Patent GB-661954-A (GB)
Title: Improvements in or relating to timing systems incorporating electrical synchronizingsystems
IPC: G04F10/06
Filed: 1948-03-19
Abstract: 661,954. Phase difference measurements. SPERRY CORPORATION. March 19, 1948 [March 24, 1947], No. 8240/48. Class 37. [Also in Group XL (c)] Relates to a system for measuring the time displacement between two pulse trains, 20 and 21, Fig. 2, part A, of equal recurrence frequency received from the two spaced transmitters A and B of a pulse hyperbolic navigation system, e.g. Loran, in which the recurrence frequencies of two locally generated A and B trains of pilot pulses 27 and 28, Fig. 2, parts D and E, are automatically controlled so that the A and B pilot pulses coincide with the corresponding A and B signal pulses, the required time interval being determined by measuring the time displacement between the A and B pilot pulse trains. In such a system each signal pulse, e.g. 20, may comprise a ground-wave pulse 22 followed by sky-wave pulses 23 and 24, in which case the pilot pulses may lock on the sky-waves giving rise to sky-wave error; also both pilot pulses may lock on the same signal pulse, i.e. double synchronization error, or the A and B pilot pulses may lock on the B and A signal pulses, respectively, i.e. inverted synchronization error, and according to the present invention means are provided for preventing these three types of error. As shown in Fig. 1, the A pilot pulse train is produced in the circuit 1 by reducing the frequency of the output from oscillator 3 in the frequency divider 4 to a value slightly less than the recurrence frequency of the signal pulses, shaping the resultant waveform at 5 to give a square wave 29, Fig. 2, part F, producing at 6 one millisecond positive gate pulses 25, Fig. 2, part B, coincident with the leading edge of the positive portions of waveform 29, and producing at 7 narrow positive A pilot pulses 27 coincident with the trailing edge of the gate pulses, the resultant A pilot pulses being applied together with signal pulses from the receiver 16 to a coincidence circuit 9. Since the recurrence frequency of the A pilot pulses is less than that of the signal pulses, the pilot pulses will drift with respect to the signal pulses and when a pilot pulse coincides with a signal pulse a control voltage is developed at 9 which is applied through the normally closed contacts of relay 46 to modulator 11 to increase the frequency of oscillator 3 so as to maintain the coincidence, the pilot pulses being then locked on the signal pulses. If a pilot pulse locks on a sky-wave pulse 23, Fig. 2, part J, the corresponding gate pulse 25 will coincide with the preceding ground wave pulse 22 and such a condition is prevented by feeding the gate pulses and signal pulses to a coincidence circuit 12 to produce a control voltage which energizes relay 46, thereby opening contact 10 and causing the pilot pulses to drift again. An identical circuit 2 is provided for producing the B square waves 30, gate and pilot pulses 26 and 28, corresponding components being indicated by suffixed references and the coincidence circuit for preventing locking on the sky-wave being shown at 15. If both the A and B pilot pulses lock on the same signal pulses, i.e. double synchronization error, the A and B gate pulses 25, 26 will also coincide, Fig. 2, part H, and this is prevented by feeding the gate pulses to a coincidence circuit 14 to produce a control voltage which energizes relay 461, thereby opening contact 10<SP>1</SP> and causing the B pilot pulses to drift. In a Loran system the B signal pulses are always later than the mid-point of the A pulse recurrence period so that when the A and B pilot pulses are correctly locked on the A and B signal pulses, the B gate pulse 26 occurs during the negative half-cycle of the A square wave 29 but if the A and B pilot pulses should lock on the B and A signal pulses respectively, i.e. inverted synchronization error, the B gate pulse 26 would occur during the positive half-cycle of the A square wave 29 and this condition is prevented by feeding the B gate pulses 26 and the A square waves 29 to a coincidence circuit 13 to energize relay 46, thereby opening contact 10, causing the A pilot pulses to drift. The coincidence circuits 12-15 may comprise a tetrode coincidence valve, the two inputs being applied to the control and screen grids, respectively, Fig. 3 (not shown). The required time interval is indicated by applying the A and B pilot pulses to a known form, of interval meter 17.