speedily solving the arithmetic of location. You will often need a long, continuous radio frontier, and you can afford it, for you can get a wide coverage from each station along the front. You can do still better with a front-line and a back-stop line. You will have some trouble with the Ionosphere. My figure of 200 miles may have startled you; but anyway 60 miles is a sure bet, and up to that range you can use tricks to avoid some of the possible difficulties with echoes from the electrically conducting region of the upper atmosphere (for which region, as it happens, I suggested the name 'Ionosphere'). But we shall have to be still cleverer. These troubles are due to the comparatively low frequencies that we must use at first; and we have to use them at first because we know enough about them, and we are in a hurry! On such frequencies we can generate high powers, build sensitive receivers, and we know what the airframe will do about reflecting them. They also risk giving away our secret of radio location. We might fool the possible enemy for a time by a quite plausible cover-story, pretending that we are peacefully surveying the Ionosphere in the interests of pure science.

"But we must get up to higher frequencies, and that means fresh work on techniques. But don't let us be foolish enough to wait for that; let us use now what we know now. To go to higher frequencies might more than halve our sensitivity until we can generate and receive them better than now. But that's not the main catch. We don't want to do just radio range finding.

"We have a very nice way (invented by myself) for seeing directly on a cathode-ray tube the directions from which brief radio impulses are arriving, but we haven't yet used it at the higher frequencies. Even if we could not do radio range finding we could do radio location immediately by a line of these instantaneous visual cathode-ray radio direction-finders, working on 50-meter wave lengths, with automatic computing and display. I am sure we can go on to make it work at ten-meter wave lengths, but that means work--and time. We have gone a long way towards the goal, even though we have not yet arrived (because we have not had to try).

"But that is by no means all we can do. We shall want to combine this direction-finding with range finding. In any case, we want to find the flying height of the enemy aircraft conveniently, and we have a way to do that, too; we have an instantaneous visual cathode-ray radio angle-of-elevation meter as well. We have not had, for our previous purposes, to make it very accurate, but we have, in fact, even used it down to ten-meter wave lengths. We are sure we can make it more accurate.

"It is not worth guessing now at the best way to combine these three direct and continuous sets of measurements--distance, direction and angle of elevation--because I am pretty sure that the only sensible thing for you to do is to ask us at Radio Research Station to put together these pieces, of the things which we have done ourselves or have learned from others, in such manner as to convert the new invention, here disclosed, into an operational process and equipment.

"It is possible I have overlooked some snags (though I do not think so); if I have, then there are two other things we can try, though I do not myself think much of them. First, there is a possible continuous-wave system which would reveal moving targets only; it would tell us directly the speed of the aircraft in line of sight, and two of my instantaneous visual direction finders would give us position fixes, but there are extra tricks needed. You will, however, see that it may, in any case, be important to get a system which 'sees' moving things only, neglecting fixed objects; and here I am offering you one way of doing that.

"Second, there is the frequency-change method, used by my colleagues Appleton and Barnett in exploring the Ionosphere. But I don't see how to get quick enough answers from it. I don't think we should do anything about it (or even think any more about it) unless we find that I have made big mistakes in the encouraging arithmetic of my main proposal.

"It is not, however, enough to locate an aircraft; we ought to know whom we are locating, so that we do not have to stop and think before we shoot. Therefore, we should try to do two more things: on the one hand we should try a new radio method of distinguishing ('discriminating') friend from foe and of distinguishing ('identifying') one kind of friend from another kind of friend. We should do it by a coded intermittent reinforcement of the radar responses we expect. And we must, of course, have good radio communication with our interceptor aircraft to control their manœuvres into a favorable position and altitude for attack.

"Lastly, we can almost certainly, again from our experience and our own novel devices at Radio Research Station, provide automatic following."

How this proposal led to the first tracking of aircraft by radar on June 16, 1935; to the first diagnosis of the strength and tactics of a formation of aircraft on July 24; to the first radar measurement of flying height in August and the first experimental verification of monostatic radar in November 1935, is a separate story. But the limitations of radar are essential to an understanding of radar in defense, and even this "plain language" translation itself needs translation.


The radar problem can perhaps be illuminated by considering the searchlight problem, in which a similar basic difficulty is attacked by different physical means.

The main difficulty about the unaided visual detection of a moderately distant aircraft in the daytime is that the sunlight which it reflects is absorbed by fog, mist, haze or cloud, when these are present, or partly scattered and partly absorbed by the molecules of the atmosphere even in "clear sky" conditions. The twofold difficulty at night is that the aircraft does not reflect a sufficient amount of the feeble moonlight or starlight falling on it to stand out more brightly than the background, and it does not stand out sufficiently darkly against the average night sky background because of this same scattering and absorption between it and the observer's eye. In both cases the basic defect is inadequate contrast between target and background, a lack of contrast due to interference by sources of light--direct light or reflected light--other than the target. The ordinary searchlight attempts to make the aircraft a predominantly strong source of reflected light by projecting strong direct lighting on to it. Quite aside from the needle-in-a-haystack aspect of searchlighting, the too familiar groping in the dark sky, the effort is in any case doomed to very restricted success. For the searchlight beam itself is heavily absorbed even in "clear air," and it makes its own contribution to diminution of contrast by having its light scattered sideways by the air molecules, which thus join the throng of interfering sources of light. So the searchlight has an acute search problem. When that is solved, it is effective at ranges of a comparatively small number of thousands of feet in cloudless weather and is ineffective when cloud intervenes. When the distant aircraft escapes from the beam there is no adequate evidence about its direction of escape (the needle is reëmbedded in the haystack). And at the best there is no direct evidence of the flying height and distance of the aircraft--simultaneous illumination by several searchlights and some quick trigonometry must intervene to establish the data needed by the gunner or the interceptor pilot.

What are the radar counterparts to these difficulties?

The basic problem of establishing adequate contrast between target and background faces the radar man also. He knows that the enemy will not send out "flashes" of invisible radio waves, because these would give away some evidence on the position of the aircraft containing the radio transmitter. So he "illuminates" the uncooperative aircraft with his own radar waves, and hopes to catch and interpret the small fraction of this invisible radio "light" which the aircraft throws back towards him. Why is there not perfect contrast between this radio reflector and its background? Not because the atmosphere scatters the radio "light" as it does visible light; the scattering of radio waves by clear air is negligible, by cloud it is substantial, but not (because it gives on the radar screen cloud echoes that are readily recognizable as cloud-like) very troublesome. Nor is the absorption of the radio "light" by the atmosphere a serious contribution to the difficulties.

Are there, then--to revert to the searchlight story--interfering sources of radio "light" in the sky? Or is it that the reflected radio "light" from the aircraft is just intrinsically too weak to be "seen" at useful distances? Not the latter; because (as the unquoted arithmetic of the 1935 memorandum showed) this could confidently be expected to be "visible" to a radio "telescope" two hundred miles away (because of one technical trick of supreme importance in radio, the ability enormously to magnify, or amplify, as the radio man prefers to say, excessively weak received radio waves). Not wholly, though largely, the former; because the sky is studded with radio stars and the sun is a radio sun, pouring the longer "radio" waves into our atmosphere along with the very short electromagnetic waves which we call light. Moreover, lightning flashes are not only light flashes, they are radio "flashes" sending radio waves towards us from quite large distances. So the radio mirror of the aircraft shows up through a feeble diffuse glow of radio "noise" (as the radio engineer, for obvious reasons, calls it), shot through at intervals by brighter radio "flashes," dispersed more or less all over the sky. And all this may be supplemented and even dominated by a "bright blaze" of radio communication waves, or of such waves deliberately sent on the wave length of the radar by an enemy undertaking radio countermeasures.

There is one more major problem in "seeing" the target from great distances. As just described, the amplification of the radar waves which are reflected by the aircraft is accompanied by the amplification of the cosmic and terrestrial radio noise. In addition, the degree of amplification that can be usefully employed is limited by the presence of radio noise generated within the radio receiver which acts as a radio eye. It is as if in the searchlight case we looked for our illuminated aircraft through binoculars inside which we had a tiny flickering lamp bulb constantly distracting us by its fluctuating internal reflections. This internally generated radio noise, due to the random movements of the electrons in the receiver circuits, can be kept low, but it cannot be eliminated, and it sets the final limit to the minimum radio "brightness" which we can observe.

It sounds formidable. But despite it all, we can extract all the information we require from a radar echo which contains only one hundredth of a billionth of a billionth of the energy which we send out to produce the echo. And although there are radio seasons and radio day-and-night, their variations are not of major importance to us, so that radar performance is substantially independent of season, day, night or weather, save for some cloud effects which are at times troublesome--and often valuable.

If any one particular aircraft were always at the same distance from our radar station when the received fraction is one hundredth of a billionth of a billionth--if, that is, the fraction returned to us were uniquely related to the distance--radar would do almost all that is asked of it by the most exacting user. But this is far from being the case. Just as the useful range of a searchlight is greatly reduced when it is turned nearly horizontally--even over a flat sea--so, though for completely different physical reasons, radar range at low angles of elevation is very greatly reduced relative to high angles. Moreover, the oddities due to the electrical conductivity of the ground, one of them being this poor low-angle performance, include also the existence of a whole series of inclined lanes in which aircraft are detectable only at much shorter range than between lanes.

And, for practical purposes, radar range is not dependably greater than "line of sight" in the special sense that if an extremely sensitive eye at the height and position of the radar antenna could not, under conditions of perfect visibility, see an extremely bright light at the height and position of the aircraft, then radar detection by any normal radar is nearly at its extreme limit of range. The radar rays can always bend a little around the bulge of the earth after grazing it, but normally only a very little. Although special distributions of temperature and humidity in the atmosphere can give exceptional radar "visibility"--sometimes to very great distances--this can by no means be depended on to happen at a selected moment. For the military user, it happens at a random and not usefully forecastable moment; it gives him only abstract satisfaction to know that a radar on the western shores of the Indian sub-continent once "saw" the hills of Aden, 1,600 miles away.

Further, just as a searchlight beam illuminates the tips of hills, the crests of waves, the walls of buildings in its path, so do the radar rays produce radar echoes from similar projections from the level surface. Thus an echo coming from an aircraft may be, and too often is, lost in "ground clutter;" or coming from a ship or a buoy or an iceberg it may be lost in "sea clutter." So on most sites the radar will require a "moving target indicator" which, on request, can be blind to reflections from stationary objects.

The objects which thus give radar echoes also cast radar shadows behind them, but fortunately the shadow "heals up" much more nearly completely for radio waves, with their relatively very great wave length, than for the short waves of visible light. Nevertheless, it is possible for the enemy to skulk in the shadows and in the low-sensitivity lanes, just as it is possible for him to dazzle the radar with "flashes" or radio jamming signals and to confuse it by radar reflections from widely spreading masses of metallic foil strips which are so effective as reflectors that they can simulate the responses which would be sent out from an aircraft formation.


So much for the things just above the average surface. What of things high above, and things below, the surface? There are frequent suggestions that the very high-flying aircraft is likely to escape radar detection, and that the very high-flying missile--V2 and its progeny--would be even more likely to do so. This is true, but not with the almost implacable certainty that protects the low flier at any substantial distance from the radar station.

There are four possible excuses for failure to locate and track the modern very-high-flying aircraft. Two of them are legitimate excuses, two are the product of faults in system design, in the sense that a failure to achieve something that is practicable, but difficult, inconvenient and expensive, is still a fault in system design.

Let's begin with the faults. The first arises from what I might call the pursuit of the celestial omnibus--the hope that a single car with infinitesimal fuel consumption can give flashing pick-up, exhilarating top speed, imperceptible braking, satin smooth riding and dependable reliability, all with the boudoir luxury of a limousine and the capacity of an inter-city coach. Radar can do nearly everything; any one radar can do only a very few things very well. What is required to maintain a continuous watch for medium-level fliers at long range, together with continuous tracking to short range, is difficult enough to embody in one radar equipment. An effective watch for low-flying aircraft over the same total area requires several separate and additional installations. And an effective watch for really high-flying aircraft poses two special design demands which are best met technically, though perhaps not economically, by still another separate radar installation. This installation must adequately "illuminate" the no-man's-land of the higher angles of elevation, and to do this it must cope with the high angular velocity of the fast aircraft at these high angles; so if it searches, it must search fast. But if it searches too fast, then--because unlike the ordinary searchlight it does not pour out its illumination in an unbroken stream, but in a succession of very brief, very intense radar "flashes," spaced very widely in time relatively to their brief duration--it may score a hit on the target with only one or two of these brief flashes in passing, and so fail to get enough energy back for detection. In other words, as the radar engineer puts it, it may achieve too few "paints" on the target. To reconcile these two sources of faulty performance is in itself difficult; to combine the successful resultant high-angle, continuous watch with a lower-angle watch by the same equipment is almost unreasonably demanding.

Now for the two "legitimate excuses." The lesser but far from negligible of these is that such special but not infrequent distributions of temperature and humidity in the atmosphere as we have already noted may "trap" a large fraction of the energy radiated from radar and target alike in a temporary "duct" at low levels. This gives a temporary (but, as was remarked earlier, unuseful because unforecastable) extension of low-angle coverage at a most dangerous expense of a restriction of high-angle coverage. This is one more of nature's contributions to the perplexities of the radar man.

The other "legitimate excuse" is man-made. Thus far we have discussed an aircraft as if it were just an aircraft, but there are aircraft and aircraft, very diverse in their merits and demerits as radar targets. The big-span, plump, many-propellered bomber of 1945 was an excellently copious reflector of radar waves; the disappearance of propellors was and is a disappointment to the radar man. The svelte figure of the jet-propelled interceptor, which should be visible on the screen of the ground-controlled interception radar so that it may be directed towards its prey, is a major worry, for the interception ought, for tidiness, to be made far beyond the radar chain and far out towards the invisible frontier.

Well before the V2's began to reach us in London, we were studying the astonishingly accurate specifications of its size and shape which became available to us from certain highly coöperative sources. We at once recognized that we were faced with an acute radar problem. We could compute with certainty that, nose-on, it would be an extremely poor radar target; it would send very little of our exploratory radar pulse energy back to us. Looked at side-on rather than head-on, it promised to be a moderately good target for our original comparatively long-wave radars of the coastal chain protecting the United Kingdom; but it still would be a very poor target for the centimeter radars that had supplemented (but not supplanted) the radars that were, with a quizzical combination of affection and disdain, called "steam radar" by all save the eight-year veterans of radar. And when the full-scale trials (in both senses) came on us, steam radar did in fact locate the V2 as it was ascending vertically, and thus side-on to England, from its launching point 120 miles away, but lost it as it turned on to the flatter and major part of its trajectory. No need for me to draw the moral or morals of this cautionary tale.

So much for troubles of ground radars. The airborne early-warning radars and the shipborne early-warning radars of the now so well advertised secret defense programs share most of these troubles, accentuate many of them, and add to them formidable problems of logistics, communications and position fixing. But they are all soluble, given money, which is in short supply; technical skill, which is in shorter supply; and operational-technical wisdom, which is in much the shortest supply of all.

It is scarcely necessary to speak here of that other over-publicized item in the defense-radar catalogue, the data-handling system necessary to reconcile and compact the information from a group of radars into constantly clear and un-stale situation maps. But without putting so much as the pressure of a little finger on the highly elastic envelope of secrecy, we must add a reminder that a country worth defending, and capable of defense, will have in its skies a wealth of aircraft which are not hostile, and that their right to live and work unharassed must be maintained. The problem of aircraft identification in radar was posed in the first "prospectus" of radar in 1935; and despite a vast expenditure of ingenuity and effort it was not satisfactorily solved by 1945. It must be solved by 1955. We may hope that there is more hopeful material in the envelope of secrecy than the Multiple Corridor Identification System, the TOMCIS cat that was let out of the bag in a recent magazine article.

We simple citizens should have some general awareness, too, of the scale and variety of the instruments of "radio warfare" that can be brought to bear on an otherwise "solid" radar early-warning radar screen. These form just the newest item of that never-ending series of counter-counter-countermeasures in the agelong contest between projectile and armor.


The sub-surface object of the greatest military importance is the submerged submarine. This is virtually radar-proof, even without anti-radar paint, the thick coating with carefully computed electrical and magnetic properties which greatly reduce its effectiveness as a radar mirror over an octave or so of radio frequencies. The anti-radar coat is valuable to the unsubmerged breathing tube of the schnorkel and to the unsubmerged periscope. The submerged craft finds a vastly more convenient and effective anti-radar coating in the mere body of salt water in which it is immersed. It isn't that all radio waves fail to penetrate a layer of salt water, for it is no secret that radio signals can be sent to submerged submarines from stations a thousand or two miles away. The trouble is that only very long wave lengths thus penetrate to any considerable depth; and very long waves, though they are reflected at boundaries between regions of differing electrical-and-magnetic properties, bring back such crude information about their travels as to be of no value as searchers. The short radio waves which are the highly qualified radar detectives cannot effectively penetrate even a few feet depth of sea water.

Can radar usefully pursue the submarine after submersion? Here all prognosis is gravely unfavorable. But fortunately the technical entrepreneurial daring which was encouraged by the triumphs of radar development offers hopes of other, non-radar, solutions.

Can the limitation to substantially rectilinear-line-of-sight radar detection range be effectively overcome? The scientific prognosis is favorable, the technical not unfavorable, the economic forbidding. A relatively new field of knowledge, which may legitimately be called radio weather and radio climatology, has been opening up at an increasing tempo in recent years. It must not be supposed to be identical with the field which is already identified as Radio Meteorology, though it is intimately linked with that field. Radio Meteorology deals, almost exclusively, with the effects of the lower atmosphere, and of its water content in particular, on the travel of radio waves and, inter alia, with the study of the lower atmosphere by the use of radio tools. What I am calling, in the absence of adequate semantic analysis, Radio Weather and Radio Climatology is the ensemble of systematized information on how the terrestrial atmosphere as a whole, up to levels hundreds of miles above the ten-mile thick weather-layer, governs the conveyance of desired intelligence from man-made devices by imposing it as a modulation on radio "carrier-waves." There are quite recently recognized grounds for expecting that radar information may usefully be gathered from greater distances than near-horizon distance; but how precise the intelligence can be, and above all how nearly the mechanism of travel of the radio waves can satisfy the inexorable military need--"the selected moment, not merely the random moment"--is still a subject for long, close and wide-flung study.


May I now, in conclusion, interview myself briefly?

Question 1: "Can a radar network be designed, on the knowledge available to us within 1953, to give certain warning at a defense command center of the passage of any aircraft over a frontier lying outward, by about six hours flying time, from any major target of attack in the continental United States?"

Answer 1: "My personal answer is broadly 'yes,' but it is subject to many reservations and explanatory supplements which cannot be set out within the limits of an article."

Question 2: "Can an aircraft crossing this frontier be identified within 15 minutes to the degree of being classed as 'very probably hostile' or 'very probably friendly'?"

Answer 2: "I do not think that an adequate or nearly adequate solution to this problem has yet been found."

Question 3: "Can the early-warning network envisaged in Questions 1 and 2 ensure the destruction of an overwhelmingly large fraction of a hostile force before it reaches a major target?"

Answer 3: "Not without the absolutely indispensable activities of an inner radar network for control of interception, and of a very large, exquisitely organized and stringently practised interceptor force."

Question 4: "Can such a destruction ratio as is envisaged in Question 3 be achieved without the network of Question 1 and the organization envisaged in Answer 3?"

Answer 4: "Quite certainly not without the latter, almost certainly not without both; almost certainly 'yes' if both are well designed and well operated."

Question 5: "Are these two radar networks and the elaborate interceptor organization indispensable to the defense of the United States, having regard to other military deterrents to aggression against the United States?"

Answer 5: "Yes."

Question 6: "Do you think the United States economy can afford the provision and operation of this threefold defense system, together with the other deterrents mentioned in Answer 5?"

Answer 6: "Yes. Expensive as they are, they are certainly within the reasonable reach of an economy with a present Gross National Product valued at a billion dollars a day. A just appreciable reduction of the material standard of living in the United States would probably be involved; but it would not be more than a minute fraction of the hardships still being suffered by every nation outside North America on both sides of the Iron Curtain. This I say not from secondhand information only, but from personal observation, in and since World War II, in every continent save Australasia and on both sides of the Iron Curtain."

Question 7: "Have you anything more to say on aids to averting successful aggression against North America?"

Answer 7: "Yes, two widely different things. First, that we must not underestimate the risk that isolated defense airfields, in the Far North for example, could become intermediate airfields used by the aggressor. Second, that as words are still triggers that release bombs, we should be diligent to use no word that tends, avoidably, to unite the ordinary citizens of Soviet and satellite lands with their dictatorial masters."

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  • SIR ROBERT WATSON-WATT, a pioneer in radar development, especially in originating and perfecting the radio-location system
  • More By Robert Watson-Watt