Patent GB-1250570-A (GB)
IPC: G01S11/08
Filed: 1969-03-03
Abstract: 1,250,570. Radionavigation. SIERRA RESEARCH CORP. 3 March, 1969 [6 March, 1968], No. 11098/69. Heading H4D.  Relates to an aircraft collision-avoidance system (CAS) involving fully equipped aircraft each transmitting a coded range pulse a predetermined time #t after the start of a uniquely assigned time slot in a sequence of n time slots (each of duration #T) comprising an epoch of duration n#T, the starting times of the epochs being maintained in absolute synchronism with coded epoch-start-sync.-pulses of recurrence period n#T transmitted from a ground master CAS station 1 so that the distance between two fully equipped aircraft is obtained at each aircraft by measuring the time interval between a local #t-delayed time-slot-start-pulse and the reception in the corresponding time slot of the range pulse signal from the other aircraft. Each aircraft may also transmit its altitude as an altitude-coded pulse signal and may also carry a Doppler discriminator for determining the closing speed of the aircraft from which range pulse signals are being received. There is also a requirement for certain types of aircraft, e.g. light aircraft and military aircraft, to carry limited CAS equipment which will enable such a limited-equipped aircraft to determine its distance from a fully-equipped aircraft but which does not transmit range pulse signals so that the limited-equipped aircraft will not be detected by a fully-equipped aircraft and according to the present invention such a limited-equipped aircraft is synchronized with the ground master CAS station 1 by providing on the aircraft local slot and epoch counters 17 and 19 driven by a stable clock oscillator 16 and presetting the slot counter 17, on receipt of an epoch-start-sync-pulse from the ground station, in accordance with the distance d of the aircraft from the CAS ground station 1 as determined on the aircraft by:- (1) a conventional secondary radar DME 5 operating in conjunction with a DME transponder 3 at the ground station 1, or (2) visual observation of the aircraft position, so that the locally generated epoch start pulses will lead the subsequent epoch start pulses received from the ground station 1 by a time period Td corresponding to the distance d. As shown the stable clock oscillator 16 is coupled to a multi-stage binary slot counter 17 producing local timeslot-start-pulses which are applied to a #t delay 26 through the epoch counter 19 and a logic unit 20 and the DME 5 gives the distance d from the ground station 1 as an analogue voltage Vd which is converted at 22 into a corresponding digital number Nd representing the equivalent number of recurrence periods of the clock oscillator 16 corresponding to the one-way propagation time Td equivalent to the distance d, the digits of the digital range number Nd being coupled to corresponding stages of the counter 17 by normally-inhibited respective AND gates 24. If the DME 5 produces a digital range output, the scale factor of said output must be converted to correspond with the frequency of the oscillator 16. Alternatively the analogue distance voltage Vd applied to the A/D converter 22 may be produced by a manually set unit 29 when the aircraft passes over a readily identifiable object of known distance d from the ground station 1. When an epoch-start-sync.-pulse is received from the ground station 1 it is selected by a decoder 12 and applied to:- (1) reset the counters 17 and 19, (2) enable the gates 24 to set the range number Nd into the counter 17 so that the next local time-slot-start-pulse is produced after a time interval of #T-Td; thereafter the counter 17 operates at its normal recurrence period of #T so that the next local epoch-start-pulse produced by the counter 19 will lead the next received epoch-start-sync.-pulse by Td thereby establishing absolute synchronism. When absolute synchronism has been established the distance D between the limited-equipped aircraft and a fully-equipped aircraft A is given by a distance counter 28 driven by the oscillator 16, the counter 28 being started by a delayed time-slot-start-pulse from the delay 26 and stopped by a subsequent range pulse received from the aircraft A and selected by a decoder 14. The output D from the counter 28 is applied to a known form of threat evaluation and display unit 30 together with the closing rate D and altitude H of the aircraft A from units 14b and 14c.