Dual satellite navigation system - Patent Review 5017926

Claims

We claim:

1. A method for determining the position of an object in a reference coordinate system using a fixed station and a plurality of earth orbit satellites wherein the positions of said fixed station and said satellites are known in said reference coordinate system with respect to a reference point and the distances from said fixed station to said satellites are also known, said method comprising the steps of:

transmitting from said fixed station a forward periodic signal synchronized to a fixed station transmission clock via first and second satellites to an object whose position is to be determined;

receiving at said object said forward periodic signal;

synchronizing at said object an object clock to said received forward periodic signal transmitted via said first satellite;

measuring at said object a first relative time difference in said forward periodic signal as transmitted via said first and second satellites and received at said object;

transmitting from said object to said fixed station via said first satellite a return signal synchronized with said object clock and modulated with information indicative of said first relative time difference;

receiving at said fixed station said return signal;

synchronizing at said fixed station a fixed station reception clock to said received return signal;

measuring at said fixed station a second relative time difference between said fixed station transmission and reception clocks;

calculating at said fixed station, from said first and second relative time differences, respective distances from said first and second satellites to said object; and

computing the position of said object in said reference coordinate system.

2. The method of claim 1 wherein said step of transmitting said forward signal comprises the steps of:

generating a first forward periodic signal having a signal carrier swept linearly in frequency in a cyclical fashion over a first predetermined frequency range with each cycle of said first forward periodic signal carrier being of a predetermined time period, said first periodic signal capable of being modulated with information;

generating a second forward periodic signal having a signal carrier swept linearly in frequency in said cyclical fashion over a second predetermined frequency range with each cycle of said second forward periodic signal carrier being of said predetermined time period, said first and second periodic signals both synchronized to said fixed station transmission clock; and

radiating said first and second forward signals from a respective fixed station antenna to a respective one of said first and second satellites.

3. The method of claim 1 wherein the step of measuring said first relative time difference comprises the step of:

measuring a percent phase offset, with respect to a forward periodic signal period, between a portion in cyclic modulation of said forward signal as received at said object via said second satellite with respect to a similar portion in cyclic modulation of said forward signal as received at said object via said first satellite.

4. The method of claim 1 wherein said step of transmitting said return signal comprises the steps of:

generating said return signal, said return signal encoded with information indicative of said first relative time difference between said forward signal as received at said object via said first and second satellites as measured just prior to transmission of said return signal; and

radiating said return signal from an object antenna to said fixed station via said first satellite.

5. The method of claim 1 wherein said step of calculating respective distances from said first and second satellites to said object comprises the steps of:

determining from said second relative time difference a round trip time value corresponding to a round trip signal propagation delay of a signal traveling from said fixed station to said object and back via said first satellite;

determining from said return signal said first relative time difference by demodulating said information modulated on said return signal indicative of said first relative time difference;

determining from said round trip time value and said first relative time difference, forward propagation times for said forward signal as transmitted via said first and second satellites to said object;

determining distances that said forward signal traversed by multiplying a value corresponding to each forward propagation time by a known value corresponding to a velocity of propagation of said forward signal; and

subtracting from values each corresponding to said distances said forward signal traversed, a respective value corresponding to said known distance from said fixed station to each respective satellite.

6. The method of claim 4 wherein said step of calculating respective distances from said first and second satellites to said object comprises the steps of:

determining from said second relative time difference a round trip time value corresponding to a round trip signal propagation delay of a signal traveling from said fixed station to said object and back via said first satellite;

determining from said return signal said first relative time difference by demodulating said information modulated on said return signal indicative of said first relative time difference;

determining from said round trip time value and said first relative time difference, forward propagation times for said forward signal as transmitted via said first and second satellites to said object;

determining distances that said forward signal traversed by multiplying a value corresponding to each forward propagation time by a known value corresponding to a velocity of propagation of said forward signal; and

subtracting from values each corresponding to said distances said forward signal traversed, a respective value corresponding to said known distance from said fixed station to each respective satellite.

7. The method of claim 4 wherein the step of calculating the distance from said first satellite to said object comprises the steps of:

commanding, on said forward signal, for said object at a particular identifiable future time referenced to said object clock to begin transmitting said return signal;

generating a round trip signal propagation time value corresponding to said second relative time difference;

dividing said round trip signal propagation time value in half to provide a first one-way signal propagation time value indicative of a first propagation time for said forward signal sent via said first satellite to said object;

multiplying said first one-way signal propagation time value by a first predetermined constant value corresponding to a velocity of propagation of an electromagnetic signal between said fixed station and said first satellite, and between said first satellite and said object, to provide a first distance value corresponding to a distance travelled by said forward signal between said fixed station and said object via said first satellite; and

subtracting from said first distance value a first known distance value corresponding to said known distance between said fixed station and said first satellite to provide a second distance value corresponding to said distance between said first satellite and said object.

8. The method of claim 7 wherein the step of calculating the distance from said second satellite to said object comprises the steps of:

demodulating said return signal to provide a first relative time difference value corresponding to said first relative time difference;

adding said first relative time difference value to said first one-way signal propagation time value to provide a second one-way signal propagation time value indicative of a second propagation time for said forward signal transmitted via said second satellite to said object;

multiplying said second one-way signal propagation time value by said first predetermined constant value corresponding to a velocity of propagation of an electromagnetic signal between said fixed station and said second satellite, and between said second satellite and said object, to provide a third distance value corresponding to a distance travelled by said forward signal between said fixed station and said object via said second satellite; and

subtracting from said third distance value a second known distance value corresponding to said known distance between said fixed station and said second satellite to provide a fourth distance value corresponding to said distance between said second satellite and said object.

9. The method of claim 4 further comprising the steps of spread spectrum modulating said return signal for transmission.

10. The method of claim 2 wherein said step of transmitting said return signal comprises the steps of:

generating said return signal, said return signal encoded with information indicative of said first relative time difference between said forward signal as received at said object via said first and second satellites as measured just prior to transmission of said return signal; and

radiating said return signal from an object antenna to said fixed station via said first satellite.

11. The method of claim 10 further comprising the steps of:

modulating information, intended for said object of a first user group, upon said first forward signal for communication to said object; and

modulating other information intended for said fixed station upon said object return signal for communication to said fixed station.

12. The method of claim 11 further comprising the steps of:

modulating information, intended for another object of a second user group, upon said second forward signal for communication to said another object;

transmitting another return signal from said another object, said another return signal containing information indicative of a measured time difference at said another object in reception of said first and second forward signals; and

modulating other information intended for said fixed station upon said another object return signal for communication to said fixed station.

13. The method of claim 12 further comprising the steps of:

relaying said first forward signal through a first transponder of said first satellite to said object and said another object;

relaying said second forward signal through a first transponder of said second satellite to said object and said another object;

relaying said return signal through a second transponder of said first satellite to said fixed station and

relaying said another return signal through a second transponder of said second satellite to said fixed station.

14. The method of claim 12 further comprising the steps of:

relaying said first forward signal through a first transponder of said first satellite to said object and said another object;

relaying said second forward signal through a first transponder of said second satellite to said object and said another object;

relaying said return signal and said another return signal through a second transponder of said first satellite to said fixed station.

15. The method of claim 12 further comprising the steps of:

relaying said first forward signal through a first transponder of said first satellite to said object and said another object;

relaying said second forward signal through a first transponder of said second satellite to said object and said another object;

relaying said return signal through said first satellite first transponder to said fixed station; and

relaying said another return signal through said second satellite first transponder to said fixed station.

16. The method of claim 1 wherein the steps of computing the position of said object relative to the position of said first and second satellites comprises:

trilaterating the position of said object by solving a set of three independent nonlinear equations having unknown object position components in cartesian x-y-z coordinates, known satellite components in cartesian x-y-z coordinates, a known approximate distance from the center of the earth to said object and measured respective distances from said first and second satellites to said object.

17. A method of object position determination using a plurality of earth orbiting satellites and a fixed station whose positions are known in an Earth-Centered-Earth-Fixed (ECEF) coordinate frame and an earth shape model in which the distance from the center of the earth to any location on the surface of the earth is known within said ECEF coordinate frame, comprising the steps of:

simultaneously transmitting from a fixed station first and second forward periodic signal, synchronized to a fixed station forward transmission clock, via first and second satellites respectively to an object whose position is to be determined;

receiving said first and second forward signals at said object, said first forward signal bearing information indicative of a future time frame which said object will begin transmitting a return signal according to an object clock synchronized to said first forward signal as received at said object;

measuring at said object a percent phase offset of a total modulation period between similar portions or received carrier modulation of said first and second forward signals, said percent offset corresponding to a first relative time difference in said first and second forward signals as received at said object;

transmitting from said object via said first satellite to said fixed station said return signal encoded with information indicative of said first relative time difference;

measuring at said fixed station a round trip delay where said round trip delay is a second relative time difference between said fixed station forward transmission clock and a fixed station return reception clock synchronized with said return signal as received at said fixed station;

calculating a first distance from said first satellite to said object according to said round trip delay;

calculating a second distance from said second satellite to said object according to said round trip delay and said first relative time difference; and

calculating a position of said object within said ECEF coordinate frame according to said first and second distances and said earth shape model.

18. The method of claim 17 wherein said object whose position is to be determined is a vehicle having a mobile communication terminal capable of two-way communications with said fixed station via at least one of said plurality of earth satellites.

19. The method of claim 18 wherein said fixed station is capable of independently initiating a vehicle location determination command and capable of two-way communications with said vehicle mobile communication terminal.

20. The method of claim 17 further comprising the step of initially determining the respective positions of said first an