Patent GB-1329866-A (GB)
Title: On-board space-division communication satellite switching systems
IPC: H04B7/204
Filed: 1970-10-14
Abstract: 1329866 Space-satellite communication systems POST OFFICE 14 Oct 1970 [15 Oct 1969] 48868/70 Headings H4K and H4L  A space-satellite has a switching network which routes segments of the received signal multiplex according to which ground stations are intended to receive the segments, segments for each ground station being switched to a corresponding directional antenna beamed to that station, control of the switching network being such that the satellite can be adapted on command to suit segments of altered duration and to suit changed allocation of segments. The satellite may receive signals from ground stations by way of a single global antenna and relay the appropriate segments of each multiplex received by way of respective ones of pencil beam millimetre wave antennae directed at the ground stations. The space-satellite relay station of Fig. 3, however, has receive antennae 21A to 21G associated with respective ground stations A to G to which the satellite transmits on separate radio beams by way of antennae 22A to 22G. A switching network 25 reorders the received segments according to destination. Synchronization of the multiplex.-An example of the frames transmitted by each station is shown in Fig. 4A while Fig. 4B shows the frames received by each station with the segments collated according to destination by the satellite switching network. The signals are pulse code modulated in time division multiplex and are coherently phase shift keyed for transmission. The satellite may initiate and broadcast a reference code for synchronizing the system but, as illustrated, one ground station of the system is nominated to transmit a reference code word SR. At the satellite a switch 28, set by a command link, connects the nominated reference station antenna to synchronizing circuits for the satellite switching network. A received reference code word is broadcast by the satellite over all transmit antennae. Following transmission of the reference code word by the nominated station, and transmission of unmodulated carrier by all other stations in corresponding positions of the multiplex, all stations transmit their own identification codes which are relayed back to the respective same stations by appropriate switching in the satellite. After such initial codes the frame transmitted by each ground station is divided into equal segments comprising groups of PCM channels allotted for transmission to specific other ground stations. Fig. 4A shows one pattern of segment allocation, the satellite control circuit being programmed over a command link to switch according to the prevailing segment duration and pattern of allocation in order to produce the received segment sequences of Fig. 4B. It is stated that more than one ground station may lie within the beam of a single pair of transmit/receive antennµ on the satellite. Space-satellite switching control.-Transistors are employed as radio frequency switches which, in a first arrangement, are connected as switching trees, one tree for each transmit antenna to which the associated tree selectively connects any one of the receive antennµ, Fig. 6, not shown. For an eight station system each tree fans out over three stages and is operated to make a selective connection in response to a 3-bit code. The sequential selections, corresponding to sequences such as shown for each station in Fig. 4B, are made from a stored array of 3-bit codes sequentially gated to operate the switching tree. Switching times, corresponding to the transition between segments or reference codes, are determined by a 1000 step counter with a cycle time equal to the multiplex frame, a series of registers equal to the number of switching operations required in the sequence being set up with timing numbers to indicate the transition points of the sequence. As the counter progresses it matches the timing numbers one after the other and at each match it steps a sequence counter to gate the next 3-bit switch tree code. The array of 3-bit codes and timing numbers is set up in accordance with instructions on a command link and is varied according to requirements. In a second arrangement the switching network comprises a matrix with a row distributing each receive antenna input and a column serving each transmit antenna output, Fig. 7, not shown. The crosspoint switches of each column are controlled by codes circulating in an associated cyclic access store, the position of the codes in the store cycle being chosen to match the transitions between segments and code words of the multiplex. To hold a cross-point operated during a multiplex segment or code word, the cross-point codes are repeated in successive compartments of the access stores so that they are repeatedly applied to a cross-point until its release is required. The access stores are set up over the system command link.