Patent GB-633547-A (GB)
Title: Improvements in radio navigation aiding systems
IPC: G01S1/20
Filed: 1946-02-13
Abstract: 633,547. Radio navigation; sawtooth and pulse generating circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Feb. 13, 1946, No. 4573. Convention date, Oct. 27, 1944. [Classes 40 (vi) and 40 (vii)] Relates to receiver means for measuring the time interval between the pulses received from two spaced stations A and B in a pulse hyperbolic navigation system of the type in which the two stations radiate synchronized pulses of the same recurrence frequency, the B pulses being radiated after the mid-point of the A pulse recurrence period. According to the invention, the receiver includes a cathode-ray tube, means for producing cathode-ray deflecting waves of the same recurrence frequency as the radiated pulses, each wave being in two parts of similar shape which produce two corresponding traces having parallel or coincident portions, means for applying the received pulses to the traces and for adjusting the starting time of the second part of each wave to align the A and B pulses, and means for subsequently applying timing markers to the traces, whereby the required time interval may be measured. In the embodiment described, the time interval actually measured is the amount by which an A pulse precedes the mid-point of a B pulse recurrence period. The complete system comprises a plurality of pairs of transmitting stations which all radiate at the same carrier frequency, the pairs of stations being characterized by their recurrence frequency, and at the receiver means are provided for varying the recurrence frequency of the indicator display to select a desired pair of stations. Method of operation, Figs. 2-8. The received A and B pulses are displayed on a cathode-ray tube on four traces cd, ef, gh and ab, produced sequentially by the horizontal deflector waveform N, Fig. 8, and the vertical trace separation waveform Q, the two fast exponential traces cd and gh which are shown coincident in Figs. 2-6 sweeping left to right and the two slow traces ef and ab which are vertically separated from each other and from the fast traces sweeping from right to left. The total period of the four traces is equal to the period of the received pulses (40,000 Ás, as shown), and the durations of the slow traces are inversely variable, i.e., ef is decreased when ab is increased, the durations of the fast sweeps being fixed so that the phase of the second fast trace gh is adjustable. The indicator is first set up with the two traces ef and ab of equal duration and the phasing is adjusted so that both received pulses A and B appear on the trace ef, Fig. 2. The phasing is then adjusted so that the B pulse falls on the fast trace cd, Fig. 3, and is further shifted and held stationary on the most expanded portion (i.e. the left end) of that trace, Fig. 4, and the phase of the second fast trace gh is then adjusted until the A pulse appears on that trace, Fig. 5, and is finally adjusted for coincidence of the A and B pulses, Fig. 6. The required time interval is now given by half the difference in duration of the two slow traces ef and ab and in order to measure this interval, calibration markers are applied to all the traces, Fig. 7 and Figs. 9 and 11 (not shown), the markers being spaced 10, 100, 1,000 and 10,000 Ás, respectively, in increasing order of amplitude, although on the slow traces only the 1,000 and 10,000 Ás markers are distinguishable; on application of the calibration markers the two fast traces are displaced vertically. The difference in duration of the two slow traces is obtained by counting the calibration markers from the left end b of the trace ab to the left end f of the trace ef and in general this comprises an integral number of 1,000 Ás markers (12, as shown in Fig. 7) plus a fraction x and the fractional interval x/2 may be obtained directly by observing the time displacement of corresponding portions of the two fast traces, e.g. by counting the markers on the upper fast trace gh in between the first 100 Ás marker nearest the left end of the trace cd, the so-called " crosshair " marker, and the first 1,000 Ás marker Mz on the trace gh to the right of the " crosshair " marker. When the indicator is set up for a pulse repetition period of 40,000 Ás, as shown, the first 100 Ás marker on the trace cd coincides with a 1,000 Ás marker but this does not obtain for other selectable repetition periods. The additional interval x/2 can be expressed as an integral number of 100 and 10 Ás intervals plus an estimated fraction of a 10 Ás interval, e.g. as shown in Fig. 7 the additional interval is 333 Ás so that the total time interval is 6,333 Ás. The integral number of 1,000 Ás intervals may be determined in other ways, e.g. by counting the number of 1,000 Ás markers from the 10,000 Ás marker to the left end of the trace ef and subtracting from ten; for the conditions shown this gives 10 - 3= 7,000 Ás, less some fraction, or 6,000 Ás plus the fraction, determined on the fast sweeps. When operating with a station pair having a pulse repetition period of (40,000 - K X 100) Ás, a correction of K x 50 Ás must be added to the time interval reading when using a map with a hyperbolic grid computed for the standard repetition period of 40,000 Ás. General description of the system, Figs. 1 and 8. The timing for the indicator is controlled by a 100 kc/s. crystal-controlled oscil. lator 11, the frequency of which is slightly adjustable by a knob 12 for the purpose of shifting the received pulses on the traces and the output is applied through a series of frequency dividers 13, 14, 33, 52 and 53, the first four of which divide by ten and the last by four to produce pulses with repetition periods of 10, 100, 1,000, 10,000 and 40,000 Ás, respectively. A feedback circuit 5.8 is provided for varying the division factor of divider 33 in integral numbers in order to select the repetition period corresponding to the required pair of A and B stations. The output from divider 53 is applied to synchronize a flip-flop 56 producing two phase-opposed square waves G and G<1>, the change-over time of the square waves t being adjustable by the control 56<1> and these two waveforms are applied through differentiating circuits 72 and 71 to give the peaky waveforms H and I the positive pips of which trigger the pre-set flip-flop 73 to give the negative pulses J. The pulses J are differentiated at 79 (waveform K) and synchronize the pre-set fiip-flop 81, producing positive pulses L which render the valve 82 conductive to discharge the capacitor C<1> exponentially to the level e<2>, Fig. 8 (N), to produce the fast sweep portions cd and gh of the waveform N. On completion of the fast sweeps, the capacitor C<1> changes through the resistors 64 and 86 to produce the slow sweeps ef and ab, which are terminated by the 920 Ás duration pulses M (derived from the J pulses by phase inversion at 77) which render the valve 74 conductive to charge the capacitor C' to the level e<1>, in readiness for the succeeding fast sweeps. The waveform N is taken from the junction of resistors 84 and 86 and applied through the amplifier 87 in push-pull to the X plates 89 of the cathode-ray tube 91. In order to separate the traces, the waveform G from the flip-flop 56 is fed through the cathodefollower 94 to the limiting and inverting valve 97 where it is mixed with the waveform O produced by the flip-flop 81, the resulting trace separation waveform Q being applied to the lower Y plate 92 of the cathode-ray tube. The received A and B pulses are applied through the receiver 102 and switch 101 (position ALIGN) to the upper Y plate 92 and in order to equalize the amplitude of these pulses which are received during different half-cycles of the control square wave G, the waveform G is applied from the cathode follower 94 to one end of the differential gain potentiometer 153, to the other end of which is applied a phase inversion G<1> of the waveform G from the inverter 152, the potentiometer slides 154 being coupled to the grid of a cathode-follower 156, the cathode load of which also forms the cathode impedance of one of the receiver IF valves, Fig. 1b (not shown), so that adjustment of the slider 154 controls the gain of the two pulses differentially. The marker pulses are produced by combining the outputs from the dividers 13, 14, 33 and 52 in the mixer 104 and applying the resultant waveform through the inverter 105 and switch 101 (position READ) to the upper Y plate. A waveform G from the cathode-follower 94 is also combined with the calibration markers at 104 to separate the two fast traces. Frequency dividers, Fig. 1a. Control pulses of repetition period (40,000-100 K.) Ás, where K is an integer are derived from the 100 kc/s. crystal oscillator 11 by means of the frequency dividers 13, 14, 33, 52 and 53, the division ratio of dividers 13, 14 and 52 being preset to ten and that of divider 53 to four. In each divider, e.g. 33, the input positive pulses are applied through a small coupling capacitor 36 and diode 37 to charge a storage capacitor 39 so that when a given number of pulses, e.g. ten, have been received the potential across the capacitor 39 is just sufficient to trigger the blocking oscillator 41. The divider 33 normally divides by ten, i.e. K=0, but in order to vary the period of the output pulses by an integral number K of 100 Ás intervals, the output pulses from divider 53 are applied to charge the capacitor 39 through a diode 48 and one of the capacitors 61a-61g selected by the switch 59, the magnitude of the selected capacitor being such that one output pulse applied through it is equivalent to K input pulses applied via diode 37 so that the blocking oscillator 41 is then triggered on receipt of 10-K input pulses. Multivibrators, Fig. 10. The cathode-coupled flip-flop 73 is a modification of the type disclosed in Specification 456,840 and one section comprises two triodes 73a and b in parallel so that pulses J of duration determined by the