Patent GB-1096422-A (GB)
Title: Cathode ray tube displays
Inventor: JACOBS CHARLES E
IPC: G01S7/20
Filed: 1964-12-23
Abstract: 1,096,422. Cathode ray tube displays; radar displays. SHAPE TECHNICAL CENTRE. Dec. 23, 1964 [Dec. 24, 1963], No. 52209/64. Headings H4D and H4T. [Also in Division G4] In a cathode-ray tube display system for radar target identification, alpha-numeric or similar marker symbols for display against selected target returns are generated electronically. In one embodiment the symbols are generated by a matrix 1, e.g. of dual-input "AND " gates or of magnetic cores, whose cross-points are connected by read-out lines in configurations corresponding to the symbol shapes. A PPI (plan position indicator) display with a recurrent radial time base is used, the matrix columns being energized in sequence during respective successive sweeps of the time base, and all matrix rows being energized sequentially during each sweep by a series of high-repetition frequency range clock pulses generated throughout each sweep. The resultant pulse-train on a selected read-out line intensifies the display tube trace to paint a symbol as a sequence of dots. The painting is initiated by the occurrence of a target return at a position on the display tube marked by an operator using a movable lightsensor or an electrical pantograph system. To ensure that the start of the scan of the rows of matrix 1 occurs at the same time (range) in each sweep, the range clock pulses are counted by a first register during each sweep and successive matrix columns are energized each time the count in a sweep reaches the same value as the count in the first painting sweep which is stored in a second register. If it is desired to paint the symbol at a smaller range than the target return, a predetermined number of additional clock pulses may be gated through to the first register. A group of symbols may be painted near a target return, the matrix 1 being scanned the required number of times and the selection of the successive symbols being recorded in a memory or in a switch panel. To prevent the initiation of a painting sequence following a fortuitous bright-up of the screen, the painting may be initiated on the occurrence of the second of two successive target returns only if these occur at the same range. A monostable circuit triggered by the second return may be used to paint a radial marker line through the target return to distinguish it from nearby returns. In order to display the symbols substantially upright on the screen irrespective of target azimuth, the matrix rows and/or columns are scanned in reverse order and the row and column energizations may be exchanged, the necessary switching being controlled according to which quadrant of the display the symbol is to appear in, and the particular quadrant being determined by a register counting the number of radial sweeps for obtaining target azimuth. Control logic prevents the quadrantal transition of the matrix scanning mode during a painting operation. To reduce distortion of symbols near the centre of the PPI display, a proportion of the range clock pulses for energizing the matrix lines are skipped so that, for example, only alternate pulses or every fourth pulse energizes a matrix line, thus expanding the symbol in the tangential direction. The proportion skipped is selected according to target range by logic circuitry including a binary divider chain (Figs. 6 and 7, not shown). In further embodiments, returns from targets equipped with coded transponders are automatically identified by marker symbols. In the embodiment of Fig. 8, all targets transmit their codes when interrogated, and the reception of a code corresponding to that set up in a code selector (12) initiates the painting of an automatically selected symbol against the corresponding target return on the screen. In the embodiment of Fig. 9, the target in a selected location, determined by a range gate, transmits its code when interrogated and the corresponding symbol is painted, the range gating and painting circuits being activated by a light-sensor (S). In the embodiment of Fig. 10 (not shown), all targets whose codes fall within a selected class are displayed and identified by appropriate symbols. In an air traffic control application, selected and identified target returns from several radar displays may be transferred to a central display. The use of time-bases other than the recurrent radial type is envisaged.