Fast acquisition position reporting system

• US 6,243,648 B1
• Filed: 07/12/1999
• Issued: 06/05/2001
• Est. Priority Date: 07/12/1999
• Status: Expired due to Term

• Current Assignee: SkyBitz, Inc. (Ametek Incorporated)
• Original Assignee: Eagle Eye, Inc.
• Inventors: Sullivan, Mark C., Kilfeather, James B.
• Primary Examiner(s): Cuchlinski, Jr., William A.
• Assistant Examiner(s): TO, TUAN C
• Application Number:
  US09/351,852
• Time in Patent Office: 694 Days
• Field of Search: 701/213, 701/207, 701/215, 342/357, 379/59
• US Class Current: 701/213
• CPC Class Codes: G01S 19/09 providing processing capability normally carried out by the receiver
• CPC Class Codes: G01S 19/256 relating to timing, e.g. time of week, code phase, timing offset
• CPC Class Codes: G01S 19/40 Correcting position, velocity or attitude
• CPC Class Codes: G01S 5/0036 of measured values, i.e. measurement on mobile and position calculation on base station
• CPC Class Codes: G01S 5/14 Determining absolute distances from a plurality of spaced points of known location

Abstract

A geographic tracking system with minimal power and size required at the mobile terminal collects observation data at the mobile terminal, forwards the data to a processor, which calculates the position. The mobile terminal needs only to gather a few milliseconds of observation data, and to relay this observation data to the processor. The range from the satellite (or other airborne transponder) to the terminal is determined using the known positions of an interrogating transmitter and a satellite, and a known terminal delay between the received signal and the transmission of the return signal, and the round trip time. An arc of locations is determined by computing an intersection of a sphere centered at the satellite having a radius given by the calculated range with a model of the Earth'"'"'s surface. Only that portion of the arc within the region bounded by the satellite beam pattern is retained. Next, the time when the mobile terminal collected the GPS signal is determined. A satellite orbit model estimates the positions of the GPS satellites at their time of transmission. Using discrete points on the arc as an initial guess, an iterative least squares technique fits the observation data to the predicted data and minimizes residual error. After convergence, this estimated position solution is then screened against the known satellite range, satellite beam boundaries, an acceptable altitude range, and a maximum residual threshold. Those position estimates not meeting these criteria are discarded. The remaining points are then subjected to a final improved position estimate and residual calculation and the best point is selected.

Claims

1. A system for determining a location of a remote terminal located on or near the surface of the Earth by communicating with an orbiting communications satellite, said system comprising:

a) a satellite gateway broadcasting an interrogation signal to the remote terminal via the communications satellite;

b) said remote terminal including;

(i) a receiver receiving the interrogation signal from the communications satellite, said receiver receiving a plurality of signals being broadcast from a plurality of GPS satellites;

(ii) a processor measuring at least one characteristic in each of the plurality of signals being broadcast from the plurality of GPS satellites at a first predetermined time relative to receipt of the interrogation signal from the communications satellite; and

(iii) a transmitter transmitting a reply signal at a second predetermined time relative to receipt of the interrogation signal to the communications satellite, wherein said reply signal includes the measured characteristic in each of the plurality of signals being broadcast from the plurality of GPS satellites;

c) an earth station receiving the reply signal from the communications satellite, and measuring time of arrival information; and

d) an operations center calculating a position of the remote terminal using time of arrival information and the measured characteristics returned by the remote terminal.

2. The system according to claim 1, wherein the measured characteristic includes code phase information.

3. The system according to claim 1, wherein the measured characteristic includes carrier phase information.

4. The system according to claim 1, wherein the measured characteristic includes Doppler-shift information.

5. The system according to claim 1, wherein the measured characteristic includes bit phase information.

6. The system according to claim 1, wherein the measured characteristic includes Fourier transform coefficients.

7. The system according to claim 1, wherein the service operations center includes a processor, said processor:

a) calculating a range between the communications satellite and the remote terminal using a time the reply signal arrived at the service operations center and a time the communications satellite broadcast the interrogation signal to the remote terminal and accounting for known delays in a communication path between the communications satellite and the remote terminal and a point at which the time of arrival information was measured;

b) calculating an intersection curve between a range sphere having a range determined in step a) and a model of the surface of the Earth;

c) determining a plurality of initial points on the intersection curve, one of said plurality of initial points being within a convergence zone of the remote terminal;

d) calculating a plurality of candidate solutions;

e) screening the plurality of candidate solution points using a predetermined criteria and discarding any candidate points not satisfying the screening; and

f) selecting a position representing a best fit of observation data using final solution residuals.

8. The system according to claim 7, wherein the predetermined criteria includes one or more selected from the group consisting of:

a solution range to the communications satellite, solution residuals, a solution altitude, a solution clock bias, and a solution proximity to a beam boundary.

9. A method for determining a position of a remote terminal on or near the surface of the Earth comprising the steps of:

a) broadcasting an interrogation signal to a remote terminal unit;

b) receiving the interrogation signal at the remote terminal unit;

c) receiving a plurality of signals being broadcast from a plurality of GPS satellites at the remote terminal unit;

d) measuring at least one characteristic in each of the plurality of signals being broadcast from the plurality of GPS satellites upon receipt of the interrogation signal from a communications satellite;

e) transmitting a reply signal at a predetermined time relative to receipt of the interrogation signal, wherein said reply signal includes the at least one measured characteristics in each of the plurality of signals being broadcast from the plurality of GPS satellites;

f) receiving the reply signal at a satellite gateway;

g) measuring time of arrival information in the satellite gateway; and

h) calculating a position of the remote terminal unit using the time of arrival information and the at least one measured characteristics returned by the remote terminal unit.

10. The method according to claim 9, further comprising the steps of:

h) obtaining current ephemeris/clock/atmospheric parameters for a plurality of GPS satellites;

i) obtaining Time of Arrival (TOA) and Round Trip Time measurements at the satellite gateway;

j) using the Time of Arrival (TOA) and Round Trip Time measurements along with the current ephemeris/clock/atmospheric parameters to calculate a GPS time of observation, a signal propagation time, a GPS transmit time, and receiver clock offset; and

k) calculating a plurality of GPS satellite state vectors at a time of transmission.

11. The method according to claim 9, further comprising the step of correcting a plurality of code phase observations for at least one of the group consisting of:

SV clock errors, relativity, and group delay.

12. The method according to claim 9, further comprising the step of correcting the GPS satellite state vectors to compensate for Earth rotation.

13. The method according to claim 9, further comprising the step of calculating a variance matrix from signal strength measurements and GPS satellite URE parameters.

14. The method according to claim 10, further comprising the steps of:

k) calculating initial position/velocity/clock bias estimates;

l) screening the initial position estimates using at least one from the group consisting of;

solution proximity to beam boundary, solution residuals, solution altitude, solution proximity to communication satellite range, and receiver clock bias solution, wherein only those candidate locations passing this screening are carried forward; and

m) calculating azimuth and elevations for each of the candidate locations.

15. The method according to claim 14, further comprising the step of correcting propagation and transmission times based on the initial positions.

16. The method according to claim 15, further comprising the step of correcting code phase observations for at least one from the group consisting of:

state vector clock errors, relativity, group delay and ionospheric and tropospheric delays.

17. The method according to claim 16, further comprising the step of correcting the plurality of GPS satellite state vectors to compensate for rotation of the Earth.

18. The method according to claim 13, further comprising the step of using the variance matrix determined from the signal strengths/URE to calculate improved position/velocity/clock bias estimates.

19. The method according to claim 17, further comprising the steps of determining a final position/velocity/clock bias estimate and converting the final position/velocity/clock bias estimate to a desired coordinate system.

20. A method for determining a position of an object on the surface of the Earth, comprising the steps of:

a) receiving an interrogation signal from a communications satellite;

b) measuring, at a predetermined time relative to receipt of the interrogation signal, a plurality of characteristics in a plurality of signals being broadcast from a plurality of GPS satellites; and

c) transmitting a response signal to the communications satellite after a predetermined delay relative to the receipt of the interrogation signal, wherein the response signal includes the plurality of measured characteristics.

21. The method according to claim 20, further comprising the step of:

d) calculating at a central site a range sphere on which the object must lay using a range to the remote object determined from measurements of a round trip travel time of the interrogation signal and the reply signal, excluding known delays and the predetermined delay.

22. The method according to claim 21, further comprising the step of:

e) using the measured characteristics to determine a plurality of points on an intersection curve determined from an intersection of the range sphere and the surface of the Earth.

23. The method according to claim 22, further comprising the step of:

f) performing a residual error test to select an optimum point from among the plurality of points which optimum point represents the location of the object.

24. A method for determining the position of a remote terminal on or near the position of the earth, comprising the steps of:

sending an interrogation signal to the remote terminal from a communications satellite;

obtaining measurements of a signal characteristic from a plurality of GPS satellites;

transmitting the measurements to a satellite gateway after waiting a predetermined time from receipt of the interrogation signal; and

determining the position of the remote terminal on or near the position of the earth from the transmitted measurements.

25. The method recited in claim 24, wherein the step of determining the position of the remote terminal comprises the step of generating an intersection curve along the surface of the earth on which the remote terminal can be located.

26. The method recited in claim 25, wherein only one intersection curve along the surface of the earth is generated.

27. The method recited in claim 25, wherein the step of determining the position of the remote terminal further comprises the steps of:

selecting initial points along the intersection curve; and

determining a set of candidate points.

28. The method recited in claim 27, wherein the step of determining the position of the remote terminal further comprises the step of screening the candidate positions according to one or more criteria.

29. The method recited in claim 28, wherein the one or more criteria are from the set of:

a solution range to the communications satellite, solution altitude, solution residuals, a solution clock bias and a solution proximity to a beam boundary of the communications satellite.

30. The method recited in claim 27, wherein the step of determining a set of candidate points comprises the step of using the initial points as seeds to a least squares fit to estimate location on the basis of the measurements.

31. The method recited in claim 30, wherein the measurements are code phase of arrival measurements.

32. The method recited in claim 31, further comprising the steps of:

calculating a range vector between the communications satellite and the remote terminal;

measuring a propagation time from the time the interrogation signal is sent until the measurements are received by the satellite gateway; and

removing known delays from a propagation time measurement.

33. The method recited in claim 28, further comprising the steps of:

using the determined candidate points as new initial points;

calculating new candidate points using the new initial points; and

screening the new candidate points according to the criteria.

34. The method recited in claim 33, wherein the steps are repeated until there is only one candidate point.

35. The method recited in claim 34, further comprising the step of displaying the one candidate point on a map.