Patent GB-1291891-A (GB)
Title: Synchronisation system
IPC: G08G5/04
Filed: 1970-04-17
Abstract: 1291891 Radionavigation SIERRA RESEARCH CORP 17 April 1970 [6 May 1969] 18504/70 Heading H4D ' Relates to a synchronization system described as applied to a radio aircraft collision avoidance system (CAS) in which each aircraft transmits during a uniquely allotted time slot of duration #t in each of a plurality of successive epochs a signal which enables another participating aircraft to determine its distance from said aircraft (i.e. one-way ranging system), the slots and epochs at each aircraft being defined by  counters 22 and 24 driven by a local clock pulse genera tor 20 which is maintained (by a unit 16 of known form) in absolute synchronism with a standard time base normally provided by ground sync.-control stations. When a group of such aircraft is not in radio contact with a ground sync.-control station, synchronization is provided by those aircraft in the group whese clocks are estimated to be sufficiently accurate for this purpose; such aircraft are designated "capable" aircraft and according to the present invention means are provided for causing the capable aircraft to act as sync. donors in an orderly manner one at a time. If there is only one capable aircraft in the group then said aircraft automatically functions continuously as the sync. donor and a switch 62 is provided for causing the local aircraft to function as a sync. donor regardless of its sync. quality. As described the capable aircraft act as sync. donors in a sequence defined by the serial numbers of the own time slots allotted to said aircraft, e.g. if there are three capable aircraft occupying slots 920, 975 and 1024 as shown in Fig. 2, then if during epoch n aircraft 920 is acting as the sync. donor, aircraft 975 and 1024 will act as sync. donors during epochs (n+ 1) and(n+2) respectively; alternatively this order may be reversed and each capable aircraft may act as a sync. donor for a plurality (e.g. two) of successive epochs. When an aircraft is acting as a sync. donor it transmits a code "0" pulse signal at a predetermined time during its own time slot and the aircraft which is to be the sync. donor during the next epoch transmits a code "1" pulse signal at the predetemined time during its own time slot.  As shown each aircraft generates:- (1) a Yes or No sync. capability signal at respective outputs 16b, 16c of the CAS unit 16 according to whether it is or is not capable of acting as a sync. donor, said capability being determined in a known manner, e.g. as dis-  closed in Specification 1,235,711;  (2) own-slot pulses OSP of duration #t by means of a selector unit 25 coupled to the time slot succession counter 24;  (3) epoch-start pulses E by means of the counter 24; (4) sync. trigger pulses STP at said predetermined times in each time slot, the pulses STP and OSP being applied to an AND gate 26 to give at said predetermined time in said own slot a sync. code trigger pulse SCTP which is applied in parallel to two AND gates 38 and 48; . and also comprises a pair of decoders 30 and 32 for decoding received code "0" and code "1" signals.  Assuming that another aircraft is acting as donor and that the local aircraft is to be the next donor, then the code "0" signal from the donor aircraft is decoded at 30 and passed by an AND gate 34 (enabled by the Yes sync. capability signal from CAS unit 16) to set a bi-stable 36 which enables the AND gate 38 to apply the sync. code trigger pulse SCTP to:- (1) a code "1" encoder 40 to generate and transmit a code "1" signal; and (2) set a bis-table 71 thereby enabling an AND gate 46 to apply the next epoch start pulse E to set a bi-stable 44 which :- (a) enables the AND gate 48 to cause the succeeding sync. code trigger pulse SCTP to generate and transmit a code "0" signal;  (b) resets the bi-stable 36 (via line 44b) thereby preventing generation of the code "1" signal,  (c) resets the bi-stable 71 (via line 44e), and  (d) enables the CAS unit 16 (via line 44a) to supply sync. signals in response to requests from other aircraft. If however another capable aircraft occupies a time slot between the slots occupied by the current donor and the local aircraft then the code "1" signal from said other aircraft resets the bi-stable 36 therebyinhibiting the gate 38 and preventing transmission of a code "1" signal from the local aircraft. '  As described above the local aircraft can only become a donor after reception from another donor aircraft of a code "0" signal which sets the bi-stable 36 but if no such code "0" signals are received during a predetermined number N= 3 of epochs as determined by a counter 52 responsive to delayed own slot pulses OSP applied via an AND gate 54 enabled by the Yes capability signal, then the counter 52 sets the bi-stable 36 thereby allowing the local aircraft to become a donor during the next epoch; as shown the counter 52 is reset by a received code "0" signal or by the local code "0" trigger pulse from the AND gate 48. The local aircraft will cease to be a sync. donor if:-  (1) it receives a code " 1" signal from another aircraft occupying a subsequent slot since the resultant output from the code "1" decoder 32 sets a bi-stable 63 thereby enabling an AND gate 64 to apply the next epoch start pulse E to reset the bi-stable 44 and so inhibit the code "0" AND gate 48; (2) it receives a code "0" signal from another aircraft since the resultant output from the bi-stable 36 triggers a monostable 60 which resets the bi-stable 44, or (3) if its sync. quality deteriorates to such an extent that a No capability signal appears on lead 16c and resets the bi-stable 44. When the local aircraft is required to operate as a sync. donor regardless of its sync. quality then the switch 62 is moved to its dotted line position so that the AND gate 34 and 54 are always enabled.