Method of locating a transmitter

• US 8,068,850 B1
• Filed: 03/04/2008
• Issued: 11/29/2011
• Est. Priority Date: 03/04/2008
• Status: Active Grant

• Current Assignee: National Security Agency
• Original Assignee: The United States of America As Represented By The Director National Security Agency
• Inventors: Kleinfelder, Walter J, Meister, James E
• Primary Examiner(s): Appiah; Charles
• Assistant Examiner(s): Peaches; Randy
• Application Number:
  US12/074,512
• Time in Patent Office: 1,365 Days
• Field of Search: 455/456.1, 455/456.6, 455/404.2, 455/432.1, 342/450, 342/457, 342/453
• US Class Current: 455/456.1
• CPC Class Codes: G01S 5/06 Position of source determined by co-ordinating a plurality of position lines defined by path-difference measurements G01S5/12 takes precedence

Abstract

A method of locating a transmitter is disclosed. A communications link is established between multiple receiving stations and a control station, and used with a calibration signal to calibrate out the time delay of the communications link. A radiated signal from an unknown transmitter is received at a receiving station and sent to the control station over the communications link. The time of arrival is recorded. Upon receiving a user input, the communication link switches from transmitting the received radiated signal to transmitting the calibration signal. The start of calibration signal is timed to begin at the same time across all of the receiving stations. The communication link time delay for each link is calculated by subtracting the start time from the received time, and the communication link time delay is subtracted from the received time recorded for the radiated signal. TDOA calculations are made to locate the transmitter.

Claims

1. A method of locating a transmitter, comprising the steps of:

a) receiving a calibration signal at a plurality of receiving stations;

b) establishing a communications link between a control station and each of the plurality of receiving stations, the communications link having a switchable input at the receiving station to transmit one of a received radiated signal and the received calibration signal, the switchable input switched in response to a user-activated signal;

c) storing a communications link delay for each receiving station at the control station;

d) receiving a radiated signal from a transmitter having an unknown location, the radiated signal received by at least two of the plurality of receiving stations;

e) transmitting a user-defined portion of the radiated signal from at least two of the plurality of receiving stations to the control station over the communications links;

f) recording the time of arrival of each radiated signals at the control station;

g) if a user-activated signal is received to switch the input of the communications links, switching the input of the communications links that transmitted the radiated signal in step (e) from the received radiated signal to the calibration signal, otherwise proceeding to step (k);

h) transmitting the calibration signal to the control station over the communications link, the transmission of the calibration signal at each of the receiving stations in step (g) timed to begin at the same time using a timing signal;

i) calculating the communications link delay for each receiving station by subtracting the start time from the time the calibration signal from each receiving station was received;

j) recording the calculated communications link delay for each receiving station at the control station;

k) for each recorded time of arrival of the radiated signal from the at least two receiving stations, subtracting the communications link delay for the respective receiving station from the recorded time of arrival of the radiated signal for the respective receiving station; and

l) locating the unknown transmitter by calculating the time difference of arrival of the radiated signal from the at least two receiving stations.

2. The method of claim 1, wherein the timing signal is obtained from a satellite available to the plurality of receiving stations and the control station.

3. The method of claim 2, further comprising the step of calculating multiple communications link delays for each receiving station and averaging the communications link delay, the average communications link delay stored at the control station.

4. The method of claim 3, wherein the communications link is one of an analog and digital communications link.

5. The method of claim 4, further comprising calculating a dispersion compensated communications link delay further comprising the steps of:

a) calculating the Fourier transform of the calibration signal received at the control station;

b) calculating the Fourier transform of the calibration signal designed to be sent by the receiving station;

c) determining the frequency response of the communications link by taking the quotient of the two calculated Fourier transforms; and

d) calculating the dispersion compensated delay times of the communications link using the frequency response of the communications link to adjust the communication link delay calculated by subtracting the start time from the time the calibration signal from each receiving station was received.

6. The method of claim 5, wherein the communications link is one of voice-over-IP and satellite.

7. The method of claim 5, wherein the communications link is a standard telephone line.

8. The method of claim 5, wherein the switchable input switched in response to a user-activated signal is switched in response to one of a manual command and an automated, periodic command.

9. The method of claim 1, further comprising the step of calculating multiple communications link delays for each receiving station and averaging the communications link delay, the average communications link delay stored at the control station.

10. The method of claim 1, wherein the communications link is one of an analog and digital communications link.

11. The method of claim 1, wherein the communications link is one of voice-over-IP and satellite.

12. The method of claim 1, wherein the communications link is a standard telephone line.

13. The method of claim 1, wherein the switchable input switched in response to a user-activated signal is switched in response to one of a manual command and an automated, periodic command.

14. The method of claim 1, further comprising calculating a dispersion compensated communications link delay further comprising the steps of:

a) calculating the Fourier transform of the calibration signal received at the control station;

b) calculating the Fourier transform of the calibration signal designed to be sent by the receiving station;

c) determining the frequency response of the communications link by taking the quotient of the two calculated Fourier transforms; and

d) calculating the dispersion compensated delay times of the communications link using the frequency response of the communications link to adjust the communication link delay calculated by subtracting the start time from the time the calibration signal from each receiving station was received.

15. A method of locating a transmitter, comprising the steps of:

a) receiving a calibration signal at a plurality of receiving stations;

b) establishing a communications link between a control station and each of the plurality of receiving stations, the communications link having a switchable input at the receiving station to transmit one of a received radiated signal and the calibration signal, the switchable input switched in response to a user-activated signal;

c) storing a communications link time difference of arrival for pairs of receiving stations at the control station;

d) receiving a radiated signal from a transmitter having an unknown location, the radiated signal received by at least two of the plurality of receiving stations;

e) transmitting a user-defined portion of the radiated signal from at least two of the plurality of receiving stations to the control station over the communications links;

f) calculating the time difference of arrival of the radiated signal from the at least two receiving stations;

g) if a user-activated signal is received to switch the input of the communications links, switching the input of the communications links that transmitted the radiated signal in step (e) from the received radiated signal to the calibration signal, otherwise proceeding to step (k);

h) transmitting the calibration signal to the control station over the communications link, the transmission of the calibration signal at each of the receiving stations in step (g) timed to begin at the same time using a timing signal;

i) calculating the communications link time difference of arrival for each pair of receiving stations that transmitted the calibration signal in step (h);

j) recording the calculated communications link time difference of arrival for each pair of receiving stations at the control station; and

k) locating the unknown transmitter by subtracting the communications link time difference of arrival for the pair of receiving stations from the time difference of arrival of the radiated signal from the at least two receiving stations.

16. The method of claim 15, wherein the timing signal is obtained from a satellite available to the plurality of receiving stations and the control station.

17. The method of claim 16, further comprising the step of calculating multiple communications link time difference of arrivals for pairs of receiving station and averaging the communications link time difference of arrivals, the average communications link time difference of arrival stored at the control station.

18. The method of claim 17, further comprising calculating a dispersion compensated communications link time difference of arrival for pairs of receiving stations, comprising the steps of:

a) for each of the pair of receiving stations sending a calibration signal, calculating the Fourier transform of the calibration signal received at the control station;

b) calculating the Fourier transform of the calibration signal designed to be sent by each pair of receiving stations;

c) determining the frequency response of the pair of communications links by taking the quotient of the calculated received calibration signal and designed calibration signal Fourier transforms; and

d) calculating the dispersion compensated communications link time difference of arrival for each pair of receiving stations that transmitted the calibration signal using the frequency response of the pair of communications links to adjust the calculated communications link time difference of arrival for each pair of receiving stations.

19. The method of claim 18, wherein the communications link is one of an analog and digital communications link.

20. The method of claim 19, wherein the communications link is one of voice-over-IP and satellite.

21. The method of claim 20, wherein the communications link is a standard telephone line.

22. The method of claim 19, wherein the switchable input switched in response to a user-activated signal is switched in response to one of a manual command and an automated, periodic command.

23. The method of claim 15, further comprising the step of calculating multiple communications link time difference of arrivals for pairs of receiving station and averaging the communications link time difference of arrivals, the average communications link time difference of arrival stored at the control station.

24. The method of claim 15, wherein the communications link is one of an analog and digital communications link.

25. The method of claim 15, wherein the communications link is one of voice-over-IP and satellite.

26. The method of claim 15, wherein the communications link is a standard telephone line.

27. The method of claim 15, wherein the switchable input switched in response to a user-activated signal is switched in response to one of a manual command and an automated, periodic command.

28. The method of claim 15, further comprising calculating a dispersion compensated communications link time difference of arrival for pairs of receiving stations, comprising the steps of:

a) for each of the pair of receiving stations sending a calibration signal, calculating the Fourier transform of the calibration signal received at the control station;

b) calculating the Fourier transform of the calibration signal designed to be sent by each pair of receiving stations;

c) determining the frequency response of the pair of communications links by taking the quotient of the calculated received calibration signal and designed calibration signal Fourier transforms; and

d) calculating the dispersion compensated communications link time difference of arrival for each pair of receiving stations that transmitted the calibration signal using the frequency response of the pair of communications links to adjust the calculated communications link time difference of arrival for each pair of receiving stations.