Patent GB-644996-A (GB)
Title: Improvements in or relating to radio direction-finding systems
IPC: G01S1/02
Filed: 1947-02-11
Abstract: 644,996. Radio navigation; valve relay circuits. PHILIPS LAMPS, Ltd. Feb. 11, 1947, No. 3953. Convention date, March 6, 1944. [Classes 40 (vi) and 40 (vii)] Relates to rotating beam beacon systems for enabling a mobile receiver to determine its bearing relative to the beacon in which the radiation polar diagram of the beacon has at least one sharply-defined minimum and the directional radiation is modulated by a first modulation frequency during the period in which the minimum of the polar diagram sweeps from one datum direction, e.g. north, to a second datum direction, e.g. south, a second modulation frequency being transmitted either omni-directionally or as a modulation of the directional radiation during the intermediate period. In such systems the changes of modulation frequency constitute datum signals and the bearing is indicated by the intervals between the occurrence of the minimum beam signal and the datum signals. When the signal/noise ratio at the receiver is low, the occurrence of the minimum beam signals is illdefined, resulting in reduced accuracy and this is avoided according to the present invention by measuring the time interval between the instant when the beam signal strength falls below a specified level and the receipt of the preceding or following datum signal and by measuring also the time interval between the instant when the beam signal strength exceeds that level and the receipt of the preceding or following datum signal, means being provided at the receiver for producing current pulses having durations respectively equal to said intervals and said pulses being fed either concurrently or alternately to an indicating device to provide a continuous indication of bearing. In one embodiment the beacon has a figure-8 radiation pattern which is alternately modulated with frequencies f and f2, the f modulation being initiated at a time O, Fig. 1, when one of the minima is pointing north and the change over to f2 modulation being effected at a time C half a revolution later when the minimum is pointing south, so that the strength of the signal received at a mobile receiver located at a bearing α from the first datum direction and at a bearing #=180 - α from the second datum direction varies as shown in Fig. 1, the receiver output being limited at a level xy greater than the noise level. At the receiver, Fig. 2, the modulation frequency f1 is selected by a filter 1 and applied to the primary of a transformer 9, one secondary of which, 13, is connected between grid and cathode of thyratron 21, causing it to strike at time O and develop a D.C. voltage of magnitude V with a polarity as indicated across the cathode resistor 25. The modulation frequency f 1 is also applied from secondary 13 through a rectifier to develop a D.C. voltage across load resistance 61 which is applied in series with the voltage across resistor 25 between grid and cathode of thyratron 23 so that thyratron 23 only strikes at time A to produce a voltage of magnitude V across cathode resistor 27 when the receiver output falls below the level xy such that the voltage across resistor 61 falls below the value V. When the receiver output again reaches the level xy at time B a voltage of magnitude V is again developed across resistor 61, but since the thyratrons 21 and 23, once having been ignited, are insensitive to grid control, they remain in a conductive state. At the time C the modulation frequency changes from f1 to f2 and the frequency f2 is applied through a filter 3 and transformer 11 to a pair of thyratrons 41 and 43 connected in a similar manner to thyratrons 21 and 23, voltages of magnitude V being developed across resistors 45, 47 and 59 during intervals CO, DO and CD and EO respectively. The voltages across cathode resistors 45 and 47 are applied in series between the grid and cathode of valve 29 which is connected in series with thyratrons 21 and 23 so that in the interval CD when a voltage is developed across resistor 45 only, valve 29 is blocked and thyratrons 21 and 23 are extinguished, but in the interval DO the voltages developed across resistors 45 and 47 balance out and valve 29 is rendered conductive so that thyratrons 21 and 23 may again be ignited in the succeeding OC period. A valve 49 connected in a corresponding manner ensures that thyratrons 41 and 43 are extinguished at time O and are capable of being ignited in the period CO. The durations and polarities of the voltages developed across resistors 25, 27 and 61 and the corresponding resistors 45, 47 and 59 are shown in Fig. 3, all the voltages having an equal amplitude V. One coil 55 of the indicating instrument 53 is connected across resistor 27 so that the duration of the current flow through the coil is equal to AC and corresponds to a bearing #+# from the second datum direction and the other coil 57 is so connected that the resultant of the voltages developed across resistors 59, 45 and 47 is applied to it so that the duration of its current flow is equal to EO and corresponds to a bearing B - #. The effect of the two currents is integrated in the meter to give a resultant proportional to 2# so that a direct indication of # may be provided. In order to obtain a steadier indication two additional coils may be connected in parallel respectively with resistor 47 and the series connection of resistors 27, 25 and 61 to produce currents of duration DO and BC respectively. The rotating figure-8 radiation pattern may be produced by rotating a frame aerial or by modulating the carrier in phase quadrature at the rotational frequency and applying the modulation products with the carrier suppressed to two orthogonal frame aerials. In order to prevent erroneous effects due to transients having a frequency spectrum including f1 and f2, the secondary windings 15 and 19 are provided on transformers 9 and 11 so that during periods of normal f reception a blocking voltage is applied to thyratron 41 to prevent it being ignited by spurious f2 components and thyratron 21 is similarly blocked during normal f2 reception. In a modification the frequency f2 is transmitted omnidirectionally and at the modified receiver, Fig. 4 [not shown), valves 41, 43 and 49 are omitted, valve 29 being unblocked and blocked respectively during the f1 and f2 transmission periods by appropriate connections from resistors 59 and 63. The coil 57 of the indicating instrument is connected in parallel with resistor 27 and coil 55 is connected in parallel with resistors 61, 25 and 27 so that the durations of the current flow in the two coils are equal to AC and BC respectively. The modified receiver may also be employed in conjunction with a beacon having a radiation pattern with a single minimum which is alternately modulated with frequencies f1 and f2 in which case a bearing indication is only produced in a semi-circular sector corresponding to the time interval OC, Fig. 1. In order to obtain a bearing in the complementary sector, the filters 1 and 3 may be interchanged by a switch so that thyratrons 21 and 23 are responsive to the f2 transmission. According to the Specification as open to inspection under Sect. 91 the f1 and f2 transmission periods in the first embodiment may overlap in time. This subject-matter does not appear in the Specification as accepted.