Patent GB-763511-A (GB)
Title: Improvements in or relating to radio direction finding systems
IPC: G01S7/06
Filed: 1954-06-21
Abstract: 763,511. Radio direction-finding. COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. June 21, 1954 [June 20, 1953], No. 18178/54. Class 40(7) The invention relatesto a development of the direction finding system described in Specification 717,651 to enable the direction of an object in space relative to the equipment to be defined in terms of angular deviations in two orthogonal planes intersecting along a line passing through the equipment. The system may be associated with a pulse radar giving a range indication to determine the position of the target. The direction-finding system of Specification 717,651 may be duplicated so that there are four aerial elements arranged 'at the corners of a square in a reflector so as to have overlapping directional patterns. Two elements forming one diagonal of the square may be associated with equipment determining the bearing of the transmitter in one plane whilst the other two aerial elements may be associated with similar equipment determining the bearing in the other plane. The same indicator may be used for the two equipments (Figs. 1 to 6. not shown). A double system of this form in accordance with the present invention is shown diagrammatically in Fig. 8. Here one of the direction-finding receivers 1 is fed from the two receiving aerials O1 and O<SP>1</SP>1 having overlapping patterns in planes parallel to one of the two planes, over lines 11 and 12 through modulators 13 and 14. These under control of saw-tooth time base generator 41 and square wave generator 31, respectively and recurrently increase the level of signals fed from one of the aerials to the receiver whilst decreasing in synchronism the level of signals fed from the other. At the same time the levels of the signals passed are gated by higher frequency pulse waveforms out of step with one another (Figs. 5 and 6, not shown). Receiver 2 is similarly fed by aerials 02 O<SP>1</SP>2 having overlapping patterns in planes perpendicular to those of the other pair. The signals in this case passing through modulators 23 and 24 controlled by square wave generator 32 of the same frequency as generator 31 and saw-tooth generator 42 of a frequency of the order of ten times that of generator 41. The two receivers are fed from a common local oscillator 5 and the video frequency signals fed respectively to amplifiers 15 and 25. These signals have an amplitude modulation as shown in Fig. 9(a) which varies in steps, the steps are rounded into alternating currents Fig. 9(b), by means of low frequency filters 16 and 26. These currents are detected at 17 and 27 to give outputs of the form shown in Fig. 9(c) having a zero level at a point in the cycle corresponding to the direction of the object. Double differentiation produces pulses at the outputs of units 17 and 27 coincident with the respective occurrences of these zero points. That from unit 27 is made of a width equal to one period (# 1 ) of recurrence of the pulse from unit 17 and both are fed to switch 8. Their joint presence controls the application of a brightening pulse to the good of cathode ray display tube 6. The time base sawtooth generators 41 and 42 apply signals to the tube deflection means so that there is produced a two dimensional display of the two angular co-ordinates of the object's position. The apparatus is effective for one object transmitting continuous waves but, if more than one object is transmitting, positions cannot be identified. If, however, a number of objects are transmitting pulse modulated signals of the same recurrence frequency, as when each is reflecting the transmission of a pulse radar, a pulse generator 7 of the same recurrence frequency and having adjustable delay means can be associated with the amplifiers 15 and 25 to render them effective to receive signals only from an object at a particular range. Where the apparatus is associated with a radar the range of an object may be shown on a type A display at the same time as its angular co-ordinates are shown on tube 6. Information can be obtained about each of two objects one of which transmits continuous waves and one pulse signals by substituting apparatus as shown in Fig. 11 for that following each receiver. In this modification bottom clipper 151 is effective to remove the continuous wave transmission signals from the input of channel 152, 161 and 171 controlling switch 81 in accordance with the location of the pulse mondulated transmitter and the pulse signals have no effect on L.F. filter 162 at the commencement of the C.W. signal information channel 163 and 172 controlling switch 82. The switches may control the intensity of the same display tube, the two spot indications being distinguishable by the use of an intermittent switch 51 in one of the channels. An intermittent indication of the C.W: transmitter could also be obtained if its transmission were frequency modulated to an extent greater than the passband of the receivers. The angular information may also be provided in the form of signals whose voltages are proportional thereto by using for each signal a multivobrator triggered on at the beginning of each saw-tooth period of the appropriate time base generator and triggered off by the pulse from unit 17 or 27 and rectifying the resultant squarewave output. Instead of triggering such multivibrators on by the time-bases they may be triggered on by a C.W. craft transmission and off by a pulse transmission to give outputs .proportional to the angular distances between the two transmitters. This information may be used for ground controlled interception purposes.