577,824. Radio direction-finding and location systems. STANDARD TELEPHONES & CABLES, Ltd. (International Standard Electric Corporation). May 14, 1942, No. 6547. [Class 40 (v)] [Also in Group XXXVI] A system for radio navigation or location comprises two spaced antenna systems 1, 2 each transmitting pulses distinguishable from those of the other, and a receiver adapted to discriminate the moments of reception of the pulses. If the pulses from the two antenna systems are emitted simultaneously, they will be simultaneously received on a longitudinal axis 3-4 bisecting and perpendicular to the line joining 1 and 2. The locus of points at which two simultaneously-emitted pulses are received with any given constant time-interval between them is an hyperbola, which at a sufficient distance from the transmitters may be regarded as coinciding with its asymptotes passing through the mid-point of the line 1-2; thus, a course-line in any desired direction may be followed by maintaining the corresponding time-interval between the received pulses. By de-timing the emitted pulses the line 3-4 on which simultaneous reception is obtained becomes an hyperbola, so that the direction of the approach path corresponding to a given time-interval may be changed in this way. The pulses may be modulated to allow the system to be simultaneously used for communication purposes. In the systems described the pulses from one transmitter are short, and from the other long, and produce indications such as 16, 17, Fig. 3, on the screen of a cathode-ray indicator, the beam of which is rotated in synchronism with the pulses. One received pulse, as shown the long pulse 16, controls the time-base circuit, and so appears at the zero of the scale, and both pulses produce radial deflections of the beam by controlling the anode voltage of the indicator. One-quarter of the pulse repetition period is made equal to the maximum difference in the times of reception of the two pulses, which occurs on the axis passing through the two transmitters 1, 2, so that the angular position of the indication 17 indicates the direction of the course. Since the same time-difference occurs on both asymptotes of an hyperbola, the indications are ambiguous as to which side of the axis 1-2 the receiver is situated, and occur always in the upper half of the oscillograph scale. To resolve this ambiguity, the axis 3-4 is periodically rotated to a position 5-6, Fig. 2, by the use of a pair of auxiliary transmitters, resulting in reverse changes in the time-interval between the reception of a long and short pulse according to whether the receiver is on one side or the other of the axis 1-2. Thus, for a receiver at 7 the interval increases from 10 to 11, whilst for a receiver at 9 the interval diminishes through zero from 14 to 15, so that the short pulse is received first. The short pulse indications corresponding to 11 and 15 appear as a flash to one side or the other of the indication 17 according to whether the latter is in the correct quadrant or not. In the latter case, the time-base voltages applied to the deflecting- plates of the oscillograph are changed over by a switch to cause the indications 16, 17 to appear in the correct positions indicated at 18, 19. The distance of the receiver from the beacon may be determined by measurement of field intensity, and indicated by the length of the indications. The application of the system to radio-location is described, two pairs of aerials respectively in vertical and horizontal alignment transmitting pulses alternately on the same frequency, or simultaneously on different frequencies. The reflected pulses are detected at a receiver situated at the centre of the aerial system, the distance of the object being determined by the time-difference between a directly-received and the corresponding reflected pulse, and its angular position in altitude and azimuth by the time-differences between the reception of the reflected pulses originated at the corresponding pair of aerials. The indications may be given by a cathode-ray tube with a circular scan of the beam. Figure 6 shows means which may be used for generating long and short pulses which are fed respectively to the two antenna systems. Negative short pulses at, say, 250 kilocycles are applied from a pulse generator to the grid of a valve 41, causing the anode current to fall to zero and consequently an abrupt increase of the grid potential and anode current of a valve 42. To the grid of the valve 42 is also applied by way of a transformer 47 an auxiliary sinusoidal potential of half the frequency of the pulses, say 125 kilocycles, which alternately advances and delays the operation of the valve 42 producing alternately long and short pulses in its anode circuit. These are amplified by a valve 43 and applied to the screen grids of two valves 48, 49, to the control grids of which the carrier current is applied. The valves 48, 49 are rendered alternately inoperative by a sinusoidal potential at 125 kilocycles applied by a transformer T to their suppressor grids, so that they pass respectively only the alternate long and short pulses to power amplifiers which supply the two antenna systems. In an alternative pulse-generator, Fig. 9 (not shown), a similar trigger circuit to that shown in Fig. 6 is used, but a sinusoidal frequency of, say, 1000 cycles per second applied to the grid of the second valve causes the time of occurrence of the trailing edges of the pulses in its anode circuit to vary at this frequency without affecting the time of occurrence of the leading edges, which latter time is determined only by the triggering effect of the 250 kilocycle pulses applied from the first valve. Short pulses of opposite sign are produced in the anode circuit of the third valve which mark respectively the leading and trailing edges of the pulses from the second valve. Those corresponding to the leading edges occur at equal intervals of time,' and are superposed on the indicator screen to produce a narrow indication, whereas those corresponding to the trailing edges occur at time-intervals which are modulated at a frequency of 1000 per second and therefore produce a wider indication resembling that of a long pulse. The pulse separator comprises a buffer valve which applies the signals of opposite sign respectively to the grids of two amplifying valves.