Patent GB-1493422-A (GB)
Title: Echo direction finding and ranging system
IPC: G01S7/04
Filed: 1975-03-26
Abstract: 1493422 Direction finding and ranging FRIED KRUPP GmbH 26 March 1975 [27 March 1974] 12723/75 Heading G1G [Also in Divisions H3-5] A direction finding and ranging system utilizing either sound or electromagnetic waves comprises a plurality of transducers arranged in a rotationally symmetrical array with respect to a plane, a transmitter, a scanner operative to scan the transducers successively to receive echo signals therefrom, a transmitting-receiving switch for connecting the transducers alternatively to the transmitter and to the scanner, a device connected to the output of the scanner for producing a plane light wave modulated in a two dimensional pattern such that the intensity or phase of the wave varies in one dimension as a function of bearing angles of the scanned transducers with respect to a reference direction and in the other dimension as a function of time of reception of the received signals, an optical correlator, to which the modulated wave is applied as an input signal and having an optical reference transmission plate, of graded density or phase variation corresponding to the pattern of modulation of said light wave when signals are received undistorted from the reference direction and in which the instantaneous values of the received signals have been subjected to amplitude staggering, which provides a transmission function representing the directional characteristic of said undistorted signals from the reference direction and a display device, which receives the light constituting the output of the correlator and provides a two dimensional display indicative of the bearing angle and the distance of a reflecting object. In Fig. 4, the transducer W array is cylindrical and transmits a linear frequency modulated pulse signal. Echo signals received by the transducers are passed through T/R switch 3 to amplifier 13 and mixer 14. The mixed down signal is passed through low-pass filter 15 to scanner 5 controlled by clockpulse generator 21 at a frequency at least twice the maximum received signal bandwidth times the number n of transducer W, substantially as described in Specification 1385709 (Division G1). C.R.T. 7 produces the two dimensional plane intensity modulated wavefront. The beam intensity varies with the received signal from scanner 5 and filter 16. Vertical deflection is generated at 18 in synchronism with the TX signal from trigger circuit 20 and horizontal deflection sawtooth generator 19 is triggered by scanner clock 21 via frequency divider 22 which divides by the number of transducers n so that horizontal deflection is triggered at the start of each bearing angle scan cycle of scanner 5. The beam from C.R.T. device 7 passes through optical correlator 8, which includes a reference plate which has an optical transmission factor corresponding to a 2-dimensional reference function, and is displayed at 10. The reference plate in the optical correlator has, as a pattern, a transmission function which corresponds to the pattern of the modulated plane wave front which is produced during an echo sounding period when the transducer array receives the transmitted signal from the reference direction, the instantaneous values of the received signals having been subjected to amplitude staggering. The transmission function also corresponds to the directional characteristic which points in the reference direction. When the modulated light wave is of incoherent light, the transmission function represents the product of the sum of the scanned undistorted signals received by each of the individual transducers, which have been subjected to a directionally dependent time delay in accordance with the disposition of the transducers in the array, and a directionally dependent amplitude staggering function. In the case of coherent light, the transmission function represents the Fourier transformation of the product. In the case of coherent light the pattern of the modulated plane wave front is compared with the pattern on the reference plate by displacing the reference plate two-dimensionally in dependence on bearing angle and time of reception until the pattern of the modulated plane wave front of the light wave and the pattern of the reference plate are coincident. The display on the display device then provides, for each reflection point of an object, a bright light spot which represents the bearing angle and time of reception of the wave energy reflected from that point, so that the object is portrayed in terms of bearing and distance. In the case of incoherent light it is unnecessary to displace the reference plate. In Fig. 5 coherent light from laser source 26 is used in 2-D optical device 7, and passes via collimator 25 and beamsplitter 24, to Titus tube 23 which is a C.R.T. producing an intensity modulated charge pattern on the screen thereof and is plane intensity modulated on reflection from the screen thereof and passes via beam-splitter 24, lens 27, reference plate 9 which consists of at the L.H. side a shutter and at the R.H. side a phase relief representing the reference function, Fig. 3c (not shown), and an amplitude date bearing a transmission function which is the conjugate complex Fourier Transform of the reference function, then to lens 28, and indicator 10, or to television camera 30 via beam-splitter 29 from which a P.P.I. display is derived at 31. In Fig. 6 the device 7 comprises a strip 32 of thermoplastics material having a photo-conductive coating which is fed between two spools 33, 34. Spool 34 is speed controlled by motor 39 and controller to be synchronized with the echo sounding signal from transmission trigger circuit 20. The strip 32 is electrically charged by device 35 and is scanned by a beam representing the instantaneous value of the received signal as displayed as the beam intensity in C.R.T. 36 in horizontal position dependance on the bearing angle #. Vertical deflection is provided by the movement of strip 32. The charge image produced on strip 32 by the beam through lens 41 is transformed by heater 37 into a relief image, the structure of which is determined by the charge structure on the strip 32, and which modulates the beam from laser source 38 which beam is then processed in optical correlator 8. In Fig. 7 an incoherent light beam from a C.R.T. 43 in device 7 is processed directly by an optical correlator 8 which comprises lens 44 having focal length f 3  and an immediately adjacent reference plate 91. An indicator 10 is disposed a distance 4f 3  from lens 44. Reference plate 91 contains a transmission function corresponding to the real part of the reference function comprising the product of the directionally dependent amplitude staggering function multiplied by the prevailing signal received, undistorted in the reference direction 11. In Fig. 8 (not shown) the arrangement of Fig. 7 is elaborated by the addition of a second beam path for a portion of the beam from C.R.T. 43 split off by a beam-splitter (46). A second reference plate (92) is included in the second path and has a transmission function corresponding to the imaginary component of the reference function. The split beams are subsequently recombined in beam-splitter (48) and passed to an indicator (10).