Patent GB-638274-A (GB)
Title: Improvements in and relating to radar and like systems for detecting and locating objects
IPC: G01S7/06
Filed: 1945-11-07
Abstract: 638,274. Radiolocation. KEYNES, R. D., PARKER, E., and WRIGHT, C. S. Nov. 7, 1945, No. 29655. [Class 40 (vii)] Relates to means for indicating the height or angle of elevation of a target in a pulse radar or equivalent infra-red or sonic system employing a cathode-ray tube type A, or P.P.I. display. According to the invention the radiated beam is movable through a, selected angle in elevation and a transparent cursor is located in front of the screen of the cathode-ray tube, the required indication being obtained by producing a relative rotational movement between the tube trace and the cursor which is synchronized with the changes in beam elevation. As applied to a display with a single stationary range trace, e.g. type A or P.P.I in which the azimuthal aerial scan has been stopped at a particular bearing, the apex 0 of a sector-shaped cursor, Fig. 3, is pivoted at the origin of the trace and the cursor is oscillated over the trace in synchronism with the changes in aerial elevation. The cursor is engraved with radial lines Oe, Oa... Od representing angles of elevation and the position of a response appearing on the screen with respect to the cursor indicates the angle of elevation of the target producing the response. Lines of constant height a a<1>, b b<1> ... are also engraved on the cursor and the spaces between them are coloured differently so that the colour of the response seen through the cursor is indicative of the height zone of the target; alternatively, the angular sectors may be coloured differently. In a system with a P.P.I. display in which the aerial rotates relatively slowly in azimuth and is simultaneously scanned rapidly in elevation, the cursor which is pivoted at the centre of the P.P.I. screen is rotated in synchronism with the azimuthal rotation of the transmitted beam and has superimposed on that rotation an oscillatory movement synchronized with the elevational scan. When the elevational scan is slower than the azimuthal scan as in helical scanning, the cursor is rotated slightly faster than the azimuthal rotation of the transmitted beam. In a preferred embodiment with a P.P.I. display, a tilted vertical scan is employed as shown in Fig. 1 in which the aerial beam scans in elevation in the plane AB inclined at 25 degrees to the vertical, the maximum elevation of the beam being 30 degrees so that the beam is shifted 15 degrees in azimuth when the top of the elevational scan is reached. The time-base deflector coil of the P.P.I. tube is rotated in synchronism with the normal azimuthal rotation of the aerial and an auxiliary deflector coil is provided to superimpose on the normal trace rotation an oscillatory movement synchronized with the elevational scan. The cursor is rotated in synchronism with the normal rotation of the aerial so that during an elevational scan period the trace moves relative to the cursor and the elevation or height is indicated as before. Since any one radial element of the tube screen is scanned a number of times corresponding to different angles of elevation, the responses should not be persistent, but in order to retain the usual persistence feature of a P.P.I, display a two-colour tube may be employed with a white direct response and a monochromatic afterglow colour which is not present in the cursor so that the afterglow indications will be visible on the entire screen except the portion being viewed through the cursor. A source of infra-red radiation can be arranged to rotate with the cursor and to shine on the screen to wipe out the afterglow just in front of the leading edge of the cursor; alternatively, a tube with a bright direct white flash and a low intensity afterglow may be employed. When an interrogator is incorporated with the system, the response from a responder may be separated from the response from the target carrying the responder by mechanically disposing the interrogator aerial with respect to the radar aerial so as to lag behind it during rotation by a small angle, e.g. 10 degrees, the cursor being provided with an additional coloured portion covering the responder response. In place of a normal cathode-ray tube with a fluorescent screen, a dark trace tube may be employed, the responses being viewed either directly or by optical projection. on to a large screen.