Patent GB-1246491-A (GB)
Title: Apparatus for synchronizing a multi access space satellite time division multiplex system
IPC: H04J3/00
Filed: 1968-11-20
Abstract: 1,246,491. Radio signalling. NATIONAL AERONAUTICS & SPACE ADMINISTRATION 20 Nov., 1968 [24 Nov., 1967], No. 55050/68. Heading H4L. Means are provided for correcting for Doppler shift and for synchronizing a master station and a plurality of slave stations in a time division multiplex communication system using a space satellite relay station. Master station, Fig. 1.-A voltage controlled oscillator 37 generates a signal which is a standard frequency minus the predicted Doppler shift and this signal controls the timing signals produced by generator 53. As described, each frame consists of a synchronizing time slot carrying a sync. burst at the standard frequency, e.g. 800 kc./s. minus the Doppler shift generated at 53 followed by a sequence of ten data slots each separated by a guard band. The sync. burst is transmitted to the satellite 59 and returned via receiver 15 to the demodulator 25 which provides a signal related to the frequency of the return signal which controls a phaselocked oscillator 27. The output of oscillator 27 controls the receiver timing signal generator 17 and is mixed at 29 with the output of oscillator 37 and the resultant sum frequency is compared at 35 with the 1600 kc./s. output of a standard oscillator 33. The output of the phase comparator 35 controls the oscillator 37 so that it is corrected for the true Doppler shift and this compensation takes place continuously. The master station now can transmit data via sampling gates such as 43 under the control of the generator 53. Data received from the satellite is selected by one or more gates 19 controlled by the receiver timing pulse generator 17 and supplied via data processor 21 to the output terminal or terminals. Slave station, Fig. 2.-The sync. burst signal which has been compensated for Doppler shift with respect to the master station is received at 69 via the satellite, demodulated at 71 and controls a phase-locked oscillator 73 the output of which as in the master station is added at 93 to the output of oscillator 101 and phase-compared at 97 with a standard frequency to compensate the frequency of oscillator 101 for Doppler shift. The timing pulse generator 103 controls an oscillator 105 whose frequency bursts occur in time steps which are gated at 107 so that the first burst appears in the first time slot of the first frame, the second burst appears in the second time slot of the second frame and so on. These bursts are transmitted and returned via the satellite 59 to the receiver 69 and after demodulation at 71 are selected by gate 77 and comb filter 85 and supplied to a delay matrix 87, Fig. 7 (not shown). The sync. burst occurring at the start of a frame is selected by gate 75 and sync. centre detector 83, which identifies the centre of the sync. time slot, and supplied to matrix 87. The matrix 87 compares the sync. centre signals and the comb filter signals and produces a phase error signal which applies a coarse control to the generator 103 and also causes it to cancel all further frequency bursts except one timed to an empty data time slot. For fine synchronization, the signals passed by filter 85 are compared at 89 with the output of sync. centre detector 83 to control generator 103 to centre the slave station in the empty time slot. Received data is passed to the data processor 91 via one or more gates 79.