GB1355167A - Collision avoidance system - Google Patents

Abstract

1355167 Automatic frequency control BENDIX CORP 24 April 1972 [19 May 1971] 18919/72 Heading H3A [Also in Division G3] In a collision avoidance system wherein a number of units, e.g. in aircraft, are allotted time slots in a repeating cycle, each unit includes a first local frequency source, e.g. a cesium beam standard, and a second local frequency source which is normally locked to the first source but takes over if the latter fails, the second source being then controlled in accordance with the average local clock error which is determined by comparing the arrival time of synchronization replies received by the unit, in response to synchronization requests, with corresponding predetermined points in the units time slots. The output from the first frequency source is normally available at terminal 10 and oscillator 20 is locked thereto by a phase locked loop circuit 14, whose output is also stored in a memory in a circuit 28. If the input 10 fails, control 34 connects output terminal 40 to the output of voltage controlled oscillator 20 via switch 36. The control 34 also changes switches 26 and 19 so that the memory in circuit 28 supplies its stored voltage to control the oscillator 20, and closes switch 30 to ensure that the memory is updated by error processor 45. The processor receives synchronization pulses by radio from a distant donor station in response to synchronization requests which are sent out at a predetermined point in each of the unit's time slots. The point of arrival of the response within the slot is used to indicate any lack of synchronism between the local unit and the donor (Figs. 2A, 2B, not shown). If the response arrives before or after the predetermined point, the number of clock pulses stored in a counter (86, Fig. 3, not shown) is correspondingly increased or decreased. The average local clock error is thus determined over a number of slot cycles and fed to a memory (56) to control the oscillator 20 via a D/A converter (58).