System and method for disambiguating shooter locations

• US 20060044943A1
• Filed: 08/24/2004
• Published: 03/02/2006
• Est. Priority Date: 08/24/2004
• Status: Active Grant

• Current Assignee: Raytheon BBN Technlogies Corp. (Rtx Corporation)
• Original Assignee: BBNT Solutions LLC (Rtx Corporation)
• Inventors: Brinn, Marshall Seth, Milligan, Stephen Douglas, Barger, James Edwin, MULLEN, RICHARD J
• Granted Patent: US 7,126,877 B2
• Time in Patent Office: Days
• US Class Current: 367/127
• CPC Class Codes: F41J 5/06 Acoustic hit-indicating systems, i.e. detecting of shock waves F41J5/056 takes precedence
• CPC Class Codes: G01S 3/808 using transducers spaced apart and measuring phase or time difference between signals therefrom, i.e. path-difference systems
• CPC Class Codes: G01S 5/22 Position of source determined by co-ordinating a plurality of position lines defined by path-difference measurements G01S5/28 takes precedence
• CPC Class Codes: Y10S 367/906 Airborne shock-wave detection

Abstract

Systems and methods for locating the shooter of supersonic projectiles based on shockwave-only measurements are described. Muzzle blast signals are neither sought nor required. The system uses at least five, preferably seven, acoustic sensors that are spaced apart at least 1 meter. The sensor signals are acquired with a time resolution in the order of microseconds and processed to find and disambiguate the shockwave arrival angle unit vector. Two different Time-Difference-Of-Arrival (TDOA) measurement techniques are described, with one technique using counters in each signal channel and the other technique using cross-correlation between signal channels. A genetic algorithm can be used to efficiently disambiguate the results.

Claims

1. A method for disambiguating a projectile trajectory from shockwave-only signals, comprising:

measuring at least an initial portion of the shockwave-only signals at five or more spaced acoustic sensors forming an antenna;

estimating a timing error distribution for the acoustic sensors;

determining from the measured initial portion of the shockwave-only signals Time-Differences-Of-Arrival (TDOA) for sensor pairs with a time resolution that is greater than the estimated timing error distribution; and

selecting the disambiguated projectile trajectory based a defined confidence level for disambiguation and on a value of a residual for the TDOA of the acoustic sensors.

2. The method of claim 1, wherein the timing error distribution is related to gain variations, sampling variations and sensor location variations of the antenna sensors.

3. The method of claim 1, wherein the confidence level for disambiguation depends on a size of the antenna.

4. The method of claim 1, wherein the disambiguated projectile trajectory is selected based on a ratio of the residuals for two ambiguous solutions.

5. The method of claim 1, wherein the Time-Differences-Of-Arrival (TDOA) for sensor pairs are determined by

designating a sensor that first encounters the shockwave as a reference sensor;

setting a first latch of a timing circuit when an amplitude of the initial portion of the shockwave-only signal at the reference sensor crosses a threshold value, said first latch activating start counters for each of the other sensors, with the counters in each of the other sensors running until the corresponding sensor encounters the shockwave;

when one of the other sensors encounters the initial portion of the shockwave-only signal, setting a second latch for the corresponding other sensor that stops the start counters for that other sensors; and

recording the TDOA values for the other sensors with respect to the reference sensor.

6. The method of claim 1, wherein a projectile trajectory is eliminated as being false if acoustic energy of the measured shockwave waveform has less than a predetermined threshold value over a predetermined frequency band.

7. The method of claim 8, wherein the predetermined frequency band comprises frequencies between approximately 700 Hz and 10 kHz.

8. The method of claim 1, wherein a projectile trajectory is eliminated as being false if a time interval where a measured shockwave waveform has a positive value is less than a minimum time or greater than a maximum time.

9. The method of claim 8, wherein the minimum time is approximately 70 μ

s and the maximum time is approximately 300 μ

s.

10. The method of claim 1, wherein the disambiguated projectile trajectory has a smaller value of the residual than any other computed projectile trajectory.

11. A method for disambiguating a projectile trajectory from shockwave-only signals, comprising:

measuring at least an initial portion of the shockwave-only signals at five or more spaced acoustic sensors forming an antenna;

determining from the measured initial portion of the shockwave-only signals Time-Differences-Of-Arrival (TDOA) for sensor pairs;

applying a genetic algorithm to an initial chromosome, that comprises projectile trajectory assumptions, for a predefined number of generations;

computing residuals for solutions obtained with the chromosomes from the generic algorithm;

performing a gradient search on a solution having a smallest residual and on its ambiguous alternate solution; and

if a ratio of the solution having the smallest residual and its ambiguous alternate solution is greater than a predefined value, designating the solution having the smallest computed residual as the disambiguated projectile trajectory.

12. The method of claim 11, wherein the predefined value of the ratio is approximately 2.

13. The method of claim 11, wherein the chromosome comprises a 4-tuple having elements selected from the group consisting of shooter azimuth, shooter elevation, missed azimuth and missed elevation.

14. The method of claim 11, wherein applying a genetic algorithm comprises applying crossover and mutation operators.

15. The method of claim 14, wherein the crossover operator comprises crossover azimuth and crossover missed azimuth operators.

16. The method of claim 15, wherein the crossover azimuth operator exchanges shooter and trajectory azimuth between two chromosomes.

17. The method of claim 15, wherein the crossover missed azimuth operator exchanges missed azimuth and missed elevation between two chromosomes.

18. The method of claim 14, wherein the mutation operator comprises field-mutation, incremental mutation, flip mutation, and no mutation.

19. The method of claim 18, wherein the field-mutation operator replaces a chromosome field with a randomly selected value.

20. The method of claim 18, wherein the incremental mutation operator induces small mutations in all fields of a chromosome.

21. The method of claim 20, wherein the small mutation operator comprises changes in all fields of a chromosome of less than ±

20 in shooter azimuth and elevation, and less than ±

0.50 for missed azimuth and elevation.

22. The method of claim 20, wherein the flip-mutation operator comprises changing the solution having a smallest residual into its ambiguous alternate solution.

23. The method of claim 20, wherein the flip-mutation operator comprises leaving a chromosome unaltered.

24. The method of claim 11, wherein determining a Time-Difference-Of-Arrival (TDOA) for a sensor pair comprises performing a cross-correlation between shockwave signals detected at the sensor pairs and selecting the TDOA that produces the smallest computed residual.