1,262,721. Multiplex pulse code signalling. DEFENCE, SECRETARY OF STATE FOR. 7 July, 1970 [7 July, 1969], No. 34236/69. Heading H4L. A local station in a time division multiple access communications system in which signals are relayed by, for example, a satellite repeater in orbit, is provided with synchronizing means in which master timing signals generated by a master station are received via the repeater, local timing signals synchronized with the reception of the master timing pulses are generated and ranging pulses with a repetition rate differing from that of the master timing pulses are transmitted until the response signals occur within a predetermined part of the time intervals between consecutive local timing signals, after which the repetition rate of the transmitter ranging pulses is made equal to that of the master timing pulses and is continually adjusted so that the response signals are received in constant timing relationship to the local timing signals. General description.-As described, a plurality of earth stations S1 to S4 communicate via a satellite repeater station, S1 acting as a master station. Waveforms (a), (b), (c), Fig. 2, represent signals at the satellite from stations S1, S3, S4 respectively, shown separately for convenience, each occupying a time slot in a frame of sixteen time slots. Station S3 is in the process of achieving synchronization and S4 is fully synchronized. The master timing pulses M11 &c. occupy the first time slot of each frame and the master station and other fully-synchronized stations also transmit ranging pulses, as shown, from the master station appearing in the second time slot followed by communication pulses, and from station S4 appearing in the fourth time slot followed by communication pulses. The master pulses and ranging pulses are distinguishable by their different carrier frequencies. At each ground station the master pulses retransmitted by the satellite are received such as M12 for station S3, and a local timing signal M12<SP>1</SP> as shown in (d) is generated and phase-locked to the master pulse. Initially the local timing pulses are given a repetition rate different from that of the master pulses to provide a searching mode and when a local timing pulse occurs within the pull-in range after a master pulse a normal phase-locking circuit becomes effective. When this has been achieved, and assuming that station S3 is to be synchronized in time slot 3, ranging pulses such as R10 (e) of 100 microseconds duration which occur at a repetition rate slightly above that of the master pulses, again to provide a search mode, are transmitted from S3, received as pulse R11 at the satellite and the satellite retransmits pulses R11 and M21 which are received as pulses R12 and M22 at S3 (f). By this time the local timing signal M22<SP>1</SP> will be phase-locked with M22 and the next ranging pulse R30 transmitted will be reduced to 50 microseconds duration (e) and can be phase-locked to its required timing in relation to the master pulse M22. In practice this process may take a relatively large number of frame periods. When the ranging pulses are synchronized they define the times of transmission of signals to the satellite and the local timing pulses define the time of reception and the station S4 may then transmit communication signals as shown for station S4 at (h) all the communication signals being transmitted on the same carrier frequency. Ground station details, Fig. 3. Aerial 10 is connected via a TR switch 11 to a radio transmitter 12 and receiver 13. The transmitter receives the outputs of gates TG1, TG2, TG3, TG3 being controlled by a switch 14 which is closed only if the apparatus is to operate as a master station. Gate TG1 supplies signals to be transmitted from a teleprinter TP1 through a sync. circuit 15, sampling gate 16 and F.S.K. circuit 17, gate TG2 receives the output of a ranging frequency oscillator 18 and gate TG3 receives the output of a master timing frequency oscillator 19. Receiver 13 supplies teleprinter TP2 via F.S.K. decoder 21. Assuming that switch 14 is closed and the system is to be set in operation, the output of a stable oscillator 30 is frequency-divided at 39, 40 to produce pulses allowing gate TG3 to pass burst of master timing frequency oscillations from generator 19 to the transmitter 12. These master pulses are returned to the master station and received at each of the slave stations and supplied via an amplifier 20 tuned to the appropriate frequency, filter 22 and detector 23 to provide a threshold voltage for search mode relay 28, Fig. 4 (not shown), controlling a pulse rate adjuster 29 receiving the output of an oscillator 30. The output of amplifier 20 is also supplied to a time discriminator circuit TD1, Fig. 4 (not shown), which is gated by pulse pairs derived from pulse rate adjuster 29 via dividers 31, 32. At this time the received master pulses will not coincide with these pulses and circuit TD1 will provide no output. Circuit 29 includes a search offset oscillator which at this stage inserts extra pulses into the output of oscillator 30 so that dividers 31, 32 operate at a slightly different speed to the dividers 39, 40 controlling the transmission of the master pulses. A channel timing pulse generator 33 consisting of a sixteen-stage shift register is stepped by pulses from the divider 31 and has its first stage set by the output of divider 32. When the station is synchronized its first stage will form a local timing signal as at (d), Fig. 2, and one of its remaining stages is selected to define the time slot appropriate to that station. When the station is synchronized the trailing edge of the first pulse of each pulse pair gating the circuit TD1 and the leading edge of the following pulse coincide with the mid-point of each received master pulse and the output of the offset oscillator is blocked. Synchronism is maintained by the addition or subtraction of pulses from the output of oscillator 30 as required under the control of circuit TD1. At a slave station such as S3 the next process is the transmission of a ranging pulse and this is received back via tuned amplifier 24 and supplied to a time-discriminator circuit TD2 similar to TD1 and supplied via a ranging frequency filter 25 to a search mode relay 36. The search mode relay controls a pulse rate adjuster 37, coupled to the oscillator 30, in a similar manner to that for the local timing pulses, Fig. 5 (not shown), but the process is slower. It also provides a signal inhibiting the gate TG1 until the station is synchronized. The output of the pulse rate adjuster 37 via dividers 41, 42 produces pulses for the transmission timing circuit 43, including a sixteen-stage shift register, controlling the gate TG2 and the transmission of the information pulses. When the ranging pulses are synchronized the search mode relay 36 provides a signal to the transmission timing circuit 43 converting the ranging pulses to narrower pulses and after a short delay removes the inhibiting signal from gate TG1.