Patent GB-1299705-A (GB)
Title: Improvements in or relating to synchronising satellite communication systems
IPC: H04J3/00
Filed: 1970-10-14
Abstract: 1299705 Multiplex pulse signalling POST OFFICE 14 Oct 1970 [15 Oct 1969] 48867/70 Heading H4L In a time division multiple access satellite communication system in which the satellite carries a repeater having pencil beam aerials so that an earth station cannot receive back its own signals, means are provided for synchronizing the signal bursts from stations so that they assume their correct time position in a frame at the satellite and do not overlap. As described, Fig. 1 (not shown), a satellite carries two repeaters having highly directional aerials, the first repeater receiving time division multiplex signals from stations A, B, C and directing these signals to stations DEF and the other repeater transmitting signals in the opposite direction in like manner. The signals from the stations are transmitted sequentially in bursts in known manner, a typical burst comprising a guard time Gt, a carrier recovery time b c  consisting of unmodulated carrier, a bit timing recovery period b b , a unique station identifying code b u , including a synchronizing pulse, and information carrying bits V m , Fig. 2 (not shown). One of the stations transmitting through each repeater is chosen to provide synchronizing information for stations using the other repeater. As described, one of the synchronizing stations is station B and in its signal burst its unique word b u  is followed by a period b s  providing each station using the other repeater with data regarding its sycnrhonization. Synchronization apparatus.-Fig. 3 shows the apparatus at station B for synchronizing stations D, E, F, assuming that station D is used as a frame reference for a frame D, E, F. Synchronizing pulses received from station D are supplied via line 19 to a correction rate circuit 17 and sync. pulses from stations E, F are supplied via line 21 to circuit 17. Correction cannot be made more often than twice the round-trip propagation time to the satellite and circuit 17 passes sync. pulses in the same frame at this frequency. Considering synchronization of station E the reference pulse from station D on line 23 is delayed by a burst period at 27 and supplied to a phase comparator 33 where it is compared with the sync. pulse from station E. The phase comparator provides an output on line 35 indicating the magnitude of the error and on line 35 or 37 indicating the direction of the error. This information is stored at 41 and encoded for insertion in station B's transmitted burst. A sync. loss detection circuit 43 provides an output if a pair of sync. pulses are not received in the appropriate frames and switches the circuit 17 so that it is operative every frame. If, after a predetermined time a pair of pulses in the same frame has not been found, synchronization is considered to be lost and this information is supplied via line 47 to be encoded at 41 for transmission to station E. At station E (Fig. 4, not shown) the information in time slot b s  of station B is decoded, stored, and used to control the basic clock frequency accordingly to delay or advance the burst by one bit until the error is zero.