Method for reducing geometrical dilution of precision in geolocation of emitters using phase circles

• US 6,285,319 B1
• Filed: 01/27/2000
• Issued: 09/04/2001
• Est. Priority Date: 01/27/2000
• Status: Expired due to Term

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Litton Systems Incorporated (Northrop Grumman Corporation)
• Inventors: Rose, Conrad
• Primary Examiner(s): Issing, Gregory C.
• Application Number:
  US09/492,155
• Time in Patent Office: 586 Days
• Field of Search: 342/424, 342/430, 342/449, 342/451
• US Class Current: 342/449
• CPC Class Codes: G01S 3/48 the waves arriving at the antennas being continuous or intermittent and the phase difference of signals derived therefrom being measured
• CPC Class Codes: G01S 5/12 by co-ordinating position lines of different shape, e.g. hyperbolic, circular, elliptical or radial

Abstract

The present invention determines the geolocation of an emitter, using at least one observer measuring signal change while moving on at least two observation tracks. The measurement of emitter signal change is begun between a first observer position and a second observer position. The second observer position is predicted at the time the last measurement from which signal change will be derived is made. The method utilizes knowledge of the measurement start and end points to determine a second pair of observer signal-change measurement start and end positions such that the emitter line-of-position (LOP) determined by the second signal change measurement will intersect the LOP associated with the first signal change measurement at possibly multiple points, but at each of these point intercept orthogonally to within the signal change measurement errors. The method determines the intersection points of the LOPs and assigns a likelihood to each intersection point for measuring the probability for each of the multiple intersection points that it is the emitter location, determines a correct emitter position from these likelihood weights, and generates an error associated with the emitter position estimate.

Claims

1. A method for determining the geolocation of an emitter, using at least one observer measuring signal change while moving on at least two observation tracks, the method comprising the steps of:

beginning the measurement of emitter signal change between a first observer position and a second observer position;

predicting the second observer position at a time the last measurement from which signal change will be derived is made;

utilizing knowledge of the measurement start and end points to determine a second pair of observer signal-change measurement start and end positions such that the emitter line-of-position (LOP) determined by the second signal change measurement will intersect the LOP associated with the first signal change measurement at possibly multiple points, but at each of these points intercept orthogonally to within the signal change measurement errors;

determining the intersection points of the LOPs;

assigning a likelihood to each intersection point for measuring the probability for each of the multiple intersection points that it is the emitter location;

determining a correct emitter position from these likelihood weights; and

generating an error associated with the emitter position estimate.

2. The method of claim 1, wherein said beginning, said predicting and said utilizing steps are performed simultaneously.

3. The method of claim 1, wherein said determining the intersection point, said assigning, said determining the correct emitter position and said generating steps are performed following said utilizing step.

4. The method of claim 1, wherein the signal change measured is the change in wavefront normal to the emitter'"'"'s signal at the bearing change angle sub-tended at the emitter and the LOP is the circle passing through the emitter and two signal change measurement points wherein a COP is a circle uniquely determined by the bearing change.

5. The method of claim 1, wherein the error in estimating the correct emitter location depends only on the range of the emitter from an intersection of the two observation tracks and not the initial relative bearing of the emitter to either of the tracks, and comprising:

generating a start and end point of the second data collection track such that the straight line connecting the start of the second track and the end of the second track measurement position is orthogonal to a line connecting the start of the first track and the end of the second track measurement positions; and

determining the start time of the second observation track from predictions of observer position utilizing velocity and acceleration measurements such that the orthogonal intersection occurs at the midpoint of the first observation track.

6. The method of claim 4, wherein the error in estimating the correct emitter location depends only on the range of the emitter from an intersection of the two observation tracks and not the initial relative bearing of the emitter to either of the tracks, and comprising:

generating a start and end point of the second data collection track such that the straight line connecting the start of the second track and the end of the second track measurement position is orthogonal to a line connecting the start of the first track and the end of the second track measurement positions; and

determining the start time of the second observation track from predictions of observer position utilizing velocity and acceleration measurements such that the orthogonal intersection occurs at the midpoint of the first observation track.

7. The method of claim 1, wherein the step of generating the estimation error comprises:

determining the range to the emitter from the intersection of the observation tracks utilizing the emitter position estimate found by a COP intersection;

generating the error in the emitter position estimate normal to the COP through the emitter associated with each observer by utilizing this range, the distance between the pair of observation points and the signal measurement error; and

combining the determined range and the generated error to form an error ellipse having the property that its major and minor axis are nearly equal such that the error ellipse is a circle centered on the true emitter position and that the contours of equal location estimation error are coaxial circles centered on the track intersection.

8. The method of claim 4, wherein the step of generating the estimation error comprises:

determining the range to the emitter from the intersection of the observation tracks utilizing the emitter position estimate found by COP intersection;

generating the error in the emitter position estimate normal to the COP through the emitter associated with each observer by utilizing this range, the distance between the pair of observation points and the signal measurement error; and

combining these two single error estimates to form an error ellipse having the property that its major and minor axis are nearly equal such that the error ellipse is a circle centered on the true emitter position and that the contours of equal location estimation error are coaxial circles centered on the track intersection.

9. The method of claim 5, wherein the step of generating the estimation error comprises:

determining the range to the emitter from the intersection of the observation tracks utilizing the emitter position estimate found by a COP intersection;

generating the error in the emitter position estimate normal to the COP through the emitter associated with each observer by utilizing this range, the distance between the pair of observation points and the signal measurement error; and

combining these two single error estimates to form an error ellipse having the property that its major and minor axis are nearly equal such that the error ellipse is a circle centered on the true emitter position and that the contours of equal location estimation error are coaxial circles centered on the track intersection.

10. The method of claim 1, wherein the second pair of observation points is determined from approximate knowledge of the emitter'"'"'s geolocation and the positions of the first two observation points such that a single unique circle that passes through the second two observation points intersects nearly orthogonally at the emitter, the first circle passing through the first two observation points, comprising:

generating a first COP from bearing change measurements;

predicting the COP passing through the approximate emitter location, orthogonal to the first COP;

determining start and end bearing measurement positions on the predicted COP;

measuring the bearing change between these positions;

forming a second COP from these bearing change measurements; and

intersecting the first and second COP to obtain precision emitter location.

11. The method of claim 4, wherein the second pair of observation points is determined from approximate knowledge of the emitter'"'"'s geolocation and the positions of the first two observation points such that a single unique circle that passes through the second two observation points intersects nearly orthogonally at the emitter, the first circle passing through the first two observation points, comprising:

generating a first COP from bearing change measurements;

predicting the COP passing through the approximate emitter location, orthogonal to the first COP;

determining start and end bearing measurement positions on the predicted COP;

measuring the bearing change between these positions;

forming a second COP from these bearing change measurements; and

intersecting the first and second COP to obtain precision emitter location.

12. The method of claim 1, wherein the observation points lie on tracks flown in transition between tactical formations.

13. The method of claim 4, wherein the observation points lie on tracks flown in transition between tactical formations.

14. The method of claim 5, wherein the observation points lie on tracks flown in transition between tactical formations.

15. The method of claim 7, wherein the observation points lie on tracks flown in transition between tactical formations.

16. The method of claim 1, wherein the step of assigning likelihood values to the intersection points comprises:

assigning a likelihood of 1 to a further intersection from the observers.

17. The method of claim 4, wherein the step of assigning likelihood values to the intersection points comprises:

assigning a likelihood of 1 to a further intersection from the observers.

18. The method of claim 5, wherein the step of assigning likelihood values to the intersection points comprises:

assigning a likelihood of 1 to a further intersection from the observers.

19. The method of claim 7, wherein the step of assigning likelihood values to the intersection points comprises:

assigning a likelihood of 1 to the further intersection from the observers.

20. The method of claim 1, wherein the step of assigning likelihood values to the intersection points comprises:

determining a second observation track such that the circle through the emitter and first pair of measurement positions and the circle through the emitter and the second pair of measurement points intercept in a second location near one of the observers, the emitter being at the further intersection from the observers with a likelihood of 1.

21. The method of claim 5, wherein the step of assigning likelihood values to the intersection points comprises:

determining a second observation track such that the circle through the emitter and first pair of measurement positions and the circle through the emitter and the second pair of measurement points intercept in a second location near one of the observers, the emitter being at the further intersection from the observers with a likelihood of 1.

22. The method of claim 7, wherein the step of assigning likelihood values to the intersection points comprises:

determining a second observation track such that the circle through the emitter and first pair of measurement positions and the circle through the emitter and the second pair of measurement points intercept in a second location near one of the observers, the emitter being at the further intersection from the observers with a likelihood of 1.

23. The method of claim 10, wherein the step of assigning likelihood values to the intersection points comprises:

determining a second observation track such that the circle through the emitter and first pair of measurement positions and the circle through the emitter and the second pair of measurement points intercept in a second location near one of the observers, the emitter being at the further intersection from the observers with a likelihood of 1.

24. The method of claim 1, wherein a bearing change is determined by measuring interferometer phase change between the observation points.

25. The method of claim 4, wherein the bearing change is determined by measuring interferometer phase change between the observation points.

26. The method of claim 1, wherein an interferometer is an ambiguous uncalibrated long baseline interferometer (LBI) with the step of:

determining COP through the initial and final observation points creating not a single unique circle but a family of phase circles representing all possible resolutions of the ambiguous LBI phase difference, where the ambiguous phase difference is measured between the initial and final observation points and only one circle in the family passes through the emitter position.

27. The method of claim 1, further comprising:

making multiple LBI phase change measurements between the initial and final measurement positions for each observer;

utilizing these multiple phase change measurements to generate two families of COP through the two pairs of initial and final measurement positions, each circle in these families corresponding to a possible ambiguity integer resolving the ambiguous LBI phase change measurement made between the measurement positions;

measuring emitter signal TDOA between the two observers;

resolving the integer ambiguities by determining the TDOA intersection statistically closest to the circle of position intersection;

determining the intercept point closest to the emitter by determining the farthest intersection point from the observer for the two resulting unambiguous circles.

28. The method of claim 26, comprising making multiple LBI phase change measurements between the initial and final measurement positions at both observers, utilizing these multiple phase change measurements to approximately locate the emitter relative to a single observer, thus resolving the phase change module 2π

ambiguity for the phase change measurements made at each observer, associating the unique COPs with the resolved phase differences to get the unique emitter geolocation.