Patent GB-1333864-A (GB)
Title: Synchronization and position location system
IPC: H04B7/185
Filed: 1971-04-28
Abstract: 1333864 Multiplex pulse signal INTERNATIONAL STANDARD ELECTRIC CORP 28 April 1971 [6 May 1970] 11842/71 Heading H4L A synchronization and range-finding arrangement in a time division, multiple access communication system is described in relation to an air traffic control system in which information is transmitted between N slave stations carried by aircraft and a ground-based master station via an orbitting satellite repeater station. Each of the stations transmits data bursts which must be compensated for doppler shift and differences in range to arrive at the repeater station in their correct time slots without overlapping. The number of slave stations (N) is an integral multiple of the number of time slots M available for the signal bursts from the stations so that each time slot is available on a push-to-talk basis for N/M slave stations. In a modification, two satellite repeater stations are used to provide an alternative method of fixing the positions of the aircraft. General description.-As shown in Fig. 2 the master station transmits a reference sync burst (A) via the satellite to each of the aircraft and this is also received back at the master station (B) from the satellite with a phase difference 2MS representing twice the distance between the master station and satellite. The reference burst is received at a slave station (C) and converted into a low level pseudo-noise-code ranging signal (D) which is transmitted via the satellite to the master station (E) where the phase in relation to the reference burst is measured. From this is subtracted the phase difference 2MS and the resultant divided by two indicates the range from aircraft to satellite. This information is coded and transmitted back to the corresponding slave station to adjust the phase of its timing signals so that its data bursts appear in the correct time slot. In a one satellite system the altitude of the slave station and the rate of change of the range obtained by measuring the doppler shift of the received carrier wave, are transmitted to the master station in slow speed data channels provided during the synchronizing interval between the data bursts. This together with the satellite-toslave station range enables the ground station to locate the position of each slave station. In a two-satellite system the satellite-to-slave station ranging equipment is duplicated for co-operation with the second satellite, the satellite-to-slave station range for both satellites and the aircraft altitude enabling the position location of a slave station to be established at the ground station. A complete frame as shown in Fig. 3A includes a master sync burst of 11 Ásecs. and M = 10 time slots, the guard time between adjacent time slots, the guard time between adjacent time slots being 0À25 Ásecs. and each time slot being divided into a synchronizing ranging and slow speed data burst interval of 1 Ásec. and a voice data burst interval of 10 Ásecs., the timing signals being derived by dividers from a 800 khz clock which has a period of 1À25 Ásecs. giving a frame period of 123À75 Ásecs. Sixteen frames are defined as one sub-frame (B) in relation to the slow speed signals and each aircraft transmits a 1 Ásec. sync pulse once per sub-frame and the master station modifies its sync signal in a basic period of one sub-frame. To avoid ambiguity in the ranging signal a minimum of twenty bits are required for coding the signal in the arrangement described and this is provided by coding the 1 sec. sync pulses and grouping 32 sub-frames into a sub-sub-frame (C). Apparatus details.-Initial access is obtained by an aircraft transmitting at low power its 1 Ásec. sync signal, its transmit timer 44, Fig. 5, being adjusted until the return signal falls in the assigned time slot. Alternatively, to speed the process, a 10 Ásec. signal providing a " coarse " search may be transmitted. The timer 44 is stepped in 10 Ásec. steps until the ranging pulse returned from the satellite is received during the 1 Ásec. sync and data interval allotted to the aircraft. This is followed by a fine search in which a low level 1 Ásec. ranging pulse is transmitted every frame to provide  frame sync and the final step consists of a search of the sixteen 1 Ásec. sync intervals of a subframe. At the ground station, Fig. 4, transmit timer 46 causes a master sync burst to be transmitted from sync burst generator 55 via phaseshift modulator 56. The return sync burst from the satellite is supplied to carrier tracking phase-locked loop 57 which in combination with  master sync envelope detector 58 and receive clock phase locked loop 59 adjusts the receive timer 45 and transmit timer 46 to compensate for doppler shift and ensure that the master sync burst occurs in the correct time slot at the satellite. At the aircraft, Fig. 5, the received master burst is detected at 61 which in cooperation with master sync envelope detector 62 and receive clock phase locked loop 63 adjusts the receive timer 47 and via transmit clock phase loop and phase shifter 65 initially adjusts the transmit timer 44. Timer 44 then activates the pseudo-noise code ranging signal for transmission via subcarrier generator and modulator 67. This coded signal is received at the ground station, Fig. 4, and supplied via sub-carrier tracking phase locked loop 68 to pseudo-noise ranging phase locked loop 51 which includes correlators co-operating with a pseudo-noise reference generator 69. The resulting phase information is supplied to a phase comparator 52 receiving an input from receiver timer 45 and operating an indicator appropriately scaled to give a direct measure of the satellite-to-aircraft range and is coded at 50 and with other data inputs is supplied via slow speed data multiplexer and modulators 49 and channel burst multiplexer and modulators 70 for transmission via phase-shift modulator 56. At the aircraft, Fig. 5, this signal is selected at 52 and supplied to command decoder 71 and the resulting phasing signal controls the transmit timer 44 directly by means of step advance or retard signals and in continuous manner through phase shifter 65. Position location can be obtained by closing switch 72 so that altimeter 55 supplies its output to one of the slow speed data channels via multiplexer 53 and by closing switch 73 so that doppler detector 54 supplies a rate-of-range change signal to another low speed data channel. These signals are recovered at the ground station in the slow speed data utilization devices 74 via demultiplexers and demodulators 47 enabling location of the aircraft in question. In the two satellite location system the doppler detector 54 is not required. High speed voice and data transmission is provided by multiplexer 70 and demultiplexer 58 at the ground station in cooperation with multiplexer 75 and demultiplexer 76 in the aircraft.