Patent GB-892674-A (GB)
Title: Improvements in or relating to radar systems
IPC: G01S1/02
Filed: 1960-01-08
Abstract: 892,674. Radar. LABORATORY FOR ELECTRONICS Inc. Jan. 8, 1960 [Jan. 8, 1959], No. 778/60. Class 40(7). The position of a fan-shaped radar beam in a plane normal to the plane in which the beam scans is changed cyclically in sequential steps during the scanning operation so that the dimension in the first mentioned plane of the sector scanned is greater than that of the sector which would be scanned if the position of the beam in this plane remained fixed. In an application of the invention to increasing the altitude coverage of a conventional airport surveillance radar (A.S.R.) having a normal coverage (e.g. 12,000 feet) as indicated in Fig. 4 for the lower beam (the beam effecting the usual horizontal scan about axis 11), the elevation angle of the beam is cyclically varied in sequential steps to increase the altitude coverage (e.g. to 24,000 feet) as shown by the upper beam position. The step variation may be effected after each complete horizontal scan producing the coverage shown in Fig. 5, in which lines 25, 26 (the 3 db points) indicate the coverage of the lower beam scanned in the direction 20, and lines 27, 28 (the 3 db points) that of the upper beam, scanned in the direction 21, both. beams overlapping by an amount 29. When the (horizontal) scanning rate is low (noise integration, on the recever display tube not being involved) the range is the same as that of the conventional system. As the scanning rate increases the percentage of the total time during which the target is "seen" is reduced and results in a reduction of effective range. For any scanning speed the "information rate" is decreased for targets outside the overlapping portion 29 of the two beams since such targets are detected only on alternate scans. Instead of effecting the step variation at the end of each scan it may, in the case of a pulse radar system, be effected at the pulse repetition frequency of the system as indicated in Fig. 6 where successive black dots indicate the position of the centre of the beam the latter being "off" during the position shift steps. This arrangement, however, whilst producing no appreciable reduction-in "information rate" suffers from the disadvantage of reduced range performance the range itself being reduced by a factor approximately equal to <SP>4</SP>#1/N - where N is the number (2 in the embodiment of Fig. 5) of discrete step variations. Other factors which reduce the effective range are scintillation effects due to the reduction by factor N of the number of hits per beam width and further the masking of the target echoes by the effect of clutter and precipitation return from N step variations. For height finding applications the scanning may be in the elevation plane and the step variation effected in the azimuthal plane (Figs. 7 and 8, not shown). In a practical arrangement (Fig. 9) a microwave switch 44 supplies energy to aerial feeds 1, 2..n and is controlled by generator 46, either directly by the master pulse generator 41 (switch 51 in "up" position) to produce the scan shown in Fig. 6, or via a rotary coupling 45 (switch 51 in "down" position) which is mechanically coupled to and rotates with the aerial 43 to produce the scan shown in Fig. 5.