Patent GB-750685-A (GB)
Title: Radar training apparatus
Inventor: CUTLER ALBERT ERNEST
IPC: G01S7/40
Filed: 1953-10-01
Abstract: 750,685. Photo-electric radar trainer. COMMUNICATIONS PATENTS, Ltd. Sept. 29, 1954 [Oct. 1, 1953], No. 28063/54. Divided out of 750,628. Class 40 (3). [Also in Group XL (c)] Radar training apparatus of the type described in Specification 750,628 is provided with means for modifying the operation of the apparatus in accordance with the altitude of the simulated radar unit above the earth's surface. The general arrangement of one embodiment includes a chart 15 of the earth's surface on which terrain features are represented by areas which reflect light in differing degrees. Areas of the chart are scanned by means of a light beam derived from a scanning spot on the screen of cathode-ray tube 13 and projected by optical system 14. Signals of an intensity determined by the light reflected by the chart to photo-electric cell 17 are passed to scanning generator and computer unit 18 which controls both the scanning of tube 13 and display tube 19 and also applies potentials to modulate the beam of the display tube to cause the required display to be produced. The tube 13 may be mounted above the chart on a gantry (not shown) and be capable of linear movement in each of three mutually perpendicular directions and angular movement about three mutually perpendicular axes to simulate the corresponding movements of an aircraft. The scanning of tube 13 is such as to simulate a radar aerial beam scanned slowly in elevation and more rapidly in azimuth to produce a pattern of the form indicated at 16. On the display tube (Fig. 1) a sector display is produced by a series of radial scans 11, radiating in the appropriate angular directions to correspond with the chart scan, from point 12 which corresponds to the craft position. The distance from 12 along these lines corresponds to the range of the object indicated. So that objects can be displayed as at their correct theoretical ranges computing means are provided to gate on the beam of the tube 13 when the range defined by the scanning potentials on the display tube is substantially equal to the simulated slant range. The ground plane is regarded as flat. The grid of the tube 19 may be similarly gated by the same means to prevent spurious traces. The computer is supplied with data representing the altitude of craft h, the angles of pitch and roll of the craft # and #, and the azimuth and elevation angle of the radar aerial T and E. Regarding the aerial beam as of small width the slant range R is given by : R=h/Cos E Sin T Cos # Sin # - Cos E Cos T Sin 6 - Sin E Cos # Cos #. If # and # are zero (as for a gyro stabilized aerial) this reduces to R= h/Sin E In practice the beam cannot be regarded as of very small width but may be assumed to be of circular cross-section. It thus forms an ellipse 20 (Fig. 3) where it cuts the ground plane and echoes from A will be received before these from B. To simulate this variation the maximum and minimum ranges R1 and R2 are computed from the expressions R1 = R - R.a.sec b and R2 = R + R.a.sec b where cos b = h/R, and correspondingly delayed pulses are used to open and close the gate. To introduce the equivalent of the difference in timing between echoes received from points A and B the scanning light beam is made much narrower than the radar beam and the spot on the face of scanning tube is made to follow a line such as to scan from A to B at the appropriate time and speed. The time taken is represented by 2 R a sec B/c where c is the speed of light, and thus must be made to vary according to the parameters concerned. It follows that the speed of travel of the scanning spot must be proportional to cos 2b/n. It is noted that a point on the ground which is in the path of the centre axis of a radar aerial beam is in the beam longer than one which is only in the path of the beam edge. This causes display traces corresponding to echoes from objects in the line traversed by the axis of the beam to be elongated in the direction of motion of the beam. To simulate this effect the narrow light beam is given a rapid transverse oscillatory movement so that it scans the ellipse 20 from A1 to B1 along a path such as is indicated in Fig. 5. A suitable computer and gate unit for providing the scan shown in Fig. 5 after the correct delay time has elapsed from the commencement of a range sweep 11 on the display tube is described with reference to Figs. 6, 7 and 8 (all not shown). The computer serves to evaluate the expressions in the equations given above and to provide the transverse oscillatory motion. It may also be arranged to modulate the intensity of the light beam inversely as its distance from the axis of the simulated radar beam as it scans over the ellipse. This provides brighter echoes from objects on the beam axis as in actual practice. In a second embodiment in which a radial scanning aircraft radar in a stabilized support is simulated the radial deflection on the tube 13 (Fig. 9) is arranged to cover a distance X in the difference in time it would take to obtain radar echoes from the points represented by O and P. Switching means prevent displacement of the scanning light beam when the display tube indication represents a range less than the altitude. The appearance of clouds or of interference of the type known as " window " can be simulated by placing pieces of material (e.g. cotton wool) on the chart 15.