Patent GB-684300-A (GB)
Title: Improvements in or relating to reflector systems for radar purposes
IPC: H01Q15/14
Filed: 1951-05-10
Abstract: 684,300. Radio navigation. SVENSKA AKTIEBOLAGET GASACCUMULATOR. May 10, 1951 [May 13, 1950], No. 11040/51. Class 40 (vii). A passive reflector marker beacon system designed to co-operate with radar apparatus comprises two or more co-operating reflectors so arranged relative to each other that an input radio energy beam from a given direction is caused to pass to and fro between two of the reflectors repeatedly, at least one of the reflectors giving off part of its energy at each reflection to return signals to a receiver provided in association with the transmitter of the beam. In an explanatory embodiment, Fig. 3, a vessel 17 carrying radar apparatus is shown moving in the same straight line as reflector units 15 and 16 which may be placed on rocks in approximately the same horizontal plane as the ship. Reflector 15 may comprise a pair of metal sheets joined at an angle in the vertical plane, reflector 16 comprises two intersected sheets in X-formation and must not obscure reflector 15 from radio waves from the vessel, for this reason it may be made of spaced sections as shown in Fig. 4a. Transmitted pulses from the vessel 17 are reflected (a) by the front reflector 16, (b) by reflector 15, (c) by 15, rear reflector 16, and 15 again, (d) by 15, 16, 15, 16, and 15, (e) by 15, 16, 15, 16, 15, 16, and 15, and so on before returning to the craft. The result is the production on the radar display on the vessel of a number of indications of decreasing intensity apparently corresponding in position to a series of points 18, 19, 20, 21, 22, &c. spaced apart by the difference 15-16. If the distance 15-16 is less than the distance the radar is capable of resolving a continuous long indication of decreasing intensity is produced. An alternative form of reflector 16 in the form of spaced W-sections is described with respect to Fig. 4b (not shown). In a variation part of reflector 16 may be made of wire network capable of passing a proportion of the energy to and from reflector 15. The arrangement of Fig. 3 is effective over a narrow range of input beam angles and Figs. 5, 6 and 7 (not shown), indicate how replacement of one or both of the reflectors 15 and 16 by partcylindrical reflectors tends to enlarge the angle within which an input beam produces multiple reflections between the reflectors. Suitable reflectors, Figs. 8 and 9 (not shown), comprise a large number of angular reflectors joined together so that their junction lines lie on a cylindrical surface, and a part cylindrical wire network. Plain or perforated metal surfaces are also suitable and cylindrical reflectors built in spaced sections to allow the passage of wave energy. If two points aligned with the direction of wave energy for which it is desired to produce multiple reflectors are not available for mounting the reflectors two reflectors, a right-angled one and an obtuse (or alternatively narrow) angled one, may be mounted one above the other along such direction and co-operate with a further right-angled reflector to one side. The Specification explains with the aid of Figs. 10, 11, and 10a (not shown), how multiple images are produced by this arrangement. An arrangement of two interlinked sets of three reflectors arranged according to this embodiment and producing indications along both sides of a channel is described with reference to Fig. 12 (not shown). Another way of interlinking two sets of three reflectors is shown in Fig. 14. In this embodiment the right-angled reflector 64<SP>1</SP> and 66<SP>1</SP> of the two sets which are placed to one side of the wave arrival path are placed differing distances from the closely adjacent pairs of reflectors 63<SP>1</SP> and 65<SP>1</SP> forming the remaining units of the sets. When waves are received from the direction of a vessel 68<SP>1</SP> (on course) energy reflected by the wide-angle reflectors of the pairs 63<SP>1</SP> and 65' is directed to the respective reflectors 64<SP>1</SP> and 66<SP>1</SP> and a particular composite pattern indicated. Vessels off course as at 86<SP>1</SP> and 93<SP>1</SP> obtain multiple reflections from only one system of reflectors, the one concerned being different in the two cases so that two different patterns are indicated on the two sides of the course line. The reflectors 64<SP>1</SP> and 66<SP>1</SP> may be on opposite sides of the course line, Fig. 15 (not shown). The Specification also describes arrangements utilizing two or more sets of reflectors spaced along a line from the expected direction of the radar system and giving coded responses, different responses to left and right of course, definitions of the two sides of a channel, and so on. These are described with reference to Figs. 16 to 21 (not shown), inclusive. It is pointed out that the multiple reflection path may be folded back on itself a number of times in zig-zag fashion by the use of additional reflectors so as to take up less ground space, Fig. 22 (not shown). Folding of the reflection path may be particularly useful where the reflector beacon is provided on a high tower and the multiple reflection is arranged to pass up and down inside the tower a number of times, thus dispensing with any reflector at a distance and making the beacon self-contained, Fig. 23 (not shown).