System for locating a source of bursty transmissions cross reference to related applications

• US 5,608,410 A
• Filed: 03/11/1994
• Issued: 03/04/1997
• Est. Priority Date: 05/07/1993
• Status: Expired due to Term

• Current Assignee: TruePosition Incorporated
• Original Assignee: Associated RT Incorporated
• Inventors: Stilp, Louis A., Knight, Curtis A., Webber, John C.
• Primary Examiner(s): Issing, Gregory C.
• Application Number:
  US08/212,552
• Time in Patent Office: 1,089 Days
• Field of Search: 342/387, 342/457
• US Class Current: 342/387
• CPC Class Codes: G01S 1/026 of associated receivers
• CPC Class Codes: G01S 3/46 using antennas spaced apart and measuring phase or time difference between signals therefrom, i.e. path-difference systems
• 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
• CPC Class Codes: H04W 64/00 Locating users or terminals or network equipment for network management purposes, e.g. mobility management

Abstract

A system for determining the location of a mobile transmitter is disclosed. The mobile transmitter receives a message signal and responds to the message signal by transmitting a responsive signal, wherein the responsive signal is indicative of the identity of the mobile transmitter. One preferred embodiment comprises at least three antenna site systems 10, a central site system 12, a database processor for use in connection with various applications, and a map system 16. The system may be employed to locate cellular telephones and/or like devices emitting bursty transmissions, e.g., over messaging or control channels.

Claims

1. A process for determining the location of a "standard" mobile transmitter not specifically designed or altered to provide signals facilitating the location of the transmitter, said standard mobile transmitter being operative to receive a message signal and to respond to said message signal by transmitting a bursty responsive transmission indicative of the identity of the standard mobile transmitter, comprising the steps of:

(a) receiving a bursty responsive transmission from said standard mobile transmitter at first, second, and third ground-based antenna sites;

(b) digitalizing the received bursty responsive transmission at each of said first, second, and third antenna sites;

(c) determining first, second, and third time differences of arrival, said first time difference of arrival representing a difference between the time of arrival of said responsive transmission at said first and second antenna sites, said second time difference of arrival representing a difference between the time of arrival of said responsive transmission at said first and third antenna sites, and said third time difference of arrival representing a difference between the time of arrival of said responsive transmission at said second and third antenna sites; and

(d) estimating the location of said standard mobile transmitter on the basis of at least said first, second, and third time differences of arrival;

wherein the step of estimating the position of the mobile transmitter comprises making a coarse estimate by creating a grid of possible latitude and longitude positions for each antenna site pair;

calculating, for pairs of antenna sites, theoretical time difference of arrival values from points on the grid;

determining a function of a sum of squares of the measured time differences of arrival minus the computed time differences of arrival; and

coarsely estimating the position as the point at which the function of the sum of squares is a minimum.

2. A process as recited in claim 1, wherein the coarse estimation step further includes deriving quality factors representing the quality of the signal received at the antenna sites of said pairs, and the determination of the function of the sum of squares includes weighting the squares of the measured time differences of arrival minus the computed time differences of arrival by the quality factor.

3. A process as recited in claim 1, further comprising providing a confidence interval along with the location estimate, wherein said confidence interval defines an area around the estimated location and there is a probability the mobile transmitter is located within said area.

4. A process for determining the location of a "standard" mobile transmitter not specifically designed or altered to provide signals facilitating the location of the transmitter, said standard mobile transmitter being operative to receive a message signal and to respond to said message signal by transmitting a bursty responsive transmission indicative of the identity of the standard mobile transmitter, comprising the steps of:

(a) receiving a bursty responsive transmission from said standard mobile transmitter at first, second, and third ground-based antenna sites;

(b) digitalizing the received bursty responsive transmission at each of said first, second, and third antenna sites;

(c) determining first, second, and third time differences of arrival, said first time difference of arrival representing a difference between the time of arrival of said responsive transmission at said first and second antenna sites, said second time difference of arrival representing a difference between the time of arrival of said responsive transmission at said first and third antenna sites, and said third time difference of arrival representing a difference between the time of arrival of said responsive transmission at said second and third antenna sites;

(d) estimating the location of said standard mobile transmitter on the basis of at least said first, second, and third time differences of arrival;

(e) determining first, second, and third frequency differences of arrival, said first frequency difference of arrival representing a difference between the frequency of said responsive transmission as received at said first and second antenna sites, said second frequency difference of arrival representing a difference between the frequency of said responsive transmission as received at said first and third antenna sites, and said third frequency difference of arrival representing a difference between the frequency of said responsive transmission as received at said second and third antenna sites; and

(f) estimating the velocity of said standard mobile transmitter on the basis of at least said first, second, and third frequency differences of arrival;

wherein the step of estimating the velocity of the mobile transmitter comprises making a coarse estimate by creating a grid or table of possible velocities;

calculating, for pairs of antenna sites, theoretical frequency difference of arrival values corresponding to said possible velocities;

determining a function of a sum of squares of the measured frequency differences of arrival minus the computed frequency differences of arrival; and

coarsely estimating the velocity as the velocity at which the function of the sum of squares is a minimum.

5. A process for determining the location of a "standard" mobile transmitter not specifically designed or altered to provide signals facilitating the location of the transmitter, said standard mobile transmitter being operative to receive a message signal and to respond to said message signal by transmitting a bursty responsive transmission indicative of the identity of the standard mobile transmitter, comprising the steps of:

(a) receiving a bursty responsive transmission from said standard mobile transmitter at first, second, and third ground-based antenna sites;

(b) digitalizing the received bursty responsive transmission at each of said first, second, and third antenna sites;

(c) determining first, second, and third time differences of arrival, said first time difference of arrival representing a difference between the time of arrival of said responsive transmission at said first and second antenna sites, said second time difference of arrival representing a difference between the time of arrival of said responsive transmission at said first and third antenna sites, and said third time difference of arrival representing a difference between the time of arrival of said responsive transmission at said second and third antenna sites; and

(d) estimating the location of said standard mobile transmitter on the basis of at least said first, second, and third time differences of arrival;

wherein the reception and processing of the responsive signal at each antenna site comprises the stems of receiving the responsive signal with an antenna array at each antenna site and providing a high quality signal for further processing;

wherein the high quality baseband signals are provided at the respective antenna sites by coherently combining multiple baseband signals so as to reduce the effects of multipath; and

further comprising the steps of;

(1) generating a substantially unimpaired representation of the baseband signal by remodulating a demodulated signal;

(2) determining which antenna in the array likely received the transmission over the shortest and most direct path;

(3) performing a correlation between the generated, unimpaired representation of the signal and each of the signals received through each of the antennas, and generating a correlation output function for each signal;

(4) determining the relative amount of multipath impairment that may have occurred at each antenna by measuring the width of each correlation output function at a predefined point on the correlation output function;

(5) coherently combining the signals that appear to have similar multipath impairments to increase the quality of the signal representation; and

(6) estimating a quality factor that is indicative of the relative amount of impairment that will remain in the signal after processing.

6. A process for determining the location of a "standard" mobile transmitter not specifically designed or altered to provide signals facilitating the location of the transmitter, said standard mobile transmitter being operative to receive a message signal and to respond to said message signal by transmitting a bursty responsive transmission indicative of the identity of the standard mobile transmitter, comprising the steps of:

(a) receiving a bursty responsive transmission from said standard mobile transmitter at first, second, and third ground-based antenna sites;

(b) digitalizing the received bursty responsive transmission at each said first, second, and third antenna sites;

(c) determining first, second, and third time differences of arrival, said first time difference of arrival representing a difference between the time of arrival of said responsive transmission at said first and second antenna sites, said second time difference of arrival representing a difference between the time of arrival of said responsive transmission at said first and third antenna sites, and said third time difference of arrival representing a difference between the time of arrival of said responsive transmission at said second and third antenna sites; and

(d) estimating the location of said standard mobile transmitter on the basis of at least said first, second, and third time differences of arrival;

wherein the reception and processing of the responsive signal at each antenna site comprises the steps of receiving the responsive signal with an antenna array at each antenna site; and

providing a high quality signal for further processing; and

wherein the step of providing a high quality signal comprises sampling a signal from each antenna in said array and formatting the samples into frames of digital data; and

providing, on the basis of the frames of data, a high quality digital baseband signal for further processing; and

further comprising extracting selected signal characteristics from the high quality baseband signal;

compressing the high quality baseband signal so as to retain the selected characteristics; and

forwarding the compressed signal to a prescribed location for further processing.

7. A process as recited in claim 6, wherein the selected characteristics include timing, power, frequency, phase, and a factor representing the quality of the baseband signal.

8. A system for determining the location of a "standard" mobile transmitter not specifically designed or altered to provide signals facilitating the location of the transmitter, said standard mobile transmitter being operative to receive a message signal and to respond to said message signal by transmitting a bursty responsive transmission indicative of the identity of the standard mobile transmitter, comprising:

(a) means for receiving a bursty responsive transmission from said standard mobile transmitter at first, second, and third antenna sites;

(b) means for digitalizing the received bursty responsive transmission at each of said first, second, and third ground-based antenna sites;

(c) means for determining first, second, and third time differences of arrival, said first time difference of arrival representing a difference between the time of arrival of said responsive transmission at said first and second antenna sites, said second time difference of arrival representing a difference between the time of arrival of said responsive transmission at said first and third antenna sites, and said third time difference of arrival representing a difference between the time of arrival of said responsive transmission at said second and third antenna sites; and

(d) means for estimating the location of said standard mobile transmitter on the basis of at least said first, second, and third time differences of arrival;

wherein the means for estimating the position of the mobile transmitter comprises means for making a coarse estimate by creating a grid of possible latitude and longitude positions for each antenna site pair;

calculating, for pairs of antenna sites, theoretical time difference of arrival values from points on the grid;

determining a function of a sum of squares of the measured time differences of arrival minus the computed time differences of arrival; and

coarsely estimating the position as the point at which the function of the sum of squares is a minimum.

9. A system as recited in claim 8, wherein the means for making a coarse estimation further includes means for deriving quality factors representing the quality of the signal received at the antenna sites of said pairs, and weighting the squares of the measured time differences of arrival minus the computed time differences of arrival by the quality factor.

10. A system as recited in claim 8, further comprising means for providing a confidence interval along with the location estimate, wherein said confidence interval defines an area around the estimated location and there is a probability the mobile transmitter is located within said area.

11. A system for determining the location of a "standard" mobile transmitter not specifically designed or altered to provide signals facilitating the location of the transmitter, said standard mobile transmitter being operative to receive a message signal and to respond to said message signal by transmitting a bursty responsive transmission indicative of the identity of the standard mobile transmitter, comprising:

(a) means for receiving a bursty responsive transmission from said standard mobile transmitter at first, second, and third antenna sites;

(b) means for digitalizing the received bursty responsive transmission at each of said first, second, and third ground-based antenna sites;

(c) means for determining first, second, and third time differences of arrival, said first time difference of arrival representing a difference between the time of arrival of said responsive transmission at said first and second antenna sites, said second time difference of arrival representing a difference between the time of arrival of said responsive transmission at said first and third antenna sites, and said third time difference of arrival representing a difference between the time of arrival of said responsive transmission at said second and third antenna sites;

(d) means for estimating the location of said standard mobile transmitter on the basis of at least said first, second, and third time differences of arrival;

(e) means for determining first, second, and third frequency differences of arrival, said first frequency difference of arrival representing a difference between the frequency of said responsive transmission as received at said first and second antenna sites, said second frequency difference of arrival representing a difference between the frequency of said responsive transmission as received at said first and third antenna sites, and said third frequency difference of arrival representing a difference between the frequency of said responsive transmission as received at said second and third antenna sites; and

(f) means for estimating the velocity of said standard mobile transmitter on the basis of at least said first, second, and third frequency differences of arrival;

wherein the means for estimating the velocity of the mobile transmitter comprises means for making a coarse estimate by creating a grid of possible velocities;

calculating, for pairs of antenna sites, theoretical frequency difference of arrival values from points on the grid;

determining a function of a sum of squares of the measured frequency differences of arrival minus the computed frequency differences of arrival; and

coarsely estimating the velocity as the velocity at which the function of the sum of squares is a minimum.

12. A system for determining the location of a "standard" mobile transmitter not specifically designed or altered to provide signals facilitating the location of the transmitter, said standard mobile transmitter being operative to receive a message signal and to respond to said message signal by transmitting a bursty responsive transmission indicative of the identity of the standard mobile transmitter, comprising:

(a) means for receiving a bursty responsive transmission from said standard mobile transmitter at first, second, and third antenna sites;

(b) means for digitalizing the received bursty responsive transmission at each of said first, second, and third ground-based antenna sites;

(c) means for determining first, second, and third time differences of arrival, said first time difference of arrival representing a difference between the time of arrival of said responsive transmission at said first and second antenna sites, said second time difference of arrival representing a difference between the time of arrival of said responsive transmission at said first and third antenna sites, and said third time difference of arrival representing a difference between the time of arrival of said responsive transmission at said second and third antenna sites; and

(d) means for estimating the location of said standard mobile transmitter on the basis of at least said first, second, and third time differences of arrival;

comprising an antenna array at each antenna site and means for providing a high quality signal for further processing at each antenna site;

wherein the means for providing a high quality signal comprises means for sampling a signal from each antenna in said array and formatting the samples into frames of digital data, and providing, on the basis of the frames of data, a high quality digital baseband signal for further processing; and

wherein the high quality baseband signals are provided at the respective antenna sites by coherently combining multiple baseband signals so as to reduce the effects of multipath; and

further comprising means for;

(1) generating a substantially unimpaired representation of the baseband signal by remodulating a demodulated signal;

(2) determining which antenna in the array likely received the transmission over the shortest and most direct path;

(3) performing a correlation between the generated, unimpaired representation of the signal and each of the signals received through each of the antennas, and generating a correlation output function for each signal;

(4) determining the relative amount of multipath impairment that may have occurred at each antenna by measuring the width of each correlation output function at a predefined point on the correlation output function;

(5) coherently combining the signals that appear to have similar multipath impairments to increase the quality of the signal representation; and

(6) estimating a quality factor that is indicative of the relative amount of impairment that will remain in the signal after processing.

13. A system for determining the location of a "standard" mobile transmitter not specifically designed or altered to provide signals facilitating the location of the transmitter, said standard mobile transmitter being operative to receive a message signal and to respond to said message signal by transmitting a bursty responsive transmission indicative of the identity of the standard mobile transmitter, comprising:

(a) means for receiving a bursty responsive transmission from said standard mobile transmitter at first, second, and third antenna sites;

(b) means for digitalizing the received bursty responsive transmission at each of said first, second, and third ground-based antenna sites;

(c) means for determining first, second, and third time differences of arrival, said first time difference of arrival representing a difference between the time of arrival of said responsive transmission at said first and second antenna sites, said second time difference of arrival representing a difference between the time of arrival of said responsive transmission at said first and third antenna sites, and said third time difference of arrival representing a difference between the time of arrival of said responsive transmission at said second and third antenna sites; and

(d) means for estimating the location of said standard mobile transmitter on the basis of at least said first, second, and third time differences of arrival;

comprising an antenna array at each antenna site and means for providing a high quality signal for further processing at each antenna site;

wherein the means for providing a high quality signal comprises means for sampling a signal from each antenna in said array and formatting the samples into frames of digital data, and providing, on the basis of the frames of data, a high quality digital baseband signal for further processing; and

further comprising means for extracting selected signal characteristics from the high quality baseband signal;

compressing the high quality baseband signal so as to retain the selected characteristics; and

forwarding the compressed signal to a prescribed location for further processing.

14. A system as recited in claim 13, wherein the selected characteristics include timing, power, frequency, phase, and a factor representing the quality of the baseband signal.

15. A system for determining the location of a "standard" mobile transmitter not specifically designed or altered to provide signals facilitating the location of the transmitter, said standard mobile transmitter being operative to receive a message signal and to respond to said message signal by transmitting a bursty responsive transmission indicative of the identity of the standard mobile transmitter, comprising:

(a) at least three ground-based antenna site systems, each antenna site system comprising;

an antenna;

a sampling subsystem operatively coupled to said antenna for sampling said bursty responsive signal at a prescribed sampling frequency and formatting the sampled signal into frames of digital data, each frame comprising a prescribed number of data bits and time stamp bits, said time stamp bits representing the time at which said bursty responsive transmission was received;

a baseband convertor operatively coupled to said sampling subsystem for providing baseband signals derived from said frames of data; and

a timing subsystem operatively coupled to said sampling subsystem for providing timing signals; and

(b) a central site system operatively coupled to said antenna site systems, comprising means for processing said frames of data to generate data identifying said mobile transmitter and the differences in times of arrival of said responsive signal among said antenna site systems; and

means for determining, on the basis of said times of arrival differences, the location of the mobile transmitter;

and further comprising location estimation means for;

(1) creating a grid of theoretical points covering a prescribed geographic area, said theoretical points being spaced at prescribed increments of latitude and longitude;

(2) calculating theoretical values of time delay for a plurality of pairs of antenna sites;

(3) calculating a least squares difference value based on the theoretical time delays and measured time delays for a plurality of pairs of antenna sites;

(4) searching the grid of theoretical points and determining the best theoretical latitude and longitude for which the value of the least squares difference is minimized; and

(5) starting at the best theoretical latitude and longitude, performing another linearized-weighted-least-squares iteration to resolve the actual latitude and longitude to within a prescribed number of degrees or fraction of a degree.

16. A system as recited in claim 15, wherein said calculating step (2) comprises accounting for any known site biases caused by mechanical, electrical, or environmental factors, said site biases determined by periodically calculating the positions of reference transmitters at known locations.

17. A system as recited in claim 15, wherein said least squares difference is given by:

space="preserve" listing-type="equation">LSD=[Q.sub.12 (Delay.sub.-- T.sub.12 Delay.sub.-- O.sub.12).sup.2 +Q.sub.13 (Delay.sub.-- T.sub.13 -Delay.sub.-- O.sub.13).sup.2 +. . . Q.sub.xy (Delay.sub.-- T.sub.xy -Delay.sub.-- O.sub.xy).sup.2 ]

where, Delay_-- T_xy represents the theoretical delay between antenna sites x and y, x and y being indices representative of antenna sites;

Delay_-- O_xy represents the observed delay between antenna sites x and y;

Q_xy represents a quality factor for the delay measurement between antenna sites x and y, said quality factor being an estimated measure of the degree to which multipath or other anomalies may have affected a particular delay measurement.

18. A system for determining the location of a "standard" mobile transmitter not specifically designed or altered to provide signals facilitating the location of the transmitter, said standard mobile transmitter being operative to receive a message signal and to respond to said message signal by transmitting a bursty responsive transmission indicative of the identity of the standard mobile transmitter, comprising:

(a) at least three ground-based antenna site systems, each antenna site system comprising;

an antenna;

a sampling subsystem operatively coupled to said antenna for sampling said bursty responsive signal at a prescribed sampling frequency and formatting the sampled signal into frames of digital data, each frame comprising a prescribed number of data bits and time stamp bits, said time stamp bits representing the time at which said bursty responsive transmission was received;

a baseband convertor operatively coupled to said sampling subsystem for providing baseband signals derived from said frames of data; and

a timing subsystem operatively coupled to, said sampling subsystem for providing timing signals; and

(b) a central site system operatively coupled to said antenna site systems, comprising means for processing said frames of data to generate data identifying said mobile transmitter and the differences in times of arrival of said responsive signal among said antenna site systems; and

means for determining, on the basis of said times of arrival differences, the location of the mobile transmitter; and

further comprising velocity estimation means for;

(1) creating a grid of theoretical points covering a prescribed range of velocities, said theoretical points being spaced at prescribed increments;

(2) calculating theoretical values of frequency difference for a plurality of pairs of antenna sites;

(3) calculating a least squares difference value based on the theoretical frequency differences and measured frequency differences for a plurality of pairs of antenna sites;

(4) searching the entire grid of theoretical points and determining the best theoretical velocity for which the value of least squares difference is minimized; and

(5) starting at the best theoretical velocity, performing another linearized-weighted-least-squares iteration to resolve the actual velocity to within a prescribed tolerance.

19. A method for determining the location of a mobile transmitter operative to receive a message signal and to respond to said message signal by transmitting a responsive signal over a predefined messaging channel, said responsive signal being indicative of the identity of the mobile transmitter, comprising the steps of:

(a) receiving said signal at least three separate ground-based antenna sites;

(b) processing said signal at each antenna site to produce frames of data, each frame comprising a prescribed number of data bits and time stamp bits, said time stamp bits representing the time at which said frames were produced at each antenna site;

(c) processing said frames of data to identify said responsive signal and the differences in times of arrival of said responsive signal among said antenna sites; and

(d) determining, on the basis of said times of arrival differences, the location of said mobile transmitter;

and further comprising estimating the location of said mobile transmitter by performing the following steps;

(1) creating a grid of theoretical points covering a prescribed geographic area, said theoretical points being spaced at prescribed increments of latitude and longitude;

(2) calculating theoretical values of time delay for a plurality of pairs of antenna sites;

(3) calculating a least squares difference (LSD) value based on the theoretical time delays and measured time delays for a plurality of pairs of antenna sites;

(4) searching the entire grid of theoretical points and determining the best theoretical latitude and longitude for which the value of LSD is minimized; and

(5) starting at the best theoretical latitude and longitude, performing another linearized-weighted-least-squares iteration to resolve the actual latitude and longitude to within a prescribed number of degrees or fraction of a degree.

20. A method as recited in claim 19, wherein said calculating step (2) comprises accounting for any known site biases caused by mechanical, electrical, or environmental factors, said site biases determined by periodically calculating the positions of reference transmitters at known locations.

21. A method as recited in claim 19, wherein said least squares difference is given by:

space="preserve" listing-type="equation">LSD=Σ

Q.sub.xy (Delay.sub.-- T.sub.xy -Delay.sub.-- O.sub.xy).sup.2

where, Delay_-- T_xy represents the theoretical delay between antenna sites x and y, x and y being indices representative of antenna sites;

Delay_-- O_xy represents the observed delay between antenna sites x and y;

Q_xy represents a quality factor for the delay measurement between antenna sites x and y, said quality factor being an estimated measure of the degree to which multipath or other anomalies may have affected a particular delay measurement.

22. A method for determining the location of a mobile transmitter operative to receive a message signal and to respond to said message signal by transmitting a responsive signal over a predefined messaging channel, said responsive signal being indicative of the identity of the mobile transmitter, comprising the steps of:

(a) receiving said signal at least three separate ground-based antenna sites;

(b) processing said signal at each antenna site to produce frames of data, each frame comprising a prescribed number of data bits and time stamp bits, said time stamp bits representing the time at which said frames were produced at each antenna site;

(c) processing said frames of data to identify said responsive signal and the differences in times of arrival of said responsive signal among said antenna sites;

(d) determining, on the basis of said times of arrival differences, the location of said mobile transmitter; and

further comprising estimating the velocity of said mobile transmitter by performing the following steps;

(1) creating a grid of theoretical points covering a prescribed range of velocities, said theoretical points being spaced at prescribed increments;

(2) calculating theoretical values of frequency difference for a plurality of pairs of antenna sites;

(3) calculating a least squares difference (LSD) value based on the theoretical frequency differences and measured frequency differences for a plurality of pairs of antenna sites;

(4) searching the entire grid of theoretical points and determining the best theoretical velocity for which the value of LSD is minimized; and

(5) starting at the best theoretical velocity, performing another linearized-weighted-least-squares iteration to resolve the actual velocity to within a prescribed tolerance.