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ENCLOSURE TO SIGNAL SCHOOL'S 1089/38 DATED 22/2/38 DETECTION OF AIRCRAFT AND SHIPS BY RADIO METHODS Introduction 1. For the defence of Great Britain against air attack it has been necessary to evolve some method of detecting aircraft at sufficient distance to allow adequate time for our fighter aircraft to intercept them. 2. Of the various methods suggested, the most promising is that depending upon the reflection of wireless waves. Analogous methods may be used in the future for the detection of ships. 3. Most of the work on this subject has been carried out at the Air Ministry Research Station, Bawdsey, which was crested for this special purpose. Every financial and administrative assistance has been given to expedite the work and very substantial progress has been made in the last two years. A party of workers at Bawdsey have also concerned themselves on behalf of the Army with the application of radio reflection methods to the problems of anti-aircraft gun laying. 4. The early detection of hostile aircraft and improvements in anti-aircraft gunnery are matters of importance to the Fleet. Experiments have therefore been commenced at H.M. Signal School to apply the same principles for Naval purposes but the work has so far been on a comparatively small scale. Range of Detection of Aircraft 5. The maximum range at which detection of aircraft by reflection of wireless waves is possible depends upon the power of the transmitter and the sensitivity of the device used for reception of the reflected rays. By an intensive campaign of research and experiment, the conditions for obtaining long ranges have been explored over a limited band of wavelengths, and it is now possible to detect aircraft at ranges of rather more than 100 miles. Such results involve the use of very tall masts carrying elaborate aerials both for transmission and reception. The methods in use will give the range correct to one or two miles up to a range of 130 miles and the bearing correct to one or two degrees up to a range of 70 miles, with a very rough estimate of height to within a few hundred feet. It must be emphasised that these ranges are only possible for high flying aircraft, say at 10,000 feet. At a height of 1000 feet, the ranges are reduced to a third. 6. Such a performance can only be achieved with relatively elaborate apparatus working under favourable conditions on shore. Nothing like this performance is possible at the present time in a ship. Limitations of mast height, permissible aerial dimensions and interference from ship's structure must be expected to reduce to a lower level the performance possible in a ship. 7. The primary problem at Bawdsey is to achieve maximum range with accurate bearing. It is essential not merely to get warning of the approach of hostile aircraft; it is equally important to get the fighters on to their objectives in the minimum time; therefore precise location is vital. As a result of the progress that has been made, work is now in hand establishing a coastal chain of Stations for air defence. Each Station will have four masts 350 feet high for transmitting and one about 240 feet high for reception. The highest powered transmitters it is possible to build will be installed. The Stations comprised in this chain are referred to as R.D.F.1., and constitute the most important and urgent commitment of the Bawdsey Research Station. Three of the Stations are already in action with crews trained at Bawdsey but using transmitters of an extemporised type not so powerful as the ones shortly to be fitted. 8. The importance of achieving a practical result by a definite date has limited the amount of research and experiment that has been possible and many important technical considerations are demanding further investigation. While it is not very likely that these will lead to a striking increase in range, they may well lead to increased reliability of detection, to greater speed of operations, and greater accuracy of location. 9. The work at Bawdsey has centred chiefly for the needs of shore defence and has not been related to ship requirements. The problem of devising apparatus suitable for a ship is more difficult than for a Station on shore. It is more difficult to get the same range and much more difficult to get bearings and elevations. Preliminary experiments carried out by Signal School in HMS Saltburn have given ranges of 15 miles for detection of an aircraft with bearings within 20 degrees. Extreme ranges of 30 miles are not uncommon but the reliable range is about 12 miles. There can be little doubt that systematic investigation on an adequate scale will increase the reliable range. The work carried out in Saltburn up to date serves chiefly to show that ship conditions are not prohibitive for ranging but are less promising for bearing and elevation. 10. In addition to developing R.D.F.1., for the main coastal chain, work is proceeding at Bawdsey on R.D.F.2., which comprises a smaller edition of R.D.F.1., situated inland and is intended to deal with those aircraft which succeed in getting through the outer screen of R.D.F.1. 11. R.D.F.2., in its technical aspects is closely allied with mobile sets intended for Army use. These sets are necessarily limited in size of mast, types of power plant, etc, and have very much reduced ranges as compared with the Stations of the R.D.F.1., chain. Sets of this type may well be used in the defence of the mobile naval base. A first set is now undergoing trials. Possible Use of RDF Gunnery Application. 12. The methods of RDF give both range and bearing. Quite recently it has been found possible to get ranges of aircraft at distances up to 6 miles correct to 50 yards. This is an achievement of great importance because it is superior to that obtained with the best optical range finders. It should be noted that the range so given is the slant range. Bearing in azimuth correct to a few degrees are possible but elevation cannot at present be measured by RDF. Nevertheless, the accuracy of the range is such as to make this an important part of anti-aircraft gunnery equipment. 13. The complete location of an aircraft sufficiently accurate for gun-laying solely by RDF is not therefore at the moment a practical possibility. The work done on this matter has been restricted to a wavelength of the order of 7 metres, but there are very good reasons for extending the work to much shorter wavelengths and it is not yet possible to say what important developments may arise in this connection. 14. A matter of importance is to define within what limits the RDF methods are to resemble searchlights with a sharply defined beam or to what extent they are to resemble flood-lighting with illumination throughout a large volume. Sharply defined beams, while having the merit of high accuracy of direction, have their own difficulties in point of search and training on a rapidly moving target. The higher the speed of aircraft the greater will be the difficulty of training on an individual target with a directive beam. The degree of directivity required influences greatly the wireless technique involved. 15. It should be noted here that while optical methods of ranging and detection are valueless in poor visibility, RDF is uninfluenced by darkness, fog or cloud. Detection of Ships 16. Since aircraft will reflect wireless waves, it is reasonable to expect that reflections can be obtained from ships. Experiments have been made with this end in view, but experience to date shows that the range of detection is very much less. A ship is a relatively small excrescence on the sea which is a large conducting surface. In this respect it is different from an aircraft surrounded by the non-conducting medium of air. Nevertheless, ships have been detected at ranges of a few miles both a Bawdsey and at Signal School. 17. No more than a start has been made in this subject and little is known at present as to the best wavelength to use, whether it should be about 1 metre as at Bawdsey or 20 cms as at Signal School, or whether longer waves, after all, may be best. RDF in Aircraft 18. In order to assist a fighter to close on its objective as rapidly as possible experiments have commenced at Bawdsey with RDF apparatus fitted in an aircraft. As might be expected, the transmitted power is small and the range at the moment only a mile or so. But as happens so often in scientific work, research directed towards one object has brought to light other possibilities not originally contemplated. It has been found, for instance, that reflections are readily obtained from the ground and that RDF provides an altimeter independent of barometric pressure. Again, the exact kind of reflection obtained from the ground depends upon the nature of the country and it is found possible to tell whether an aircraft is flying over open country or over a town or is crossing a coast line. This is a matter of great significance. Detection of a town from above cloud level is now more than a possibility. 19. In the course of work on detection of aircraft from aircraft, it has been found possible to get reflections from ships. The movements of the Home Fleet to Invergordon on the 4th and 5th of September 1937, afforded an opportunity of examining the magnitude of the echoes obtained from ships. On the evening of the 3rd September, HM Ships Rodney and Courageous and four destroyers were located ten miles south west from Beachy Head and a number of observations were made. The aircraft's flying height was about 1300 feet and substantial echoes were obtained at ranges up to 5 miles. 20. Further flights were made on 4th September in about latitude 52 degrees north over HMS Courageous and destroyers. It was then found that detection could still be obtained at heights of 3000 and 5000 feet though there was some falling off in amplitudes at 5000 feet. The size of echo is related to the size of the ship. HM Ships Rodney and Courageous gave the largest echoes and the destroyers substantially smaller echoes. 21. These results have a most important bearing on air reconnaissance in general and in particular on the possibility of shadowing the movements of a ship at night or in conditions of poor visibility. RDF provides an aircraft with a means of detecting a ship anywhere within an area of about 75 square miles immediately beneath it; no matter what the visibility. These ranges are very likely to be increased by improved transmitters and receivers. It is not unreasonable to expect the range to increase to 10 miles in the near future, when the area of effective detection by a single aircraft will increase to 300 square miles. Subsidiary Problems 22. The development of RDF for its primary purpose of defence has brought to light a number of important problems demanding serious attention before an accurate estimate can be formed of the real scope and power of RDF methods under conditions of actual warfare. There is, for example, the possibility that transmitters for RDF will be liable to give away a ship's position. It would be very easy to exaggerate this danger. Any use of W/T is attended with some risk and, in view of the very short wavelengths likely to be used for RDF, it would be unwise to be unduly pessimistic on this score. The answer can only be obtained by practical trial. 23. The aerial requirements in a ship for satisfactory RDF will probably be exacting and will almost certainly conflict with some existing or contemplated W/T arrangement. 24. A matter receiving attention at Bawdsey is "recognition". It is important to be able to decide between friendly and hostile aircraft. There is some hope that this can be done by methods which depend upon getting a distinct type of reflection from friendly craft. 25. A further subject on which experiment is proceeding is that of deliberate interference to RDF by an enemy specially equipped to produce electrical disturbances which upset an RDF receiver. The extent to which RDF can be rendered inoperative by deliberate jamming is now the subject of a full scale trial. Up to date, jamming has not proved so easy as might have been anticipated. It must be remembered that the act of jamming is itself a warning of attack. Conclusions 26. RDF is a subject in its infancy. Its potential importance can only be estimated, but enough has been done to show that a certain performance is possible. At the present time, the work proceeding on behalf of the Navy is tentative and restricted. If the full value to the Fleet of this important development of science is to be made available, the scale of attack must be increased. There is good reason to believe the subject is now receiving attention abroad, and since it is impossible to confine any discovery or invention to one nation for long, it is of vital importance that, at any given moment, this country should be ahead of any other in the practical application and use of this new scientific aid.
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