AIRCRAFT CLOCK MONITORING AND TIME PROPAGATING

• US 3,705,404 A
• Filed: 11/17/1969
• Issued: 12/05/1972
• Est. Priority Date: 11/17/1969
• Status: Expired due to Term

• Current Assignee: John P. Chisholm
• Original Assignee: John P. Chisholm
• Inventors: Chisholm, John P.
• Primary Examiner(s): Borchelt, Benjamin A.
• Assistant Examiner(s): Berger, Richard E.
• Application Number:
  US04/877,188
• Time in Patent Office: 1,114 Days
• Field of Search: 343/112R,112TC,112CA,103,7.5
• US Class Current: 342/465
• CPC Class Codes: G01S 11/08 using synchronised clocks

Abstract

A synchronized clock time sharing method and system for navigation and collision avoidance, including multiple fixed position units and multiple mobile units participating by exchange of signals therebetween with reference to their positions in two time propagation grids one comprising families of hyperbolic time difference curves and the other comprising polar coordinate curves. The units use these curves and their transmissions and their various positions in the grids to monitor each other'"'"''"'"'s clocks and/or establish position in the grid of a selected unit.

Claims

1. In a system including multiple fixed units and at least one mobile unit, said fixed units having time clocks substantially synchronized to a system wide time base and each mobile unit having means for transmitting pulse signals from time to time, the combination of means for determining the times at which the mobile units transmit their pulse signals without requiring cooperation from the mobile units beyond the occasional transmission of their pulse signals, comprising:

a. at least three fixed units spaced apart and located in a geographical area and forming the foci of coordinate hyperbolic grids representing the loci of points having constant differences in signal propagation times to the foci, and at least one of said fixed units forming the center of a polar coordinate grid of propagation times to at least that one fixed unit;

b. receiver means at said fixed units for receiving said pulse signals; and

c. common computer means connected to said receiver means and responsive to the relative times of arrivals of said signals to locate a point in said hyperbolic time grid representing the position of the transmitting mobile unit, and said computer means being operative using said point and said polar time grid to determine the time according to the clock within said one fixed unit when the mobile unit transmitted its signal.

2. In a combination as set forth in claim 1, both the fixed units and the mobile units having time clocks substantially synchronized to said system wide time base, and said mobile units having means for transmitting said pulse signals and information indicating the times at which said signals were transmitted according to the mobile unit'"'"''"'"'s time clock.

3. In a combination as set forth in claim 2, means for comparing the time of transmission of a mobile unit pulse signal as determined according to the clock within one fixed unit with the time of transmission of the signal as indicated according to said information based upon the mobile unit'"'"''"'"'s time clock to determine the accuracy of synchronization of that time clock with respect to the clock in said one fixed unit.

4. In a combination as set forth in claim 3, communication means between the fixed unit associated with the polar time grid and said mobile units and operative for instructing the mobile units when their own clocks are determined to be inaccurate.

5. In a combination as set forth in claim 3, said communication means including clock error telemetry means.

6. In a combination as set forth in claim 1, both the fixed units and the mobile units having time clocks substantially synchronized to said system wide time base, said base including a repeating cycle of time slots occupied by the mobile units, and the mobile units respectively including means to transmit their pulse signals at predetermined moments in the slots known to all participating units.

7. In a combination as set forth in claim 6, means for comparing the time of transmission of a mobile units pulse signal as determined by said computer means with said predetermined moment at which the mobile unit transmitted the signal according to its own clock to determine the accuracy of the latter clock.

8. In a combination as set forth in claim 7, communication means between the fixed unit associated with the polar time grid and said mobile units and operative for instructing the mobile units when their own cycles are determined to be inaccurate.

9. In a combination as set forth in claim 8, said communication means including clock error telemetry means.

10. In a combination as set forth in claim 1 and including at least two mobile units, means for locating a fourth fixed unit with respect to the grids established by said at-least-three other fixed units and said fourth fixed unit being coupled to said common computer means, comprising means at the fourth fixed unit for receiving the mobile unit'"'"''"'"'s transmitted pulse signals;

means including said computer means and said other fixed units for determining the positions in the hyperbolic grids said at-least-two mobile units and for informing said fourth fixed unit of their positions;

means operative at the fourth fixed unit for determining time differences relative to the propagation of the pulse signals from said located mobile units to said fourth fixed unit; and

said computer means including means using at least two thus-determined time differences to locate the fourth unit in the grid.

11. In a combination as set forth in claim 1 and including at least one mobile unit whose position in said grids has been determined and whose times of transmission have been determined in the time base of said at-least-three other fixed units and including a remote fourth fixed unit which is coupled to said computer means and whose position in the grids is known, means for synchronizing the fourth unit'"'"''"'"'s clock with the time base to which said other fixed units are synchronized comprising, means at the fourth fixed unit for receiving the mobile unit'"'"''"'"'s transmitted pulse signal;

means including said computer means for determining the actual time of arrival of the received signal according to the fourth unit'"'"''"'"'s clock time, for computing when the pulse signal should arrive at the fourth unit according to its position in said grids and the time base of the three fixed units, and for computing the error in the clock at the fourth unit by comparing said actual time of arrival witH said computed time of arrival; and

means for correcting the fourth fixed unit'"'"''"'"'s clock to eliminate such error.

12. In a combination as set forth in claim 11, said fourth fixed unit and said common computer means including means for extending said coordinate grid of hyperbolic curves using as foci said other fixed units and said fourth fixed unit by establishing additional grids representing the loci of points having the same differences in propagation times as between said fourth fixed unit and other fixed units.

13. In a combination as set forth in claim 1, at least two groups of fixed units each group having its own common computer means, and the respective computer means being coupled to a central computer serving the several groups, and the time grids of adjacent groups having areas overlapping each other whereby mobile units operating in the overlapping areas can have their clocks monitored by either group, and means for propagating the time base from one group to another comprising means in the latter group from synchronizing its local time base to that of a mobile unit synchronized to the former time group.

14. The method of determining the time of transmission of a pulse signal from a mobile unit to a selected fixed unit which is one of at least three fixed units having clocks synchronized to a common time base, including the steps of transmitting a pulse signal from the mobile unit and receiving it at the fixed units, locating the mobile unit by computer-solving simultaneous hyperbolic relationships based on differences in times of arrival of said signal at said fixed units, computer-converting this location into polar coordinates extending from the selected one of said fixed stations to determine travel time of the pulse signal thereto, and correcting the actual time of arrival at the selected fixed station of said signal by said travel time to determine the actual time of transmission by the mobile unit.

15. The method of locating in a system of hyperbolic time difference grids whose foci comprise multiple ground station units a remote fixed unit whose position with respect thereto is unknown but which is coupled to computer means shared in common with said ground station units, including the steps of navigating multiple mobile units within said grids and in direct communication with said ground stations and said fixed unit;

synchronizing said ground stations and mobile units to a common time base;

transmitting accurately timed pulse signals from the mobile units and receiving said signals at the ground stations and at said fixed unit;

computer calculating the locations of said mobile units in said hyperbolic grids using differences in the times of arrival of their pulse signals at said ground stations; and

computer calculating the position in said time difference grids of said fixed unit by using the differences in times of arrival of at the fixed unit of the pulses from the mobile units using their known positions as foci to establish hyperbolic relationships and extending said grids to include said fixed unit.

16. The method of synchronizing a time clock in a fixed unit whose position is known in a grid of hyperbolic time difference relationships including multiple ground stations which together with said fixed unit comprise the foci of said hyperbolic relationships and said ground stations having clocks synchronized with a common time base and these stations and the fixed unit being coupled to a common computer, including the steps of navigating at least one mobile unit within said grid and in direct communication with said ground stations and said fixed unit;

transmitting pulse signals from said mobile unit and receiving said pulse signals at said ground stations and said fixed unit;

computer calculating the location of said mobile unit in said hyperbolic grid using the differences in times of arrival of its pulse signals at said ground stations;

computer-calculating the time with reference to said common time base when the mobile unit tranSmitted its signals and the arrival time thereof at said fixed unit; and

correcting the fixed unit clock to remove the difference between said calculated arrival time and the time according to the clock at the fixed unit when said signals arrived.