Patent GB-916045-A (GB)
Title: Speed measuring system
Inventor: JR FRANK RAMSEY DICKEY
IPC: G01S15/60
Filed: 1959-01-27
Abstract: 916,045. Radar and sonic speed measuring systems. GENERAL ELECTRIC CO. Jan. 27, 1959, No. 2883/59. Class 40 (7). [Also in Group XXXVI] Relates to a system for measuring the relative velocity v of first and second bodies in a direction transverse to the line joining the two bodies, said first body having an extensive irregular surface such that wave energy transmitted from the second body and reflected by the first body is modified in amplitude, phase or frequency by the reflection process, said modification varying in time in an irregular manner due to the relative motion. According to the present invention the modified reflected signal is received on the second body at two points spaced apart a distance s in said transverse direction and the two received signals are detected and subjected to a relative time delay #, the valve of s or # being adjusted to make the relatively delayed signals correspond with one another as determined by a correlation sensing means; the required   velocity v is then given by v = s/2#. The correlation sensing means may comprise a subtractor producing a difference signal or a multiplier, Fig. 3 (see below), producing an output correlation function p(#); in the former case s or # is adjusted to produce a zero output and in the latter case s or # is adjusted to produce a maximum value of the correlation function p(#). The invention is described as applied to a radio system for determining the ground speed of an aircraft, additional spaced receiving aerials being provided for measuring the drift angle, Fig. 5; the transmission may be unmodulated C W or interrupted C W. Other application suggested are measuring the speed of (1) a railway train passing a fixed observation point, (2) material on a moving belt, and (3) a surface vessel relative to the sea bed by means of sonic or ultrasonic waves. In a first embodiment, Figs. 1 and 2 (not shown), the transmitting aerial is mounted midway between the two receiving aerials with the base line of the three aerials parallel to the longitudinal axis of the aircraft, the time delay # is fixed and the longitudinal position of the forward aerial is adjusted automatically or manually by a rack-and-pinion to produce a null output from a subtracting correlationsensing circuit; the null detector may comprise an A.C. moving coil device having sufficient inertia to provide an average indication. The transmitting aerial may be quite small since its beam should be relatively broad, e.g. 40 degrees, and its position relative to the receiving aerials is not critical. Simple linear signal detectors may be employed if direct feed-through from the transmitter to the receiving aerials is prevented by isolating the transmitting and receiving aerials or by rendering the transmitter and receivers alternatively operative at a rate dependent on the height of the aircraft. Alternatively, transmitter feed-through may be prevented by using coherent synchronous detectors but special measures must be taken to prevent errors due to any phase difference between the two received signals caused by the base line of the receiving aerials not being horizontal, e.g. due to changes in the elevational attitude of the aircraft. Fig. 4 shows such a system employing synchronous detectors 44, 49 supplied respectively with reference signals (f0-f1) and (f0+f1), (f1 = 30 c/s.), derived from the transmitter frequency f0 by a motordriven rotary phase-shifter 58, the outputs from the synchronous detectors 44, 49 being applied to a multiplier 52 via high-pass filters 45, 50 to remove any residual components due to direct leakage from the transmitter 41 and a fixed time delay 47 being inserted between the filter 45 and the multiplier 52. It is shown that the amplitude of the output component from the multiplier 52 of frequency 2 f1 = 60 c/s. is proportional to the correlation function p(#) and this component is selected by a band-pass filter 53, rectified at 55 and applied through a low-pass filter 56 to an output meter 57. Fig. 5 shows an arrangement for measuring both ground speed and drift. It comprises four receiving aerials A, B, C, D mounted on a horizontal platform 65 which is rotatable about a vertical axis by means of a servomotor 68. The aerials C and D spaced symmetrically on either side of the platform axis 67 are alternately connected to a synchronous detector 79 (corresponding to detector 44 in Fig. 4) by means of a switch 78 and the aerials A and B spaced along the platform axis 67 are alterately connected to a synchronous detector 73 (corresponding to detector 49 in Fig. 4) by a switch 72, the two switches 72, 78 being operated cyclically in phase quadrature at a low frequency #2 (e.g. 2 c/s.) by phase quadrature outputs #2 and #2/90 from a source 91. The outputs from the detector 73, 79 are relatively delayed by means of a magnetic drum delay device 81 driven by a variable speed motor 82 such that the relative delay # is inversely proportional to the speed of the motor 82 and the relatively delayed outputs are applied through a multiplier 84, band-pass filter 85 and rectifier 86 (corresponding to components 52, 53, 54 in Fig. 4) and are applied in parallel to two synchronous detectors 87, 89 together with phase quadrature switching voltages f2 and f2/90 from the source 91, the output from the detector 87 being applied to control the speed of the motor 82 and the output from the detector 89 being applied to control the motor 68 controlling the orientation of the platform 65. If the axis 67 of the platform 65 is aligned with the ground track of the aircraft the time variation of the signals received by the aerials C and D will be substantially identical and if the relative delay # produced by the drum 81 is such that maximum correlation would be obtained by multiplying either of the signals from the aerials C or D with a signal received by an aerial located midway between the aerials A and B then the values of the actual correlation factors # A (#), p B (#) produced during alternate switching half-cycles of the switch 72 will be less than maximum but they will be equal since the aerials A and B are symmetrically displaced along the axis 67 on either side of the position giving maximum correlation. The effective   distance s in the ground speed formula v = s/2#   is thus the distance between a point midway between the aerials A and B and a point midway between the aerials C and D. Accordingly, if the value of # is incorrect then #A(#) and # B (#) will differ and the output from the rectifier 86 will have a modulation component of frequency #2 which is in phase or out of phase with the switching frequency f2 and a corresponding error signal is produced by the detector 87 to correct the speed of the motor 82. Similarly, if the platform axis 67 is not aligned with the ground track, the output from the rectifier 86 will have a modulation component of frequency #2 which is in phase or out of phase with the switching frequency f2/90 and a corresponding error signal is produced by the detector 89 to correct the orientation of the platform 65, the drift angle being then equal to the angle between the platform axis 67 and the longitudinal axis of the craft. In alternative arrangements (not shown) (1) the aerials A and B are fixed and the aerials C and D are displaced transversely to the line 67, or (2) the receiving aerial system comprises a fixed aerial and an aerial rotating at a fixed rate, e.g. 2 c/s. In the Fig. 5 arrangement the pair of aeriels C, D may be replaced by a single aerial if drift angle determination is not required. In the multiplier circuit shown in Fig. 3, for forming the product el, e2 of two input voltages e1 and e2, the voltages e1 and e2 are applied respectively to transformers 24 and 25, each having two secondary windings which are interconnected so that the sum voltage (e]4+ e2) and difference voltage (e1 - e2) are applied respectively to full-wave square-law bridge rectifiers 26, 27 producing respectively positive and negative outputs (e1+e2)<SP>2</SP> and (e1-e2)<SP>2</SP> which are combined to give the required product el, e2.