Patent GB-579813-A (GB)
Title: A system of radio vision and location
IPC: G01S13/26
Filed: 1941-09-30
Abstract: 579,813. Radiolocation and television systems. FORMAN, J., and PYE, Ltd. Sept. 30, 1941, Nos. 12624 and 15037. [Class 40 (v)] In a system for locating, or forming images of, distant objects by reflected electromagnetic waves, scanning is effected by passing a " polychromatic " radio beam (i.e. one comprising a range of frequencies) through a refracting prism which disperses the beam to create a radio frequency-spectrum. In the systems described, a short (e.g. 3 centimetres or less) wave narrowbeam transmitter emits pulses, each of which during transmission is modulated through a continuous range of frequencies. The pulses are passed through a refracting prism, Fig. 2b (the refracting medium being, for example, arsine, phosphine, or ammonia gas, or pitch) and emerge with a wave-front, each point of which has a corresponding frequency, indicated by''the discrete frequencies f1, 2, 3, and which is inclined relatively to the perpendicular to the direction of propagation 'owing to the sequential emission of the different frequencies. A reflecting object lying in the beam therefore reflects as the pulse passes it a transitory signal, the frequency of which corresponds to the position of the object in the frequency spectrum extending transversely to the direction of propagation of the pulses. The reflected signals are detected by a receiver, the tuning system of which is given sweeps of the same frequencyrange, time-period, and repetition-rate as those of the transmitter, but with a delay period relatively to them. Reflecting objects situated in the space scanned by the frequency-spectrum, in a plane perpendicular to the direction of propagation and at a distance corresponding to the delay period, will therefore reflect signals, each of which arrives at the receiver at the moment it is tuned to the frequency of that signal, so that the temporal distribution in the sweep period of the received signals corresponds to the spatial distribution of the objects transversely to the beam. In one system described, the high-frequency line-scanning produced by the frequencyspectrum is supplemented by a low-frequency scanning movement of the beam in a direction perpendicular to that in which the spectrum extends, produced either electrically or mechanically. A " plane of vision " in space is thus scanned, an image of the reflecting objects in. which may be produced on the screen of a cathode-ray tube. Systems are also described in which the plane of vision is subjected to a double scan by the use of two transmitting and receiving systems operating over different wavebands and providing two frequency scans in mutually perpendicular directions. When, as in radiolocation, it is desired to detect objects at any distance, instead of in one plane only, the systems are modified to cause the receiver to sweep repeatedly and continuously a number of times through its tuning range after each pulse has been emitted, so causing reception from a succession of planes of vision at distances determined by the time-intervals between the pulse and the individual sweeps. Fig. 6 shows a system providing a single plane of vision. Pulses from a generator 10, after passing a shaper 9, frequency-modulate a transmitter 8 supplying a radiating system 7 comprising an antenna and reflector system providing a narrow beam and a refractor which disperses' it. The frequency-modulation is preferably produced by applying the shaped pulse to the grid, bias, or anode circuit of a non-stabilised oscillator of the negative grid type. To maintain constancy of amplitude of the emitted waves with change of frequency, the system is arranged so that the amplitude of oscillation increases as the frequency shifts into the absorption band of the refracting medium. A tilting mechanism 18, controlled by a sawtooth generator 19, produces the lowfrequency sweep of the beam. The generator 19 also controls one pair of deflecting plates of a cathode-ray tube 20, to produce a low frequency scan of the spot corresponding to that of the beam from the transmitter. The pulse generator 10 also feeds a variable delay unit 11, adjustment of which determines the distance of the plane of vision, and from which the delayed pulses pass to a shaper 12 and attenuators 13, 14, one being fed to frequencymodulate the receiver 15 and the other to the other pair of deflecting plates of the tube 20 to produce the high-frequency line scanning of the spot. The receiving aerial 16 may be omnidirectional, or a beam system which is tilted by means 17 similarly to the transmitting system. The received signals are applied to the grid 23 of the tube, which is normally biassed to cut-off point, to cause the production of a spot on the screen. For a maximum range of operation the maximum rate of pulse emission is 5000 per second, and if the frame rate is 20 per second the image will have 250 line definition. The definition actually obtained depends on the rate of change of frequency and on the circuit selectivity constants. In a similar system, Fig. 7 (not shown), for giving the effect of a series of planes of vision, the frequency-modulation of the receiver and the high-frequency sweep of the cathode-ray beam are controlled, not by pulses from the generator 10, but by pulses from a separate rapid sawtooth generator which are continuously repeated. A pulse time of 0.1 microseconds results in a shift forward of 15 metres of the plane of vision at each successive frequency sweep, effectively covering the whole space'included in the cone in which the transmitted pulses operate. The distance of a reflecting object may be estimated from the size of the image, or found by the use of known time-measuring circuits. Systems employing two' mutually-perpendicular frequency scans of the plane of vision are described. Two transmitters with cylindrical reflectors, giving beams of flat sections extending in mutually-perpendicular directions are used. The transmitters emit pulses which are frequency-modulated over different wavebands in the two cases, and the reflected signals are detected in two receivers, the tuning system of each of which is given frequency sweeps over the same frequency band as that of the corresponding transmitter. Fig. 9a shows the arrival of a wave-pulse from each transmitter at the plane of vision ABCD, the pulses being indicated for simplicity as consisting of discrete' frequency strips f1, 2, 3 and f7, 8, 9 respectively. As the wave pulses, inclined to the plane of vision as shown in mutually perpendicular directions, sweep past the plane of vision they produce a frequency scan f1, 2, 3 in the direction AD and a scan f7, 8, 9 in the direction AB. A reflecting object produces, therefore, two signals, one in each wave-band, which are detected by the corresponding receivers and together by their frequencies indicate the position of the object in the plane of vision. If, as shown, the wave-fronts arrive simultaneously at the point A they will arrive simultaneously at all points on the diagonal AC, but not at other points in the plane, so that the two signals reflected from an object not located on this diagonal will not be received simultaneously. A system is described in which, therefore, the two received signals are stored for subsequent simultaneous application to the vertical and horizontal deflecting plates of the cathode-ray indicator. In this system, Fig. 10 (not shown), pulses from a single generator, after passage through a shaper, control, substantially as in the system of Fig. 6, two transmitters so that they emit simultaneously pulses in different frequency bands, but alike in form and duration. The same pulses, after passage through a variable delay unit and a shaper, also produce the tuning sweeps of the two receivers. As the reflected pulses arrive at the corresponding receivers they trigger relays through which two condensers are charged to the instantaneous voltages of the shaped pulse, these voltages corresponding to the moments of reception of the two signals and hence to the frequencies of these signals and to the two co-ordinates of the position of the reflecting object. The originating pulse from the generator, after passage through a delay unit set to a delay time equal to that necessary for the wave-front to reach and return from the maximum range, then triggers a third relay which connects the charged condensers to the deflecting plates of the cathode-ray tube, and simultaneously controls the grid of the tube to cause emission to occur, the co-ordinates of the resulting spot thus corresponding to those of the reflecting object in the plane of vision. A modification, Fig. 11 (not shown), for scanning a series of planes of vision is described, in which a rapid saw-tooth generator produces continuouslyrepeated frequency-sweeps in the receivers, the flyback causing a pulse which triggers the relay connecting the signal-storing condensers to the cathode-ray tube and simultaneously renders the tube emissive. Another system is described in which two mutually-perpendicular frequency scans of the kind indicated in Fig. 9a provide a complete matrix scan of the plane of vision. As above pointed out, only objects located in the diagonal AC of the plane of vision give rise to simultaneously-occurring signals in the two wavebands, so that by the use of a coincidence circuit at the receivers only objects so located will be indicated, giving rise to a scan of this diagonal. By maintaining the frequency range f1, 2, 3 of one transmitter and receiver constant, and by employing a series (for the purpose of illustration, five) of frequency bands of successively-increased wavelengths f5, 6, 7, f6, 7, 8, f7, 8, 9, f8, 9, 10, f9, 10, 11 for the second transmitter and receiver, each pair of simultaneous pulses from the two transmitters results in a similar diagonal scan, successive scans being, however, owing to the frequency shifts, relativel