Patent GB-539952-A (GB)
Title: A method of and means for measuring the distance between two points in space by means of the reflection of waves
IPC: G01S13/34
Filed: 1940-04-01
Abstract: 539,952. Locating objects by reflected waves. GUANELLA, G. April 1, 1940, No. 5858. Convention date, March 31, 1939. [Class 37] [Also in Group XXXV] Oscillations are, emitted and received back after reflection at a frequency which periodically varies non-uniformly and continuously in accordance with a low frequency u0 and at least one periodically variable control magnitude is generated corresponding to the beat frequency between the transmitted and reflected waves. A component of this control magnitude is tested by means which are selective on a harmonic of the low frequency u0 so as to obtain the difference in the time of travel of the reflected and direct waves. A radio transmitter T, Fig. 1, and a receiver R are mounted close to one another on an aeroplane or at any other location. A portion e1 of the' frequency modulated oscillations e0 generated by the transmitter T is directly applied to the receiver R, whilst another portion e2 of the oscillations is reflected from a distant point X and returned to the receiver R with a time delay. The signals e1, e are mixed in the receiver R so that the frequency w of the output signal e is equal to the beat frequency w3 between the signals e1, e2. Due to the non- uniform periodic variation of the transmitting frequency, this beat frequency also has a periodic character from zero to a maximum and back to zero. The signal e is applied to a frequency de-modulator K and produces a control magnitude E having an amplitude depending on the momentary beat frequency between the signals e1, e2. The amplitude of at least one component or harmonic u of this control magnitude is measured by an indicating instrument J, which may be calibrated in units of distance, while the effect of all the remaining components and interfering signals is suppressed by a filter S. The oscillations utilized for the distance determination may be electromagnetic waves, or acoustic or mechanical oscillations of super-audible frequency. A system for determining the altitude of an aeroplane above the ground X by means of ultra short electrical waves is shown in Fig. 3. The transmitter T 2 comprises a threeelectrode valve 11 arranged in braking field connection to produce micro-ray waves. The grid is maintained at a high positive potential by means of a battery in series with a choke coil 14, while the plate is maintained at zero or slightly negative potential by a second battery in series with a choke coil 13. The frequency of such an oscillator is dependent on the grid potential and frequency modulation in correspondence with a low frequency potential u0 is effected by superimposing this potential on the grid potential by means of a transformer 12: The high frequency oscillations are transmitted through a Lecher wire system to a directional radiator T1 and are reflected by the ground X to the directional receiver R1 in the form of oscillations e2 with a delay depending on the travelling time to and from the ground. The circuit of the receiver R2 is similar to that of the transmitter T2 and produces beats of frequency w3. The beat frequency potential is applied through an amplifier R3 to an amplitude limiting system L and then to a converting system K which produces a potential E5 proportional to the momentary frequency w3. A component u5 of the potential E5 is segregated by means of a filter N1 consisting of a pair of series resonant circuits 29, 30 tuned above and below the frequency of the component u5. This component is measured by means of an A.C. instrument, or by a D.C. instrument J1 after rectification by a rectifier H. This instrument may be directly calibrated in terms of distance. By impressing a portion of the potential u5 on a network N2 comprising a series condenser 9 and parallel resistance 10, a potential u10 is produced proportional to the rate of change of u5. This is measured on an instrument J2 calibrated in speed. In the arrangement shown in Fig. 4 the transmitter T2 is regeneratively coupled and the tuning condenser 31 is periodically varied by means of a motor 50 so as to obtain a sinusoidal or other frequency modulation of the emitted waves e0. The tuning condenser 37 of the receiver R2 is coupled to the condenser 31 so that the tuning of the receiver is synchronized with that of the transmitter; The intermediate frequency signal e3 3 is rectified to produce a potential E3 by means of a rectifier H2. The signal e 3 is also impressed on a filter or frequency demodulator F comprising a pair of resistances 39 and a parallel inductance 40 so as to produce a potential e 5 having an amplitude proportional to the frequency of the signal e3.The signal e5 is rectified by a double-wave rectifier G and passed through a filter N by which a component u5 is segregated. This is passed through an amplifier or rectifier H1 and applied to one coil 49 of a crossed coil ratiometer J. The other coil 48 is supplied with the potential E3 and the meter J indicates the distance to be determined. Steering mechanism st may be controlled by contacts closed by the meter J. In a modification the frequency modulation is effected by means of a toothed wheel rotated in front of an iron-cored inductance and a differential relay is employed which is automatically balanced by means of a potentiometer. Fig. 6 shows an arrangement for selectively measuring a component of the control potential with a wattmeter type instrument. A control circuit C comprises a valve 83 having its grid connected to the junction point between a condenser 84 and a resistance 85. This results in a wattless re-action, and the grid and plate potentials are in quadrature. The plate current is in quadrature with the plate voltage so that the valve acts as a capacitative reactance the value of which depends on the degree of amplification, which may be varied in accordance with the low frequency u0 impressed on the grid by way of a choke coil 86. The control circuit C is shunted across an oscillator O so that the oscillations generated are frequency modulated in accordance with the low-frequency variations u0. The oscillations are amplified by a power amplifier T2 and converted in a transmitter T1 into supersonic oscillations or compressional waves e0 to be transmitted through water or other medium. A portion e1 of the oscillations reaches the receiver R1 directly while another portion arrives as a retarded oscillation e2 after reflection. The receiver R1 may be a microphone which translates the compressional waves into electrical oscillations which are combined and amplified in the receiver R2. The rectified oscillations of intermediate or beat frequency are amplified by means of an amplifier R3, producing a potential e3 having a frequency equal to the beat frequency w 3. Amplitude control is obtained by rectifying a portion of the potential e 3 by means of a circuit Z and obtaining a potential E3 which is applied to the valve 91. The potential e 3 is applied to a filter or demodulator F comprising a series condenser 98 and a parallel resistance 99 to convert the frequency modulated potential into an amplitude modulated potential e 5, which is rectified by a rectifier G to obtain a control potential E5 varying in accordance with the low frequency potential u0. This potential is applied to a frequency multiplying circuit H0 comprising an input transformer 87 and a pair of rectifiers 88, 88<1> whereby an output potential u1 is obtained composed of harmonics of the low frequency potential u0. A component u 2, such as the second harmonic, is segregated from the potential u1 by means of a filter N comprising a parallel condenser 89 and series inductance 90. An instrument J is of wattmeter type and is responsive only to frequencies which are simultaneously impressed on the fixed coils 102, energized by the potential E5,and the movable coil 101 energized by the potential U 2, so that only the component of E5 having the same frequency as u2 will be effective, and an indication is given in terms of distance. Radio or other radiant energy signals may be employed instead of acoustic signals. Fig. 7 shows an arrangement for measuring the distance of reflecting points in a cable. The oscillation generator O comprises a regeneratively coupled valve 121, the grid circuit comprising a condenser 122 shunted by an inductance coil 123. The grid and plate circuits are coupled through a common iron core pre-magnetized by D.C. from a battery 128, on which is superimposed a low-frequency modulating potential u0 so as to cause a periodic variation of the generator frequency. The oscillations are passed through an amplifier T and impressed through a repeater 124 and the primary of a further repeater 125 of a de-coupling system TR on the line or cable Y1, Y2. A short-circuit at the point X represents a point from which the oscillations are reflected and applied as retarded oscillations e through the repeater 125 to the modulator M1. The resulting beat frequency e3 is passed through an amplifier V1 and a portion of it produces an A.V.C. potential by means of a system Z1. The potential e3 is applied to a de-modulating circuit F to convert the frequency modulated potential into a corresponding amplitude modulated signal e5 which is passed through a rectifier G to give a control potential E 5. The network F comprises a series resistance 140 and a parallel inductance 141, the former being provided with a variable contact 126 for adjusting the signal amplitude of the potential E 5. A harmonic u1 of the modulating frequency u0 is produced by means of a multiplication system H0 and is passed through an amplifier V 2 and maintained at constant amplitude by a volume control circuit Z2. The amplified potential U 2 of constant amplitude is applied to one pair of input terminals of the balanced modulator M and is also applied to the other pair of terminals in series opposition to the output control potential E5. Any difference between the potential u2 and