Patent GB-1469536-A (GB)
Title: Position determining system using sound transducers
IPC: G01S11/14
Filed: 1974-03-25
Abstract: 1469536 Ultrasonic position determination system HONEYWELL Inc 25 March 1974 [30 March 1973] 13110/74 Heading G1G The Specification describes an apparatus for determining the position of two objects having sound transducers mounted thereon, e.g. for determining the line of sight of an aircraft pilot wearing a helmet equipped with two ultrasonic transmitters, in order to control the direction of a directional system slaved thereto, e.g. a weapon or television camera system. The apparatus comprises ultrasonic receivers mounted e.g. in the cockpit; a data processing unit which measures the transit times of sound signals, which may be pulsed, between the transmitters and receivers; a means to determine the speed of sound, operating iteratively to obtain (a) a quantity which may be the distance between the transducers mounted on the helmet, computed on the basis of the transit times and an assumed value of the speed (b) a correction factor from this quantity and the known true distance between the transducers and (c) a more accurate value of the speed; and a further means for computing the relative position of the transmitting and receiving transducers from the measured transit times and the speed of sound. Basic Mechanical Elements. An observer 10, Fig. 1 e.g. in an aircraft cockpit, wears a helmet 12 with a convex visor 14 on which a reticule 16, which may be a cross-hair, is projected, in front of one eye, to mark the observers Line of Sight 22, LOS. 75KHz ultrasonic transmitters 18 and 20 are placed in a predetermined relationship with respect to reticule 16, at a known distance S apart, the direction of S may be e.g. parallel to LOS 22. A receiver assembly comprising transducers 26, 28, 30 is mounted in a known disposition, e.g. on a portion of the aircraft cockpit. Position Calculation. The associated computer system, using a cartesian co-ordinate system to represent the position of each sound transducer, utilizes the measured transit times of pulses alterna tively transmitted by each transmitter 18, 20 as data for a computer programme which uses equations given in the Specification to calculate the co-ordinates of the transmitters, and thus the direction ratios of the distance S. These equations include as a factor the speed of sound, the accuracy of which must be maintained in conditions of  varying propagation velocity. Electrical System. In Fig. 3 the received signals are filtered at 36, and passed to pulse detection circuitry 40, where a logic signal is generated on detection of a predetermined portion of each received pulse, for each receiving transducer. Start/stop logic 44, provides simultaneously, a signal 50 to transmitter drive electronics 54, and a signal to oscillator and counters 46 by link 48, at the beginning of a transit time measurement. On reception of a logic signal from pulse detector 40 corresponding to a particular receiver transducer, start/stop logic 44 stops the corresponding counter in block 46. Transmitter drive electronics 54 provide alternate sequenced excitation of ultrasonic transmitters 60 which generate, via pulse transformers, bursts of ultrasonic energy. Oscillator and counters 46 provide a measurement first of the three transit times from one transmitter which are read by computer 64, and then after a further sequence the three transit times from the other transmitter, so that computer 64 has the six transit times necessary for calculation of the coordinate positions. For a particular calculation, the value of the speed of sound, CN used, is a nominal value or the value used on the previous computation. The computer is programmed with the value of S and uses this to calculate the LOS direction ratios using the Formulae:- where the suffixes A, B correspond to the coordinates of transmitters A and B, and A x , A y , A z  are the direction ratios of the angle between the LOS and each of the co-ordinate axes, which are used in the slaved directional system. The value of A x , A y , A z  are also fed by link 66 to a computer 68 which calculates a distance ratio A o  given by: When the value of the speed of sound CN is accurate the computed value of A o  is unity. If A o  # 1, unit 72, for the detail see Fig. 4, (not shown) performs an algorithm: which is used in an iterative process to produce successively improved values of CN in subsequent computations of the LOS in computer 64. When A o  = 1, the direction ratios have become true direction cosines of the LOS. A flow chart Fig. 5 (not shown) shows the steps in the iterative process of calculation of the LOS.