COMPUTER DETERMINING THE LOCATION OF OBJECTS IN A COORDINATE SYSTEM

• US 3,659,085 A
• Filed: 04/30/1970
• Issued: 04/25/1972
• Est. Priority Date: 04/30/1970
• Status: Expired due to Term

• Current Assignee: Basil E. Potter, James A. Cadzow, John P. Chisholm, Theodore K. Bosworth
• Original Assignee: Basil E. Potter, James A. Cadzow, John P. Chisholm, Theodore K. Bosworth
• Inventors: Potter, Basil E., Bosworth, Theodore K., Chisholm, John P., Cadzow, James A.
• Primary Examiner(s): Gruber, Felix D.
• Application Number:
  US05/033,205
• Time in Patent Office: 726 Days
• Field of Search: 343/112,112TC 340/172.5 235/150.1,150.2,150.22,150.23,150.26
• US Class Current: 701/518
• CPC Class Codes: G01S 2205/03 Airborne
• CPC Class Codes: G01S 5/0244 Accuracy or reliability of position solution or of measurements contributing thereto
• CPC Class Codes: G01S 5/0268 by deriving positions from different combinations of signals or of estimated positions in a single positioning system

Abstract

A disclosure in which one or more objects, such as aircraft or other vehicles, moving in a system of coordinates transmit pulse signals to a large number of fixed position receiving stations all linked to common computer means, and the computer means uses one of several well known techniques such as multilateration, time-of-arrival, differential time-of-arrival, etc., of the pulse signals to solve for position of the transmitting object, and in which weighting is used to minimize errors. Such weighting includes selective of optimum receiving stations from a larger number of available stations according to geometric criteria, weighting the value of the data delivered by the various stations in order to weight most heavily the best data chosen according to predetermined criteria, and minimizing the errors by one of several different techniques including the use of iterative computations converging upon a location which continuously grows more accurate, or by other error minimizing techniques such as the technique of least squares fitting.

Claims

1. The method of determining successive positions in a system of multiple coordinates of one or more objects each emitting pulse signals receivable at a multiplicity of m pulse-signal receiving points whose coordinate positions in a fixed pattern are known, the coordinates of each object being defined by the times of arrival of said pulse signals at at least some of said m receiving points and the coordinates of each previously determined position being known, the method including the steps of:

a. receiving said emitted pulse signals at said receiving points and converting them to electrical data representing the times of reception of the signals thereat;

b. automatically selecting for processing the data available at a number n of receiving points, where n is less than m, the selection of which points is based upon the known coordinate positions of the respective points and upon said previously determined object positions; and

c. automatically processing said data from said n selected points and computing a new position of each object by solving simultaneous equations for its coordinates.

2. The method as set forth in claim 1, including the steps of automatically repeating said receiving and selecting and processing steps, basing repeated selections of data to be processed upon each new coordinate position determined.

3. The method as set forth in claim 1, including in said selecting step the step of selecting for processing the data from a first receiving point which is closest to the known previous position of each object and then selecting the data from at least three other receiving points which are near said first point.

4. The method as set forth in claim 1, including the steps of storing data simulating topographical features in the vicinity of the pattern of receiving points, automatically computing a straight line between each selected point and the known previous position of the object, automatically determining whether each line will intersect a topographical feature and selecting receiving points where it will not.

5. The method as set forth in claim 4, including the steps of storing data represEnting calculated error profiles of the receiving points for various possible positions of the object, and said selecting step including the selecting of data from miminum-error receiving points based on the known previous position of the object.

6. The method as set forth in claim 1, including for the purpose of said selecting, the step of selecting data from an arbitrary first group of at least n receiving points in the absence of prior information as to the location of an object, and said processing step including automatically computing from the data from these points an initial approximation of position of the object.

7. The method as set forth in claim 1, including the step of selecting more than n receiving points, automatically determining a coefficient for each such point based on the strength of the signals respectively received thereat, and automatically processing the data from selected points having the best signal-strength coefficients.

8. The method as set forth in claim 1, including the step of selecting more than n receiving points, automatically testing data from said points to determine when any two such points yield data representing simultaneous reception of pulse signals thereat, and in response to such determination automatically selecting different receiving points.

9. The method as set forth in claim 1, including in the selecting step the step of selecting at least n receiving points based upon the condition that the points selected include points which are most nearly mutually orthogonally disposed with respect to the position of the object to be located.

10. The method set forth in claim 1, where one object to be located is an aircraft flying in said coordinate system over an x, y plane at an altitude z, said selecting step including selecting at least three receiving points such that at least two of them are widely spaced from the projection of the aircraft position onto the x, y plane to provide good x, y position data, and selecting at least one other receiving point located nearly beneath the aircraft to provide good z-position data.

11. The method as set forth in claim 10, including selecting plural points substantially beneath the aircraft, automatically computing the elevation angles therefrom to the known previous position of the aircraft;

and automatically weighting the data from those points according to a weighting coefficient proportional to the sine of the respective elevation angles when computing z-coordinate position.

12. The method as set forth in claim 1, wherein at least one object emits its pulse signals at a constant repetition rate, including the further steps of automatically determining from said data the repetition rate of said signals, automatically determining from said data and from said rate anticipated moments when further signals will be received, and automatically receiving further signals only during brief guardband intervals around said anticipated moments.

13. The method as set forth in claim 1, wherein plural objects emit pulse signals, including the step of automatically computing said anticipated moments with respect to signals emitted by one of the plural objects;

and automatically identifying data related to signals received from said one object based upon the moment during which it is received.

14. The method as set forth in claim 1, wherein plural objects emit their pulse signals at substantially constant but mutually unsynchronized repetition rates, including the further steps of automatically determining from received data the repetition rates of the various objects'"'"''"'"' signals, automatically determining from data related to each of the objects'"'"''"'"' signals the anticipated moments when further signals should be received from the respective objects, automatically receiving further signals with respect to each of the emitting objects only during brief guardband intervals encompassing said anticipated moments, And automatically identifying the various objects on the basis of the anticipated moments and the actual moments of reception of their signals.

15. The method of determining from prior known positions a succession of new positions in a system of three-dimensional coordinates of one or more pulse signal emitting aircraft flying over receiving points located on the ground, the method being operative for aircraft flying at altitudes not exceeding a predetermined maximum altitude, the method including the steps of:

a. deploying a multiplicity of m of said receiving points in fixed ground positions to form an extensive grid thereof in which the spacings between adjacent points are no greater than twice said maximum altitude and storing the coordinates of said receiving points;

b. receiving pulse signals from each aircraft to be located with respect to said receiving points and automatically converting said signals into electrical data representative of the times of reception of said signals thereat;

c. automatically selecting for processing the data which becomes available at a selected number n, where n is less than m, of said multiplicity of receiving points based upon the prior known aircraft position and upon the stored coordinates of the respective receiving points; and

d. automatically processing said selected station data including computing a new position for each of said aircraft by solving simultaneous mathematical expressions for its coordinates.

16. The method as set forth in claim 15, said selecting step including selecting on the basis of a prior known position of an aircraft to be located and on the basis of the stored coordinates of the receiving points a first receiving point located substantially below the aircraft and at least three other receiving points located near thereto.

17. The method as set forth in claim 15, based upon an arbitrary subdivision of the airspace above the ground grid into subvolumes defined in terms of coordinates of said system;

automatically computing and identifying groups of n receiving points having coordinates such that they are favorably located respectively to provide optimum electrical data based upon emitted pulse signals from an aircraft flying in each of said subvolumes and automatically storing a tabulation of the coordinates of said subvolumes together with the coordinates of said corresponding groups of points;

automatically determining which subvolume an aircraft occupies by comparing the coordinates of its prior known position with the coordinates of said subvolumes; and

said selecting step automatically selecting the corresponding group of receiving points to receive the next emitted signals from that aircraft and furnish data related thereto.

18. The method as set forth in claim 17, including automatically identifying at least one alternate group of receiving points for each computed group;

automatically determining whether signals received at said group are of strength exceeding a predetermined level, and if not, automatically selecting an alternate group to receive the next emitted pulse signals from the aircraft.

19. The method as set forth in claim 15, including the steps of storing in coordinate form a topographical representation of the ground contour in the vicinity of the grid of receiving points;

automatically making a determination based upon the said prior known position of the aircraft whether an imaginary line extending from the aircraft to each selected receiving point would intersect a topographical feature, and automatically selecting receiving points free of such intersections.

20. The method as set forth in claim 19, including automatically computing and storing data identifying alternate receiving points corresponding with other selectible receiving points;

automatically determining whether signals received by selected receiving points are of strength, exceeding a predetermined level, and if not, automatically selecting aLternate points to receive the next emitted pulse signals from the aircraft.

21. The method as set forth in claim 15, including the steps of automatically introducing error perturbations of known magnitude into said step of processing by computing a new aircraft coordinate position to determine the sensitivity of the computation to error in each of the individual coordinates;

automatically computing weighting coefficients based on the effects of said introduced perturbations and operative to optimize the accuracy of future new positions solved for.

22. The method as set forth in claim 21, including the step of automatically including in said simultaneous expressions said weighting coefficients to reduce the ultimate residual positional error when the expressions are solved.

23. The method as set forth in claim 22, including the step of automatically including in said expressions weighting coefficients which are constant for the respective x, y and z coordinates and which weight most heavily the z-coordinate errors.

24. The method as set forth in claim 22, based upon the arbitrary subdivision of the airspace above the grid according to different altitude levels, the steps of automatically calculating and storing data representing for each of these levels separate optimum z-coordinate weighting coefficients;

and automatically selecting a z-coordinate weighting coefficient for a present computed solution on the basis of the last-determined aircraft z-coordinate.

25. The method as set forth in claim 24, including the steps of automatically determining weighting coefficients for each aircraft position solution using as the basis previously determined coordinate positions;

automatically determining the residual altitude error; and

automatically selecting for subsequent solutions coefficients which are proportional to a power of the residual altitude error.

26. The method as set forth in claim 22, including automatically selecting coefficients which are proportional to the square or to a higher power of non-altitude residual errors.

27. The method as set forth in claim 15, including automatically determining weighting coefficients for each aircraft position solution of said expressions using previously computed aircraft position to determine altitude, range, and elevation angle from each selected receiving point;

and automatically calculating coefficients which are proportional to the square of the sine of the aircraft'"'"''"'"'s elevation angle for each such station.

28. The method as set forth in claim 15, including the steps of automatically selecting at least two receiving points widely spaced from a projection of the aircraft position onto the ground to provide good x, y position coordinates, and automatically selecting at least one other point on the basis of the computed x, y coordinates such that the latter point is located nearly beneath the aircraft to provide good z-position coordinates.

29. The method as set forth in claim 15, including the steps of automatically determining a position of the aircraft by automatically solving simultaneous mathematical expressions obtained by equating the square of the ranges to the aircraft position from the receiving points to the sum of the square of expressions relating coordinates of the aircraft position and signal receiving point locations plus error terms, and automatically solving these expressions for coordinates of an aircraft position which will minimize the error terms.

30. The method as set forth in claim 29, including the step of automatically determining weighting coefficients for said error terms and including them in the expressions solved to obtain said coordinates which minimize the weighted error terms.

31. The method as set forth in claim 29, including the steps of automatically solving said expressions equated to error terms which are equated to zero where the expressions also include increments added to said aircraft coordinAtes to yield error components diminishing after each solution of the expressions to provide an iterative process yielding convergent solutions.

32. A system for determining a series of space coordinate positions for one or more periodic pulse-signal emitting aircraft flying over a grid of fixed ground stations deployed in the same coordinates and having means to receive said signals, and the stations being coupled with central data processing equipment including a computer programmed to select optimumly located stations to yield data for computing each new aircraft position and further including memory means, said system comprising:

a. means in the central processing equipment coupled to said computer and operative for accepting demodulated signals resulting from reception of said pulse signals from an aircraft in at least some selected stations;

b. means in the central processing equipment for receiving said demodulated signals and operative for determining from the latter whether a sufficient number n of demodulated signals have been received from said stations to permit computing a new aircraft position, for so indicating to the computer;

c. means in the central processing equipment responsive to said demodulated signals from said stations and operative to generate data representing the relative times of reception of each of the pulse signals at said selected stations, and store the data temporarily in said memory means; and

d. means in the central processing equipment responsive to said data representing the times of reception to generate guardband gating signals anticipating the times of reception of the next periodic aircraft signals and operative to inhibit said means for accepting signals from passing further signals to the central processing equipment except during those times.

33. The system set forth in claim 32, including a pulse-emitting test beacon fixed in a known position in the grid relative to said ground stations, and operative when actuated to emit pulse signals receivable by the stations for testing said ground stations and the accuracy of the system in computing the position of the coordinates of the test beacon.

34. The system as set forth in claim 32, including data link means providing two-way communication between each station and the central processing equipment, and including means responsive to selection by the latter of preferred stations to control the operative condition of the various stations.

35. The system as set forth in claim 32, including means at each station operative when enabled for transmitting to the central processing equipment signals indicating the time of arrival at that station of the aircraft pulse signal and for transmitting information indicating its signal strength;

said means in the central processing equipment further comprising means responsive to these transmissions from a station for determining whether all selected stations have received pulse signals of strength exceeding a predetermined minimum and whether these stations have received n separately distinguishable pulse signals within a predetermined span of time representing the maximum difference in time of reception of a pulse signal received at any two stations which might be selected at the same time, and means responsive to both determinations in the affirmative to so inform the computer.

36. The system as set forth in claim 35, wherein the computer is programmed to select another group of stations which are alternate with respect to the selected group, means in the system responsive to failure of any of the latter determinations to so inform the computer and thereby actuate it to select an alternate group.

37. The system as set forth in claim 32, means at wherein each station operative when enabled for transmitting to the central processing equipment signals indicating the moment of reception at that station of the aircraft pulse signal;

said means to count out the relative times of pulse signal rEception comprising n-1 register counters at the central processing equipment;

a source of clock pulses;

means responsive to reception of said signals at the central equipment to gate pulses into said counters to accumulate counts therein proportional to the differences in times of arrival of the same aircraft pulse signal at the selected stations; and

means responsive to reception of n signals from the stations to set these counts into said memory means in the computer for use in computer-determining said new position coordinates.

38. The system as set forth in claim 37, wherein each aircraft periodically transmits its pulse signals at a constant repetition rate;

said means in the central processing equipment to generate guardband gating signals including means responding to the first pulse signal received by any selected station as a result of each aircraft transmission and further including means responsive to the periodic occurrence of such first pulse-signals to accurately determine the pulse repetition rate of that aircraft'"'"''"'"'s transmissions;

means responsive to these first pulse signals and to said counts of the counters representing the differences in times of reception of the same aircraft pulse signals at the other selected stations for generating groups of guardband gating signals which are each wider than said pulse signals and which have the same spacings as those pulse signals;

means for offsetting these guardband gating signals to occur earlier in real time by one-half of a guardband gating signal width, and means for gating signals from the selected stations into the central equipment only during said guardbands.

39. The system as set forth in claim 38, including means operative during initial acquisition of pulse signals from an aircraft whose position is unknown for gating signals from stations into the central equipment in the absence of generated guardband signals.

40. The system as set forth in claim 38, wherein plural, aircraft are flying over said grid emitting pulse signals at constant repetition rates which are mutually unsynchronized;

multiple means for generating separate groups of guardband gating signals corresponding respectively with said multiple aircraft, each group anticipating the arrival of pulse signals from different aircraft.