Redundant aircraft clock synchronization

Claim 2

In an air traffic control system of the type including common master clock means counting out repeating cycles of time slots and including multiple participating aircraft each having local time keeping means including means for counting out similar cycles of slots and each aircraft uniquely occupying a designated one thereof, the system comprising in each aircraft: a. first and second time keeping means, said first time keeping means comprising a first time clock and said second time keeping means including at least a separate time cycle and slot counter; b. synchronizing means cooperative with the common master clock means and operative at all times for synchronizing the aircraft''s first time clock therewith; c. first correcting means operative when enabled to set the local second time keeping means to match the local first time clock; d. means for exchanging pulses with said another participating aircraft and including a second correcting means operative when enabled to synchronize the local second time keeping means with that aircraft''s time keeping means; e. first comparator means for comparing the local first and second time keeping means to determine their degree of mutual synchronization; f. second comparator means for comparing the local second time keeping means with the corresponding means in said another aircraft to determine the degree of their mutual synchronization based on the exchange of pulses therebetween; and g. means selectively operable in response to the degrees of synchronization determined by said first and second comparator means to selectively enable said first and second correcting means.

Claim 3

In a system as set forth in claim 1, said master clock means being located at participating ground stations common to the system and said first time keeping clock including oscillator means and counter meAns in each aircraft substantially continuously synchronized thereto, predetermined tolerance means associated with each comparator means which also includes means for delivering outputs from the comparator means indicating respectively whether or not the local second time keeping means is within tolerance when compared with the local first clock, and also whether or not it is within tolerance when compared with the second time keeping means in said another aircraft, and selective enabling means responsive to out-of-tolerance outputs from both local comparator means to enable the first correcting means to set the local second time keeping means to match the local first clock.

Claim 4

In a system as set forth in claim 2, said second time keeping means comprising a second clock oscillator driving said time cycle and slot counter independently of said first time clock.

Claim 5

In a system as set forth in claim 2, said second time keeping means comprising said time cycle and slot counter coupled to be driven by the oscillator means of the first time keeping means.

Claim 6

In a system as set forth in claim 2, said selective enabling means being responsive to outputs from the two comparator means indicating the local second time keeping means to be within tolerance as determined by both comparator means to disable said first correcting means and to enable said second correcting means.

Claim 7

In a system as set forth in claim 2, said selective enabling means being responsive to outputs from the two comparator means indicating the local second time keeping means to be within tolerance with respect to the local first clock but outside of tolerance with respect to the second time keeping means in said another aircraft to disable said first correcting means and to enable said second correcting means.

Claim 8

In a system as set forth in claim 2, said selective enabling means being responsive to outputs from the two comparator means indicating the local second time keeping means to be outside tolerance with respect to the local first clock but within tolerance with respect to the second time keeping means in said another aircraft to disable said first correcting means and to enable said second correcting means.

Claim 9

In a system as set forth in claim 2, said local second time keeping means including its own independent controlable-rate clock oscillator and said second comparator means determining an error signal, and said second correcting means including oscillator correcting means responsive to said error signal and operative to correct the rate of the latter clock oscillator.

Claim 10

In a system as set forth in claim 2, said local second time keeping means including said time cycle and slot counter coupled to be driven by pulses from the oscillator means of the first time keeping means, and said second correcting means including means responsive to said error signal to selectively delete or add drive pulses with respect to those provided by the oscillator means.

Claim 11

In a system as set forth in claim 2, alarm means respectively coupled to each comparator means and each rendered operative by an output indicating the local second time keeping means to be outside of tolerance as determined by the associated comparator means.

Claim 12

In an air traffic control system of the type including common master clock means counting out repeating cycles of time slots and including multiple participating aircraft each having local time keeping means including means for counting out similar cycles of slots and each aircraft uniquely occupying a designated one thereof and exchanging synchronizing signals with another aircraft uniquely occupying other designated slots and variously acting as synchronizor aircraft and as synchronizee aircraft, the system comprising in each aircraft: a. first and second time keeping means, said first time keeping means comprising a first time clock and said second time keeping means including at least a separate time cycle and slot counter; b. synchronizing means cooperatIve with the common master clock means and operative at all times for synchronizing the aircrafts''s first time clock therewith; c. first correcting means operative when enabled to set the local second time keeping means to match the local first time clock; d. means for exchanging pulses with the local aircraft''s designated synchronizor aircraft during their time slots and including a second correcting means operative when enabled to synchronize the local second time keeping means with the synchronizor aircraft''s time keeping means; e. means operative during slots occupied by synchronizee aircraft to transpond to pulses transmitted thereby to permit the synchronizee aircraft to improve the degree of synchronization of their second time keeping means with the second time keeping means in the local aircraft; f. comparator means for comparing the local second time keeping means with the local first time clock to monitor the difference therebetween; and g. means selectively operable in response to the degrees of synchronization determined by said comparator means to selectively enable one of said correcting means.

Claim 13

In an air traffic control system of the type including common master clock means counting out repeating cycles of time slots and including multiple participating aircraft each having local time keeping means including means for counting out similar cycles of slots and each aircraft uniquely occupying a designated one thereof and exchanging synchronizing signals with another aircraft uniquely occupying other designated slots and variously acting as synchronizor aircraft and as synchronizee aircraft, the system comprising in each aircraft: a. first and second time keeping means, said first time keeping means comprising a first time clock and said second time keeping means including at least a separate time cycle and slot counter; b. synchronizing means cooperatIve with the common master clock means and operative at all times for synchronizing the aircrafts''s first time clock therewith; c. first correcting means operative when enabled to set the local second time keeping means to match the local first time clock; d. means for exchanging pulses with the local aircraft''s designated synchronizor aircraft during their time slots and including a second correcting means operative when enabled to synchronize the local second time keeping means with the synchronizor aircraft''s time keeping means; e. means operative during slots occupied by synchronizee aircraft to transpond to pulses transmitted thereby to permit the synchronizee aircraft to improve the degree of synchronization of their second time keeping means with the second time keeping means in the local aircraft; f. comparator means for comparing the local second time keeping means with the local first time clock to monitor the difference therebetween; and g. means selectively operable in response to the degrees of synchronization determined by said comparator means to selectively enable one of said correcting means.

Claim 14

In a system as set forth in claim 11, slot logic means in each aircraft driven by the local second time keeping means to cyclically select the local aircraft''s own slot and the slots of predetermined synchronizor and synchronizee aircraft, said means to transpond being enabled by said slot logic means during each selected synchronizee slot; means enabled by said slot logic means during the local aircraft''s own time slot for transmitting pulses for marking its own position and for interrogating the transpond means in its own synchronizor aircraft; means responsive to the synchronizor''s transponded reply and responsive to the time of reception of the synchronizor''s position marking pulse according to the local aircraft''s second time keeping means for determining the difference between the second time keeping means in the local aircraft and in the synchronizor aircraft, and said second correcting means including means responsive to said determined difference for correcting the local second time keeping means to reduce said difference.

Claim 15

In a system as set forth in claim 11, said comparator means including means for determining when said difference between the two local time keeping means is within a predetermined tolerance, and operative in response thereto to disable said first correcting means.

Claim 16

In a system as set forth in claim 13, alarm means responsive to a difference which exceeds said predetermined tolerance as determined by said comparator means.