System and method for disambiguating shooter locations

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

measuring at least an initial portion of the shockwave-only signals with a plurality of spaced acoustic sensors forming an antenna;

determining from the measured portion of the shockwave-only signals Time-Difference-Of-Arrival (TDOA) values for sensor pairs with an accuracy sufficient to determine a gradient of curvature of the shockwave wavefront across the antenna;

determining the gradient of curvature of the shockwave wavefront; and

determining from the determined gradient of curvature the disambiguated projectile trajectory.

2. The method of claim 1 , wherein the accuracy sufficient to determine the gradient of curvature depends on a size of the antenna.

3. The method of claim 1 , wherein the accuracy sufficient to determine the gradient of curvature depends on a spatial distribution of sensor locations.

4. The method of claim 1 , wherein measuring the initial portion of the shockwave-only signals includes measuring acoustic energy of a received signal and excluding the received signal from being a shockwave-only signal if the acoustic energy of the received has less than a predetermined threshold value over a predetermined frequency band.

5. The method of claim 4 , wherein the predetermined frequency band comprises frequencies between approximately 700 Hz and 10 kHz.

6. The method of claim 1 , wherein measuring the initial portion of the shockwave-only signals includes measuring a time interval where a received signal has a positive value and excluding the received signal from being a shockwave-only signal if the time interval is less than a minimum time or greater than a maximum time.

7. The method of claim 6 , wherein the minimum time is approximately 70 μs and the maximum time is approximately 300 μs.

8. The method of claim 1 , wherein determining the Time-Differences-Of-Arrival (TDOA) values for sensor pairs comprises

designating a sensor of the plurality of sensors that first encounters the shockwave wavefront 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 counters for each of the other sensors; and

stopping the counters in each of the other sensors when the respective other sensors encounter the shockwave wavefront;

wherein a count between start and stop of the counters represents the TDOA between the other sensors and the reference sensor.

9. The method of claim 1 , wherein determining the Time-Differences-Of-Arrival (TDOA) values for sensor pairs comprises

measuring at least the initial portion of the shockwave-only signals at the acoustic sensors with a first clock rate;

correlating the measured portions between the acoustic sensors with a second clock rate greater than the first clock rate; and

determining a Time-Difference-Of-Arrival (TDOA) from a time shift producing a maximum value of the correlation.

10. The method of claim 9 , and further computing from the correlated portions a residual for each sensor and selecting the smallest residual as producing optimum TDOA values.

11. A method for disambiguating a trajectory of a supersonic projectile from shockwave-only signals with an antenna having a plurality of acoustic sensors, comprising:

detecting shockwave-only signals at the plurality of acoustic sensors;

determining from a correlation between the shockwave-only signals detected at the plurality of sensors Time-Difference-Of-Arrival (TDOA) values for sensor pairs;

for an expected closest distance between a projectile trajectory and the antenna, determining if a precision of the Time-Difference-Of-Arrival (TDOA) values is adequate for determining a gradient of curvature of a shockwave wavefront across the antenna;

determining the gradient of curvature of the shockwave wavefront; and

determining the disambiguated projectile trajectory from the gradient of curvature.

12. The method of claim 11 , wherein the correlation is performed with a timing resolution which is greater than a timing resolution used to measure the portion of the shockwave-only signals.

13. The method of claim 11 , wherein detecting shockwave-only signals comprises measuring at least an initial portion of a received shockwave-only signals, measuring acoustic energy of the received signal, and excluding the received signal from being a shockwave-only signal if the acoustic energy of the received has less than a predetermined threshold value over a predetermined frequency band.

14. The method of claim 11 , wherein detecting shockwave-only signals comprises measuring at least an initial portion of a received shockwave-only signals, measuring a time interval where a received signal has a positive value, and excluding the received signal from being a shockwave-only signal if the time interval is less than a minimum time or greater than a maximum time.

15. The method of claim 11 , wherein detecting shockwave-only signals comprises eliminating detected signal not originating from a shockwave.

16. The method of claim 15 , wherein a detected signal is deemed to be a shockwave-only signal if it satisfies all of the following criteria:

(a) the detected signal has a minimum and a maximum peak signal amplitude, with at least the minimum peak signal amplitude exceeding a predetermined threshold value;

(b) the detected signal has a sharp transient from zero to the maximum peak signal amplitude, without being preceded by another signal with a value that is not significantly smaller than the peak signal value;

(c) a time between the minimum and maximum peak signal amplitudes of the detected signal is greater than a predetermined time interval;

(d) a magnitude of the minimum peak signal amplitude is substantially identical to the maximum peak signal amplitude;

(e) the detected signal has a sharp transient from the minimum peak signal amplitude to zero; and

(f) a time between the maximum peak signal amplitude and a zero-crossing between the maximum peak signal amplitude and the minimum peak signal amplitude, and between the zero-crossing and the minimum peak signal amplitude are substantially identical.

17. A system for disambiguating a trajectory of a supersonic projectile from shockwave-only signals, comprising:

an antenna having a plurality of acoustic sensors; and

a processor configured to

receive signals at the acoustic sensors and determine if the received signals are shockwave-only signals;

perform a correlation between the shockwave-only signals;

determine from the correlation Time-Difference-Of-Arrival (TDOA) values for sensor pairs;

determine for an expected closest distance between a projectile trajectory and the antenna, if a precision of the Time-Difference-Of-Arrival (TDOA) values is adequate for determining a gradient of curvature of a shockwave wavefront across the antenna;

determine the gradient of curvature of the shockwave wavefront; and

determine the disambiguated projectile trajectory from the gradient of curvature.

18. The system of claim 17 , wherein the antenna is mounted on a vehicle.

19. The system of claim 17 , wherein the antenna comprises a frame and elastomeric couplings attached to the frame, with the elastomeric couplings holding the acoustic sensors.

20. The system of claim 19 , wherein the acoustic sensors having a mechanical resonance frequency of less than approximately 1 Hz.