Patent GB-504767-A (GB)
Title: Improved method of and means for determining distances by means of electromagnetic waves
IPC: G01S13/46
Filed: 1938-02-18
Abstract: 504,767. Measuring distances by reflected electromagnetic waves ; phasedifference measurements. STANDARD TELEPHONES & CABLES, Ltd. Feb. 18, 1938, No. 6171. Convention date, May 15, 1937. [Class 37] [Also in Group XL] The distance between two objects, for example, the altitude of an aircraft, is determined by transmitting ultra-short waves from one of the objects, receiving at this object at a plurality of points waves reflected from the second object, and comparing the phaserelations of the reflected waves to obtain an indication of the distance. The invention is particularly applicable to measuring aircraft altitudes up to three times the wing span thereof, and may be combined with known means for measuring greater altitudes wherein the phase relation between the modulating component of a transmitted and reflected wave serves to indicate altitude. Fig. 1 shows an aircraft provided at each wing extremity with two antennµ 42, 43, 44, ultra-short waves being transmitted from the antennµ 42 (acting as a single transmitting antennµ) and received after reflection from the earth by the other pair of antennµ 43, 44. As shown in Fig. 5A, the antennµ are of the dipole type, and the individual antennµ of each pair are disposed half a wavelength apart so that the transmitting antennµ discriminate against transmission in their own plane thereby preventing direct transmission to the receiving antennµ 43, 44. The antenna 44 may be disposed below antenna 43, Figs. 3, 4 (not shown), to increase the accuracy of the measurement of the phasedifference of the reflected waves received thereby and hence the altitude. Since the wings 41 must be horizontal for accurate measurements, switching arrangements may be provided for alternately transmitting from the wings on the port and starboard sides, and receiving on the other, a steady reading on the measuring instrument indicating that the wings are horizontal. Fig. 6 shows diagrammatically the ultrashort wave transmitter 60 connected to the dipole antennµ 42. The receiving antennµ 43, 44 are connected, respectively, to heterodyne receivers 51, a local oscillator 54 being common to both receivers. The phase difference between the beat frequencies corresponding to the two reflected waves is indicated by a synchroscope 56, for example, a known rotary synchroscope, Fig. 9 (not shown), which may read altitude, directly. In a preferred form of synchroscope the two beat frequency signals from the receivers 51 are applied, respectively, to the control grids of known tetrode " limiters " 87, Fig. 10, which convert the signals into successions of squaretopped impulses as shown in Fig. 11, at A, B respectively. These impulses are applied through transformers 91 to the control grids 97, 98, of a pentode 95 so arranged that the anode current will increase only when both control grids 97, 98 are positive, that is, during the intervals over which the impulses at A and B, Fig. 11, are co-existent. The increase in the anode current of the pentode 95 then consists of a series of pulses as shown at C in Fig. 11, and varies linearly with the phase displacement between the impulses A, B. A milliammeter 96 in the anode circuit may therefore be calibrated to read altitude directly. Local oscillator. One method of obtaining the beating or local oscillator frequency for the receivers 51 is shown in Fig. 8. Four insulated plates 72 ... 75 form with a common plate 76 four condensers which are energized through a transformer 82 and phase splitting device so as to produce a rotating electrostatic field at the frequency of transmitter 60. A plate 71 between the plate 76 and plates 72 . . . 75, is driven by an electric motor 70 so that the potential applied by the plate 71 to the receivers 51 has a frequency equal to that of the transmitter 60 plus or minus the number of cycles per sec. corresponding to the speed of the motor 70. In an alternative arrangement, the beat note frequency desired, say #f, is generated. by an oscillator 65, Fig. 7, and modulates ultra-short wave energy, at frequency f from the transmitter 60 in a modulator 66. A filter 67 then selects one side band of the modulator output, for example f-#f, which beating frequency wave is then applied to the receivers 51. To indicate altitude at heights greater than three times the wing span of the aircraft, a long wave oscillator 100, Fig. 12, is provided, the output from which modulates the ultrashort wave output of an oscillator 141 in a modulator 140, the output of which is radiated from the antennµ 42. A demodulator 103 demodulates the reflected wave received et 44 to reproduce the long wave component and a synchroscope 102 compares the phases of the long wave modulating component of the reflected wave obtained from the demodulator 103 and long wave energy obtained directly from oscillator 100, this phase difference being an index of the altitude. Preferably, the long wave modulating frequency is such that the normal cruising height of the aircraft is about one eighth of the wevelength so that a synchroscope as shown in Fig. 10, will be operating at about midscale where the instrument has its grµtest accuracy. This apparatus is combined as shown in Fig. 12 with the apparatus shown in Fig. 6 for measuring heights less than three times the wing span, the local oscillator 52, Fig. 12, corresponding to 54 in Fig. 6. For widely varying cruising heights of the aircraft, the trsmssnitted ultra-short waves may be modulated by a number of longer waves which after reception and demodulation are separated by filters and each supplied to an appropriate synchroscope where its phase is compared with that of the appropriate local oscillator. A single synchroscope could of course be used with switching arrangements to connect it into the several circuits as desired. To detect the presence of rising ground ahead of the aircraft, the antennµ 42, 43, 44, may be tilted so as to function with waves rejected from an approaching hill. Other bodies such as other aircraft may be similarly detected. U.S.A. Specifications 2,045,071 and 2,045,072 are referred to.