579,154

PATENT SPECIFICATION

Application Date: March 29, 1940. No. 5687/40

Complete Specification Left: March 28, 1941.

Complete Specification Accepted: July 25, 1946.

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PROVISIONAL SPECIFICATION

Improvements in or relating to the Detection and/or Observation of Reflecting Objects by Means of Radiation

I, ALAN DOWER BLUMLEIN, of 37, The Ridings, Ealing, London, W.5, a British subject, do hereby declare the nature of this invention to be as follows:

The present invention relates to the detection and/or observation of reflecting objects by means of radiation such as electromagnetic, supersonic radiation or the like.

It has already been proposed to radiate short bursts of electromagnetic waves and to receive said waves after reflection by reflecting objects such as aircraft in order to detect the presence of such aircraft or to determine their distance by measurement of the times interval between the transmission and reception of said waves.

It is desirable, in order to increase the effective power of said waves, to employ directional transmission, and to cause said waves to sweep through the solid angle within which it is desired to search. The directional transmission system must therefore be readily movable, and consequently it has to be relatively small and it is therefore preferable to employ a very short wave radiation of the order of a few centimetres in order that good directivity may be obtained.

It is known that it is difficult to generate and to control oscillations of such short wavelength, and consequently the generation of short bursts of such oscillations gives rise to serious practical difficulties.

It is the object of the present invention to provide an improved system of the kind referred to in which the above mentioned difficulty is overcome.

According to one feature of the present invention, there is provided a method of detection and/or observation of a reflecting object which comprises radiating waves having a given frequency f1 and a given frequency f2 in successive time intervals receiving said waves after reflection by said objects by means of a receiver in which the received waves are heterodyned by oscillations from the source of said radiated waves, whereby said reflected waves of frequency f1 and f2 are heterodyned by oscillations of frequency f2 and f1 respectively to a common intermediate frequency.

Preferably said radiation is ultra short wave electromagnetic radiation and is provided by a magnetron or Barkhausen Kurz thermionic valve oscillator, the frequency of oscillation of which is changed from f1 to f2 and back to f1 at regular time intervals by changing the voltage applied to one or more electrodes of said valve. If desired, the invention may be applied to the transmission and reception of supersonic waves, for example, for the detection by reflection of submarine objects.

The decrements of the intermediate frequency circuits and/or detector circuit of said receiver are preferably arranged to be of the order of one third of the time period between successive changes in the frequency of said oscillations in order to decrease the differences in the amplitude of the received reflected signals from objects at different distances from the transmitter, and said receiver may also be provided with automatic frequency control means controlling the amplitude of the voltage changes applied to effect the change of oscillation frequency of said thermionic valve oscillator.

According to a further feature of the present invention, there is provided a method of detection and/or observation of a reflecting object which comprises transmitting radiation in a narrow beam, changing the direction of said beam cyclicly within a predetermined angle, receiving the radiation reflected by said object, causing said received radiation to control the intensity of a cathode ray beam in a cathode ray tube, and causing said cathode ray beam to scan a line on the screen of said tube, the angular position of which varies continuously during a predetermined time interval, whereby the distance of said object is indicated by the angular position of a light or dark line on said screen.

One way in which my invention may be carried into effect will now be described by way of example.

In order to carry my invention into effect I provide a directional short wave transmitter and a short wave receiver preferably provided with cathode ray indicating apparatus. The transmitter is preferably arranged to operate at a mean frequency of 6000 megacycles/sec. And may consist of a magnetron or Barkhausen Kurz oscillator operating at this frequency and coupled to a dipole aerial tuned to this frequency and mounted at the focus of a parabolic reflector so as to radiate in a substantially parallel beam. Associated with the transmitter means are provided for changing the frequency of the radiated waves at regular intervals, which may be of 200 microseconds duration. Said means preferably comprises a multivibrator generating square topped pulses t a frequency of 2500 cycles/sec., so that teach positive or negative pulse has a duration of 200 microseconds, and these pulses are preferably aplied to one of the electrodes of the thermionic valve of the oscillator to modify the frequency of generated oscillations alternately by plus and minus 10 megacycles/sec. every 200 microseconds, so that the frequency of the oscillations transmitted will be alternately 6000 megacycles/sec. and 6010 megacycles/sec. In successive 200 microsecond intervals.

Means are also provided for moving said parabolic reflector within the solid angle to be explored so that the radiation is successively directed in all directions within said solid angle.

The receiver may comprise an aerial adjacent the transmitting aerial connected to a rectifier, the output of the rectifier being connected to an intermediate frequency amplifier which serves to amplify the heterodyne signal produced by the beats between the signal picked up direct from or fed from the transmitter and the received reflected waves. Alternatively, a single aerial may be used for transmitter and receiver, the rectifier being connected to this aerial as well as the transmitter. Furthermore, the transmitter oscillator valve may be used as the rectifier, the intermediate frequency components of its anode current being amplified by intermediate frequency amplifier. In the case mentioned above the intermediate frequency of the receiver will be 10 megacycles/sec., although of course any other suitable intermediate frequency may be employed.

The system operates as follows. When the transmitted waves fall upon a reflecting object, waves are reflected back to the receiving aerial. Provided that the time taken for the transmitted waves to travel to said reflecting object and back to the receiver is less than 400 microseconds, i.e. the reflecting object is not more distant from the site of the transmitter and receiver than 60 kilometres, the received waves will differ in frequency by 10 megacycles/sec. from the transmitted waves for at least a part of each 200 microsecond interval and will therefore be heterodyned to 10 megacycles/sec. in the said receiver, thus giving a rise to a signal at the frequency to which the intermediate frequency of the receiver is tuned, which signal may be rectified and may serve to indicate the presence of said reflecting object.

It will be appreciated that the system described above has the advantage that the received signal is heterodyned to a much lower frequency at which it may be conveniently amplified without requiring oscillators of extremely high frequency stability for the transmitter and for the local oscillator of the receiver. The only requirement as to frequency stability is that the drift of frequency of the oscillator serving according to the invention for transmission and for reception shall remain during an interval of 200 microseconds within the band width of the intermediate frequency amplifier of the transmitter.

Further, the change of frequency of the oscillator by 10 megacycles/sec. at regular intervals of 200 microseconds is readily accomplished with the desired frequency stability above referred to, whereas, as previously stated, the nature of known high frequency generators is such that the generation of oscillations of a given frequency for short time intervals is exceedingly difficult, and the appreciable time required to build up and damp out such oscillations imposes an undesirable lower limit to the distances from which reflected signals may be received, since such signals cannot conveniently be received while the transmitter is in operation at the same frequency.

It will be understood that the time during which the reflected signal will be heterodyned at the intermediate frequency of 10 megacycles/sec. at the receiver will increase as the distance of the reflecting object increases up to a given distance dependent upon the duration of the time interval (in the example given 200 microseconds) during which the transmitted waves remain at a constant frequency. Thus, if the reflecting object is at a distance of 300 metres, the signal will return in two microseconds, and therefore only the signal radiated during the last two microseconds each 200 microsecond time interval will be heterodyned to the desired intermediate frequency. On the other hand, if the reflecting object is at a distance of 10 kilometres, the signal radiated during the last 67 microseconds of each 200 microsecond time interval will be heterodyned to the desired intermediate frequency. Thus, when the reflecting object is at a greater distance, and the reflected signal is therefore weaker, the signal is received for a greater proportion of the 200 microsecond time interval, and this effect may be utilised according to a further feature of the invention, to compensate the decrease in the intensity of the received signals as the distance of the reflecting object increases.

In order to utilise this effect, the band width of one or more of the intermediate frequency and/or the post detector circuits of the receiver may according to another feature of the invention be reduced so that the energy of the received signals may be effectively integrated. Thus, the intermediate frequency circuits may be given a long decrement, preferably equal to about one third of the period between successive frequency changes of the transmitted waves, so that the weaker signals may build up oscillations over a relatively long period to an amplitude comparable with the amplitude built up in a much shorter period by the stronger signals from a nearer reflecting object. As, however, an intermediate frequency circuit having decrement of the order of 70 microseconds has a relatively narrow pass band, this necessitates a very high order of stability of the transmitter frequency. It may therefore be preferable in practice, particularly when moving objects are to be observed, to use intermediate frequency circuits of relatively short decrement, say 7 microseconds, and consequently relatively wide pass band, and to provide a circuit of narrow pass band and long time constant following the detector. Such a post detector circuit may be arranged to alter the overall decrement of the receiver from 7 microseconds to say 70 microseconds so that the waveform of received signals correspond to those obtained through a single time constant of 70 microseconds. Said last mentioned circuit may comprise a resistance in series and a series connected resistance and condenser in shunt in the output circuit of the detector, the shunt circuit having in this example a time constant of 7 microseconds.

It will be appreciated that the other circuits of the receiver, with the exception of the circuits above referred to, and preferably arranged to have a relatively wide pass band of the order of two megacycles/sec.

In order to improve the stability of the intermediate frequency in the receiver an automatic frequency control circuit may be provided, according to another feature of the invention. A discriminator circuit of known type may be provided in the intermediate frequency amplifier of the receiver prior to the long decrement circuit, if any, and may be arranged to develop a D.C. bias proportional to changes in the intermediate frequency of the received signals. This bias may then be applied to the multivibrator controlling the frequency of the transmitter so as to control the amplitude of the pulses generated by the multivibrator and hence the frequency of the transmitted waves. This control should preferably be arranged to be slow in operation so as to be actuated by signals of relatively short duration recurring at relatively long intervals such as will be received when the transmitted radiation only strikes a reflecting object once in every complete cycle of movement within the solid angle which is to be explored.

It will be appreciated that the pulses delivered to the transmitter by the multivibrator should be as rectangular as possible in order that the changes in the frequency of the transmitted waves may take place as rapidly as possible, since no observation can be made during this change of frequency and consequently the minimum distance of which a reflecting object can be observed is determined by the time required to effect this change of frequency. Preferably, this change of frequency should be arranged to take place in less than half a microsecond so that the said minimum distance is not greater than 75 metres. It may also be desirable to filter said pulses so as to remove components having frequencies equal to the intermediate frequency of the receiver so as to minimise spurious signals during the periods when the transmitter frequency is being changed.

The received signals are preferably observed by means of a cathode ray tube. Signals derived from the output circuit of the receiver are preferably fed to the control electrode of the cathode ray tube so as to control the intensity of the cathode ray beam, and said beam is preferably scanned across the fluorescent screen of the tube in synchronism with the movement of the reflector and transmitting aerial, so that when the said reflector is directed towards a reflecting object, the intensity of the trace on the screen is modified and the angular position of the said reflecting object thus indicated by a scale on the said screen.

In order to indicate the distance of a reflecting object, according to a further feature of the invention, the signals may be applied to the control electrode of a cathode ray tube and a relatively high frequency deflecting oscillation, which may have a frequency of three megacycles/sec., may be applied to both pairs of deflecting plates, the relative amplitude of the oscillations applied to said deflecting plates being varied cyclicly so as to cause the cathode ray beam to trace on the screen a line which rotates through a given angle in a given time and then returns rapidly and recommences its rotation. For this arrangement the main scan which follows the movement of the reflector is preferably arranged in a step by step manner so as to give discrete circular patterns. The said given time is preferably arranged to be equal to the time interval between successive frequency changes of the transmitter, so that the angular position of the brightest or darkest line traced by the beam indicates the time taken for the signal to reach and to return from the reflecting object, and the screen may thus be calibrated to indicate the range of the said reflecting object.

Although the invention has been described with reference to the detection and/or observation of reflecting objects such as aeroplanes, it will be appreciated that it is also applicable to the detection and/or observation of any objects which will reflect short wave electromagnetic radiation and may be applied, for example, to the navigation and aircraft or ships under conditions of bad visibility.

It will also be appreciated that the various features of the invention may be used separately or in combination.

Dated this 28th day of March 1940

F.W. Cackett

Chartered Patent Agent

COMPLETE SPECIFICATION

I, ALAN DOWER BLUMLEIN, of 37, The Ridings, Ealing, London, W.5, a British subject, do hereby declare the nature of this invention and to what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:

The present invention relates to the observation of reflecting objects by means of wave radiation such as electromagnetic or the like.

It has already been proposed to radiate short bursts of electromagnetic waves and to receive said waves after reflection by reflecting objects such as aircraft in order to detect the presence of such aircraft or to determine their distance by measurement of the times interval between the transmission and reception of said waves.

It is desirable, in order to increase the effective power of said waves, to employ directional transmission, and to cause said waves to sweep through the solid angle within which it is desired to search. The directional transmission system must therefore be readily movable, and consequently it has to be relatively small and it is therefore preferable to employ a very short wave radiation of the order of a few centimetres in order that good directivity may be obtained.

Further, in order to determine the distance of reflecting objects near to the transmitter, it is necessary that the transmitted waves shall be cut off very rapidly at the end of each burst so that the reception of the reflected waves shall not be masked by the residual transmitted waves.

The transmission and reception of such short bursts of high frequency oscillations gives rise to serious practical difficulties due, for example, to lack of frequency stability and finite decay time of the transmitter.

It is the object of the present invention to provide an improved system of the kind referred to in which the above mentioned difficulty is overcome.

According to one feature of the present invention, there is provided apparatus for the observation of a reflecting object comprising transmitting means for radiation waves given frequency f1 and f2 in successive time intervals and receiving said waves adapted to receive waves reflected by said object and to heterodyne received waves of frequency f1 and f2 with waves of frequency f2 and f1 respectively to a common intermediate frequency.

Preferably, said receiving means is provided with means adapted to integrate said intermediate frequency signals and/or signals derived therefrom, whereby variations in the output from said receiver due to variation in the distance of said object are reduced.

If desired, said receiving means may be provided with frequency responsive means adapted to develop a control signal responsive to variations in said intermediate frequency and said transmitting means may be provided with frequency control means adapted to vary the difference between frequencies f1 and f2 in response to said control signal in such manner that, in operation, said intermediate frequency is maintained substantially constant.

Said transmitting means may be provided with a directional aerial adapted to be moved so as to direct said radiated waves in different directions within a given solid angle and said receiving means may be provided with a cathode ray tube arranged to have its beam intensity controlled by the output of said receiving means and the direction of its beam controlled by the movement of said directional aerial in such manner as to be always related in the direction in which said waves are radiated, the arrangement being such that the direction of said reflecting object can be indicated of said reflecting object can be indicated by the position of bright or dark spot on said screen due to the change of beam intensity caused by the reflected waves.

According to another feature of the present invention, there is provided apparatus for indicating the relative time of two signals comprising a cathode ray tube, means for controlling the intensity of the beam of said tube in according with the later of said two signals, means for deflecting said ray in one direction at a relatively high frequency, means for similarly deflecting said beam in a direction at right angles to said first mentioned direction, means for controlling the relative amplitude of said deflections in such manner that in operation, said beam is caused to trace a line on said screen, which line rotates through a given angle in a given time and then returns to its initial inclination and recommences rotation and means for initiating said rotation under the control of the earlier of said two signals, the arrangement being such that, in operation, the angle through which said line rotates before its brightness is modified by said later signals indicates the relative timing of said signals.

If desired, said beam may be additionally deflected intermittently so as to displace the mean position of said trace in successive time intervals, whereby the relative timing of a pair of signals in said successive time intervals may be indicated simultaneously on said screen.

In order that the invention may be more fully understood it will now be described by way of example with reference to the accompanying drawing, which shows a schematic arrangement of apparatus for indicating the direction and distance of a reflecting object.

Referring to the drawing, it will be seen that the apparatus comprises a directional short wave transmitter and a short wave receiver provided with cathode ray indicating apparatus. The transmitter comprises the short wave oscillator 1 coupled to the dipole aerial 2 situated at the focus of a parabolic reflector 3 and arranged to be modulated in frequency by the modulator 4. The oscillator 1 and aerial 2 are preferably tuned to a high frequency such as 6000 megacycles/sec. and the modulator preferably comprises a multivibrator arranged to generate square topped pulses at a frequency of 2500 cycles/sec., so that each positive and negative pulse has a duration of 200 microseconds and causes the frequency of the oscillator to be changed alternately by plus and minus 10 megacycles every 200 microseconds. Thus, the frequency of the radiated oscillations is alternately 5995 and 6005 megacycles/sec. in successive 200 microsecond periods.

The oscillator 1 may be a Barkhausen Kurz or magnetron type of oscillator in which case the modulating voltages provided by the modulator 4 will be applied to the anode of the oscillator valve. Alternatively, a Klystron oscillator may be used, preferably of the kind in which the electron stream is caused to pass in succession through two rhumbatrons which are coupled together, and the output from the modulator 4 is applied to the rhumbatrons so as to cause excitation of the two desired frequencies in succession. If desired, however, the oscillations may be initially generated at a relatively low frequency modulated, for example, by means of a back coupled thermionic valve arranged to introduce a variable reactance into the oscillator circuit in well known manner, the modulated oscillations being then multiplied in frequency to give the desired high frequency oscillations for transmission.

Means (not shown) are provided for moving the reflector 3 cyclicly within the solid angle which is to be explored. The motion of said reflector may be compounded for two sinusoidal oscillations at right angles to each other, or alternatively, the reflector may be caused to describe a successively expanding and contracting spiral motion.

The receiver may comprise the dipole aerial 5 adjacent the transmitting aerial 2, the rectifier 6 and the intermediate frequency amplifier 7 which serves to amplify the 10 megacycles/sec. heterodyne signal produced by the beats between the waves received directly from the transmitter and the waves reflected back to the receiver by the reflecting object.

The dipole aerial 5 is tuned to 6000 megacycles and is coupled to the rectifier 6, the output circuit of which is tuned to 10 megacycles/sec. and coupled to the amplifier 7. The aerial 5 is placed as to receive from the transmitting aerial 2 waves of sufficient strength to enable the rectifier 6 to provide an intermediate frequency signal of a convenient amplitude.

Alternatively, a single aerial may be used for transmitter and receiver, both the rectifier 6 and the oscillator 1 being connected to this aerial. Furthermore, the transmitter oscillator valve may be used as the rectifier, the intermediate frequency components of its anode current being selected and fed to the intermediate frequency amplifier 7.

It will, of course, be appreciated that waves may be injected into the rectifier 6 directly from the transmitter instead of being received from the transmitting aerial.

The system operates as follows. When the transmitted waves fall upon a reflected object, waves are reflected back to the receiving aerial. Provided that the time taken for the transmitted waves to travel to said reflecting object and back to the receiver is less than 400 microseconds, i.e. the reflecting object is not more distant from the site of the transmitter and receiver than 60 kilometres, the received waves will differ in frequency by 10 megacycles/sec. from the transmitted waves for at least a part of each 200 microsecond interval and will therefore be heterodyned to 10 megacycles/sec. in the said receiver, thus giving a rise to a signal at this frequency in the output circuit of the intermediate frequency amplifier 7, which signal may be rectified in the rectifier 8 and may be utilised to indicate the presence of a reflecting object.

It will be appreciated that the system described above has the advantage that the received signal is heterodyned to a much lower frequency at which it may be conveniently amplified without requiring oscillators of extremely high frequency stability for the transmitter and for the heterodyne oscillator of the receiver. The only requirement as to frequency stability is that the drift of frequency of the oscillator serving according to the invention for transmission and for heterodyne reception shall remain during an interval of 200 microseconds within the band width of the intermediate frequency amplifier 7. This change of frequency of the oscillator by 10 megacycles/sec. at regular intervals of 200 microseconds is readily accomplished with the desired frequency stability above referred to.

It will be understood that the timing during which the reflected waves will be heterodyned at the intermediate frequency of 10 megacycles/sec. at the receiver will increase as the distance of the reflecting object increases up to a given distance dependent upon the duration of the time interval (in the example given 200 microseconds) during which the transmitted waves remain at a constant frequency. Thus, if the reflecting object is at a distance of 300 metres, the waves will return in two microseconds, and therefore only the signal radiated during the last two microseconds each 200 microsecond time interval will be heterodyned to the desired intermediate frequency. On the other hand, if the reflecting object is at a distance of 10 kilometres, the waves radiated during the last 67 microseconds of each 200 microsecond time interval will be heterodyned to the desired intermediate frequency. Thus, when the reflecting object is at a greater distance, and the reflected waves is therefore weaker, they are received for a greater proportion of the 200 microsecond time interval, and this effect may be utilised according to a further feature of the invention, to compensate the decrease in the intensity of the received waves as the distance of the reflecting object increases.

In order to utilise this effect, the band width of one or more of the intermediate frequency and/or the post detector circuits of the receiver be reduced so that the energy of the received signals may be effectively integrated. Thus, the intermediate frequency circuits may be given a long decrement, preferably equal to about one third of the period between successive frequency changes of the transmitted waves, so that the weaker signals may build up oscillations over a relatively long period to an amplitude comparable with the amplitude built up in a much shorter period by the stronger reflected waves from a nearer reflected object. As, however, an intermediate frequency circuit having decrement of the order of 70 microseconds has a relatively narrow pass band, this necessitates a very high order of stability of the transmitter frequency. It is therefore preferable in practice, particularly when moving objects are to be observed, to use intermediate frequency circuits of relatively short decrement consequently relatively wide pass band, e.g. 2 megacycles/sec. and to provide a circuit of narrow pass band and long time constant following the detector. Such a post detector circuit may be arranged to reduce the overall decrement of the receiver to 70 microseconds so that the waveform of received waves correspond to that which would be obtained with intermediate frequency circuits having time constants of 70 microseconds Thus the output circuit of the detector 8 may be connected to the high impedance input circuit of a following amplifier through a series resistant and a further resistance in series with a condenser connected in shunt with said input circuit. The time constant of said condenser with the said resistances in series should be 70 microseconds and the time constant of said condenser with said further resistance should correspond to the decremental time of the circuits of the intermediate frequency amplifier 7. It will be understood that this circuit may, if desired, be included in any modulation frequency amplifiers following said detector.

In order to improve the stability of the intermediate frequency in the receiver an automatic frequency control circuit may be provided. A discriminator circuit 9 of known type may be coupled into a stage of the intermediate frequency amplifier 7, prior to the long decrement circuit, if any, and arranged to develop a D.C. bias proportional to changes in the intermediate frequency of the received signals. This bias may then be applied to the multivibrator 4 controlling the frequency of the transmitter so as to control the amplitude of the pulses generated by the multivibrator and hence the difference between the two frequencies of the transmitted waves. This control should preferably be arranged to be slow in operation so as to be actuated by signals of relatively short duration recurring at relatively long intervals such as will be received when the transmitted radiation only strikes a reflecting object once in every complete cycle of movement of the reflector 3 within the solid angle which is explored.

It will be appreciated that the pulses delivered to the transmitter by the multivibrator should be as rectangular as possible in order that the changes in the frequency of the transmitted waves may take place as rapidly as possible, since no observation can be made during this frequency and consequently the minimum distance of which a reflecting object can be observed is determined by the time required to effect this change of frequency. Preferably, this change of frequency should be arranged to take place in less than half a microsecond so that the said minimum distance is not greater than 75 metres. It may also be desirable to filter said pulses so as to remove components having frequencies equal to the intermediate frequency of the receiver so as to minimise spurious signals during the periods when the transmitter frequency is being changed.

The received signals are preferably observed by means of a cathode ray tube 10a. Signals derived from the output circuit of the detector 8 or a following amplifier are preferably fed to the control electrode of the cathode ray tube 10a so as to control the intensity of the cathode ray beam, and deflecting means (not shown) are provided to cause said beam to move across the fluorescent screen of the tube in synchronism with the movement of the reflector 3. Thus, when the said reflector is directed towards a reflecting object, the intensity of the trace on the screen is modified and a bright or dark spot appears at a point on said screen which is related to the direction of said object.

According to a further feature of the invention, the distance of the reflecting object may be indicated by the angular position of a line on the screen of a cathode ray tube. A further cathode ray tube 11 is provided having two pairs of deflecting plates 12, 13 respectively, arranged at right angles to each other, and coupled to the output circuits of the gain controlled amplifiers 14, 15 respectively. A source 16 of sinusoidal oscillations having a frequency which may conveniently be 3 megacycles/sec. feeds the input circuits of said amplifiers 14 and 15, which preferably include thermionic valves adapted to have their amplification controlled by variations of grid bias. Grid bias for controlling the gain of said amplifiers 14 and 15 in opposite senses is provided by the sawtooth generated 10 which is controlled by pulses from the multivibrator 4. The control electrode of the cathode ray tube 11 is coupled to the output circuit of the detector 8 or a following amplifier.

The arrangement operates as follows. In the course of the 200 microsecond time period between successive frequency changes of the oscillator 1, the high frequency oscillations applied to the two pairs of plates 12, 13 of the cathode ray tube 11 v4ary in amplitude in opposite senses in such a manner that the cathode ray beam traces on the screen of said tube a straight line which rotates through a right angle in 200 microseconds. As a sawtooth waveform is used to control the gain of the amplifiers 14 and 15, said line will commence to rotate when the oscillator changes its frequency and will rotate with substantially uniform angular velocity for approximately 200 microseconds, after which it will return rapidly to its starting position and gain commence rotation. In the absence of any received signal, the brightness of the trace will be constant but if waves are returned to the receiver by a reflecting object, the brightness of the trace will be modified momentarily after a time equal to the time required for the transmitter waves to travel out to the reflecting object and to return to the receiver. Thus, the angular position of the bright or dark line due to said modification will give an indication of the distance of the reflecting object and the screen may be calibrated to give direct readings of distance in terms of angle. If desired, the trace may be arranged to rotate through 180 degrees by including a stage with a push-pull input and a parallel output in one of the amplifiers 14 and 15 and controlling the gain of each of the push-pull valves in opposite senses. The scanning oscillations are thus transmitted initially at maximum amplitude then decrease to zero amplitude and then increase to a negative maximum, so that a change of phase of 180º takes place when the amplitude passes through zero.

The bias on the cathode ray tube 11 is preferably adjusted so that the trace is normally blacked out and only becomes visible when a reflected signal is received. Thus, all that will be seen will be a bright line indicating by its slope the distance of the object.

The distance and direction of the reflecting object may be indicated simultaneously on the same cathode ray tube by superimposing the two deflections above referred to for separate indication of direction and distance. Preferably, the deflection which follows the movement of the reflector 3 is modified so that the mean position of the cathode ray beam is held fixed for each 200 microsecond period in a position corresponding to the mean angular position of said reflector during said period. The requisite deflecting potentials may be derived from a number of contacts associated with said reflector which are arranged to feed to the plates of the cathode ray tube potentials related to the angular position of said reflector. With this arrangement, the cathode ray beam will trace rotating lines on the screen of the tube in successive 200 microsecond intervals about spaced points corresponding to the successive mean angular positions of the reflector 3.

The scanning oscillations controlling the rotation of the cathode ray beam are sufficiently reduced in amplitude to avoid overlapping between the traces swept out by said lines rotating about said spaced points in successive 200 microsecond time intervals. In the absence of any reflected waves, and assuming that the beam is not blacked out, the screen will thus exhibit a large number of circular areas traced out by said rotating lines, said areas being displaced from each other along the path of the ray corresponding to the path traced by the reflector, the number of such areas being equal to the number of 200 microsecond intervals comprised in the time period required for said reflector to complete the exploration of the solid angle within which it is arranged to operate, which period may be example be of the order of 1 second. If reflected waves are received, each circular area in the proximity of that point of the screen corresponding to the direction of the reflecting object will exhibit a bright or dark line, the angular position of which line will indicate the distance of said reflecting object. Preferably, the screen is arranged to be blacked out in the absence of reflected waves, in which case all that will be visible on the screen will be a few inclined bright lines which indicate the distance of the object by their inclination and its direction by their location upon the screen.

Although the invention has been described with reference to the observation of reflecting objects such as aeroplanes, it will be appreciated that it is also applicable to the observation of any objects which will reflect radiation of any suitable kind, such as electromagnetic or supersonic waves, and may be applied, for example, to the navigation of aircraft or ships under conditions of bad visibility.

It will also be appreciated that the method described for the indication of the range of one reflecting object may be applied generally to the indication of the differences of timing between pulses.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

  1. Apparatus for the observation of a reflecting object comprising transmitting means of radiating waves having given frequencies f1 and f2 in successive time intervals and receiving means adapted to receive waves reflected by said object and to heterodyne received waves of frequency f1 and f2 with waves of frequency f2 and f1 respectively to a common intermediate frequency.
  2. Apparatus according to Claim 1, wherein said receiving means is provided with means adapted to integrate said intermediate frequency signals and/or signals derived therefrom, whereby variations in the output from said receiver due to variation in the distance of said object are reduced.
  3. Apparatus according to Claim 1 or 2 in which said receiving means is provided with frequency responsive means adapted to develop a control signal responsive to variations in said intermediate frequency and in which said transmitting means is provided with frequency control means adapted to vary the difference between frequencies f1 and f2 in response to said control signal in such manner that, in operation, said intermediate frequency is maintained substantially constant.
  4. Apparatus according to any of the preceding claims in which said transmitting means is provided with a directional aerial adapted to be moved so as to direct said radiated waves in different directions within a given solid angle and in which said receiving means is provided with a cathode ray tube arranged to have its beam intensity controlled by the output of said receiving means and the direction of its beam controlled by the movement of said directional aerial in such manner as to be always related to the direction in which said waves are radiated, the arrangement being such that the direction of said reflecting object can be indicated by the position of bright or dark spot on said screen due to the change of beam intensity caused by the reflected waves.
  5. Apparatus for indicating the relative timing of two signals comprising a cathode ray tube, means for controlling the intensity of the beam of said tube in according with the later of said two signals, means for deflecting said beam in one direction at a relatively high frequency, means for similarly deflecting said beam in a direction at right angles to said first mentioned direction, means for controlling the relative amplitude of said deflections in such manner than in operation, said beam is caused to trace a line on said screen, which line rotates through a given angle in a given time and then returns to its initial inclination and recommences rotation and means for initiating said rotation under the control of the earlier of said two signals, the arrangement being such that, in operation, the angle through which said line rotates before its brightness is modified by said later signal indicates the relative timing of said signals.
  6. Apparatus according to Claim 5 in which said beam is additionally deflected intermittently so as to displace the mean position of said trace in successive time intervals, whereby the relative timing of a pair of signals in said successive time intervals may be indicated simultaneously on said screen.
  7. Apparatus according to Claims 1, 2, 3 or 4 in which apparatus according to Claim 5 is provided for indicating the distance of said reflecting object.
  8. Apparatus according to Claim 4 in which apparatus according to Claim 6 is provided for indicating the distance and direction of said reflecting object, said additional intermittent deflection of said ray being arranged to be controlled in accordance with the movement of said directional aerial so that in operation, the mean position of said trace in successive time intervals is related to the direction in which said waves are radiated.
  9. Apparatus for the observation of a reflecting object, or for indicating the relative timing between two signals, substantially as described with reference to the accompanying drawing.

Dated this 27th day of March 1941

F. W. Cackett

Chartered Patent Agent