Patent GB-890791-A (GB)
Title: Improvements in and relating to navigational aids
Inventor: ASPDEN HAROLD
IPC: F41G7/30
Filed: 1957-03-21
Abstract: 890,791. Radio navigation. ENGLISH ELECTRIC CO. Ltd. June 18, 1958 [March 21, 1957; July 3, 1957; July 29, 1957], Nos. 9360/57, 21112/57 and 24031/57. Class 40(7). [Also in Group XX] Relates to an interception control system for directing a controlled aircraft or missile along an optimum path to achieve an intercepting position relative to a moving target in which the instantaneous positions of the interceptor and the target are displayed as corresponding image points A and T on a display surface located at a control point, e.g. on the ground, and the control signals transmitted to the interceptor are produced by a human controller or automatically by known photo-electric scanning means. According to the present invention the display is produced by a radar and a reference grid such as the one shown in Fig. 1 of the first Provisional Specification is superimposed on the display, said grid comprising (1) a family of curves indicating pre-calculated optimum paths covering a wide range of possible positions of the interceptor, each curve indicating the bearing to be followed relative to the target course, and (2) an origin point T and reference direction arrow R, the relatitve position and orientation of the grid and display being adjusted continuously to maintain the origin point T of the grid coincident with the display image point T and to maintain the grid reference direction arrow R parallel to the direction on the display corresponding to the direction of target movement.  As described the display surface comprises the screen of the cathode-ray tube Plan Position Indicator (P.P.I) of a pulse radar and the grid may be (1) marked on a transparent member superimposed on the screen of the cathode-ray tube, (2) combined optically with the radar display or (3) produced electronically on the radar display. The grid shown in Fig. 1 of the first Provisional Specification is for use when (1) the target is moving at a uniform speed along a rectilinear path, (2) the intercepting position E is located behind the target and (3) the optimum path is a straight line followed by a turn on to the target course. The effective distance between the intercepting position E and the target T may be varied during operation by selecting different points along the grid reference direction R as the origin point T. The positions of the interceptor at which the final turn should be effected are indicated by the line DED<SP>1</SP> and the positions at which the interceptor should accelerate from cruising speed to combat speed are indicated by the lines BC and B<SP>1</SP>C<SP>1</SP>. The sector to the rear of the target between the points C and C<SP>1</SP> indicates an area in which the possibility of a successful interception depends upon the relative speed and acceleration of the interceptor and target and the sector ahead of the target between the points B, B<SP>1</SP> indicates an area in which the interceptor is in danger of destruction by the target. A graphical method of constructing the reference grid is described with reference to Figs. 1-3 (not shown) of the second Provisional Specification. Optical projection of grid. As shown in Fig. 4 of the second Provisional Specification the grid is formed as a negative transparency on a slide 7 and an image of the grid is projected by means of lenses 6, 8 and a lamp 5 of adjustable brilliance on to a translucent screen 9 identical geometrically with the screen 10 of the radar cathode-ray tube 1, the screens 9 and 10 being viewed in superposition by a semi-transparent reflecting sheet 2. The relative position and orientation of the radar and grid images are adjusted by lateral movement and orientation of the slide 7 or projection unit 4 and the scale of the grid image is adjusted by movement of the slide 7 along the axis of the optical system. Electronic production of grid. In Fig. 1 (not shown) of the third Provisional Specification a picture of the grid is televised and the resultant video signals are combined with the radar video signals and applied to a rotating coil C.R.T. P.P.I. display, the television scan being of the same form as and synchronized with the radar scan; alternatively the radar signals modulate one beam of a double-beam display tube and the television signals modulate the other beam. The position and orientation of the displayed grid is controlled by adjusting the television camera or the grid picture being televised.  In a modification, Fig. 2 (not shown) of the third Provisional Specification, a single beam display tube and the camera tube each have fixed X and Y scan deflection members and the rotating radial display timebase is produced by applying a sweep waveform to a resolver ganged to the rotating aerials, the sine and cosine outputs from the resolver being applied to the fixed X and Y deflection members. The camera tube timebase is produced in a similar manner and the position and orientation of the displayed grid is controlled by applying shift voltages to the camera tube deflection members and by adjusting the relative angular displacement of the rotors of the display and camera tube resolvers. As shown in Fig. 3 of the third Provisional Specification two radar P.P.I. tubes are employed each having fixed X and Y deflection members and a resolver as described above. A fixed reference grid is produced on the screen of the right hand tube by any of the methods described above and the position x and y and orientation # of an electronic cursor produced on the left-hand tube is adjusted by a reference grid control unit to coincide with the position and direction of movement of the target image point T, the control unit also controlling the position and orientation of the radar display on the righthand tube in a corresponding manner so that the target image on this tube is stationary at the centre of the screen and the target course corresponds to a fixed direction on the screen. The cursor line for the left-hand tube is produced by generating in a function generator two signals x +r cos # and y+ r sin # where x and y are adjustable D.C. components representing the X and Y displacements of the mid point of the cursor, r is an A.C. component of pre-determined amplitude determining the length of the cursor and # is the adjustable angle of the cursor. The signal x + r cos # is applied together with the X sweep waveform to an amplitude comparator to produce a pulse each time the two inputs are of equal amplitude and a corresponding pulse train is produced by comparing the signal y + r sin # with the Y sweep waveform, the two pulse trains being applied to a coincidence detector whose output is combined with the radar video signals and applied to the left-hand tube. If one end of the cursor is to be used as the position reference, point then the output signals from the function generator are rectified before application to the amplitude comparators. The orientation of the radar display on the right-hand tube is adjusted by displacing the rotor of the righthand tube resolver and the position of the display is adjusted by applying shift voltages (x cos # + y sin #) and (y cos # - x sin #) to the X and Y deflection members, said shift voltages being derived from the outputs from the control unit. Specification 844,283 is referred to.