Dual satellite navigation system and method

Claims

We claim:

1. A method of airborne object position determination using a pair of earth orbiting satellites (S1 and S2) and a fixed station whose positions are known in an Earth-Centered-Earth-Fixed (ECEF) coordinate frame and in which the distance from the center of the earth to any location on the surface of the earth (L3.sub.s) is known within said ECEF coordinate frame, comprising the steps of:

simultaneously transmitting from a fixed station first and second forward periodic signals, synchronized to a fixed station forward transmission clock, via first and second satellites (S1 and S2) 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 at 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 of received carrier modulation of said first and said 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 (L1) from said first satellite to said object according to said round trip delay;

calculating a second distance (L2) from said second satellite to said object according to said round trip delay and said first relative time difference;

measuring an altitude (L3.sub.a) of said object from a location on the surface of the earth;

combining said measured altitude (L3.sub.a) with a known distance from said location to the center of the earth (L3.sub.s) to obtain a third distance (L3); and

calculating a position of said object within said ECEF coordinate frame according to said first, second and third distances.

2. The method of claim 1 wherein said step of measuring said altitude further comprises the steps of:

providing a baro altimeter upon said object capable of measuring atmospheric pressure; and

converting said measured atmospheric pressure to an altitude measurement with respect to mean sea level.

3. The method of claim 2 wherein said known distance is the distance from the center of the earth to mean sea level.

4. The method of claim 1 wherein said step of measuring said altitude further comprises the step of providing a radar altimeter upon said object capable of directly measuring distance from said object to said location.

5. The method of claim 4 wherein said known distance is the distance from the center of the earth to the surface of the earth, said surface of the earth being according an earth shape model, and a correction for local terrain differences from said earth shape model.

6. The method of claim 1 wherein said step of calculating said object position within said ECEF coordinate frame comprises the steps of:

providing Cartesian coordinates (x.sub.S1, y.sub.S1, z.sub.S1) of said first satellite (S1) in said ECEF coordinate frame;

providing Cartesian coordinates (x.sub.S2, y.sub.S2, z.sub.S2) of said second satellite (S2) in said ECEF coordinate frame;

providing said first (L1), second (L2) and third distance (L3);

solving a set of three independent nonlinear equations having unknown object position components (x.sub.V, y.sub.V, y.sub.V) in Cartesian coordinates in said ECEF coordinate frame from said known satellite components (x.sub.S1, y.sub.S1, z.sub.S1) and (x.sub.S2, y.sub.S2, z.sub.S2), and said first (L1), second (L2) and third distances (L3), wherein: ##EQU2##

7. The method of claim 1 wherein said object whose position is to be determined is an airborne vehicle having a mobile communication terminal capable of two-way communications with said fixed station via at least one of said earth satellites.

8. The method of claim 1 wherein said fixed station is capable of independently initiating a vehicle location determination command and capable of two-way communications with said airborne vehicle mobile communication terminal.

9. A system for determining the position of an airborne object, travelling above the surface of the earth, in an Earth-Centered-Earth-Fixed (ECEF) reference coordinate system, in which the center of the earth is a reference point (x.sub.0, y.sub.0, z.sub.0) within said reference coordinate system, using a fixed station located at a known position and a pair of earth orbit satellites (S1 and S2) whose positions, respectively (x.sub.S1, y.sub.S1, z.sub.S1) and (x.sub.S2, y.sub.S2, z.sub.S2), are also known in said reference coordinate system, comprising:

first communications means located at said fixed station for, generating a periodic signal synchronized to a first communications means transmission clock and transmitting said periodic signal to said object via first and second satellites (S1 and S2);

second communications means located at said object for, receiving said periodic signal, synchronizing a second communications means clock to said received periodic signal transmitted via said first satellite (S1) and received by said second communications means, measuring a first relative time difference between said periodic signal as transmitted via said first satellite (S1) and said second satellite (S2) and received by said second communications means, generating a return signal indicative of said time difference, and transmitting said return signal via said first satellite (S1) to said first communications means;

altimeter means located at said object for measuring a distance (L3.sub.a) above the surface of the earth said object is located; and

wherein said first communications means further is for, receiving said return signal, synchronizing a first communications means reception clock to said received return signal, measuring a second relative time difference between said first communications means transmission and reception clocks, determining from said first and second relative time differences and said known positions of said first and second satellites a distances between said first satellite (S1) and said object and a distance between said second satellite (S2) and said object, combining a known distance (L3.sub.S) between said reference point (x.sub.0, y.sub.0, z.sub.0) and a location on the surface of the earth beneath said object with said distance (L3.sub.a) to provide a distance (L3), and for determining object position (x.sub.V, y.sub.V, z.sub.V) from said known positions of said first and second satellites (x.sub.S1, y.sub.S1, z.sub.S1) and (x.sub.S2, y.sub.S2, z.sub.S2), said distances between said first and second satellites to said object (L1) and (L2) and said combined distance (L3) from said object to said reference point.

10. The system of claim 9 wherein said first communications means determines object position by computing object position components (x.sub.V, y.sub.V, y.sub.V) in said ECEF coordinate frame from said known satellite components (x.sub.S1, y.sub.S1, z.sub.S1) and (x.sub.S2, y.sub.S2, z.sub.S2), and said distances (L1), (L2) and (L3), wherein: ##EQU3##

11. The system of claim 9 wherein said altimeter means comprises a baro altimeter.

12. The system of claim 9 wherein said altimeter means comprises a radar altimeter.

13. The system of claim 9 wherein said first and second communications means are capable of two-way communications with respect to one another via said satellites.

14. The system of claim 9 wherein said first communications means is capable of independently initiating an object position determination.

15. A method for determining the position of an aircraft using a pair of earth orbiting satellites and a fixed station whose positions are known in an Earth-Centered-Earth-Fixed (ECEF) coordinate frame, comprising the steps of:

transmitting from a fixed station whose position is known in an Earth-Centered-Earth-Fixed (ECEF) coordinate frame a first forward signal containing data in a time frame data format coordinated with respect to a fixed station transmission clock;

transmitting from said fixed station a second forward signal, said first and second signals of a same carrier waveform synchronized with one another;

relaying said first forward signal by a first satellite whose position is known in said ECEF coordinate frame to an aircraft whose position is to be determined,

relaying said second forward signal by a second satellite whose position is known in said ECEF coordinate frame to said aircraft;

receiving said first forward signal at said aircraft;

synchronizing at said aircraft an aircraft clock with said first forward signal as received at said aircraft wherein said first forward signal bears data representative of a command for said aircraft to transmit a return signal at a predetermined time frame in said time frame data format, said aircraft clock offset in time with respect to said fixed station transmission clock as a function of a propagation delay of said first forward signal;

receiving said second forward signal at said aircraft;

measuring an offset in carrier waveform of said first and second forward signals as received at said aircraft, said measured offset corresponding to a first relative time difference in reception of said first and second forward signals at said aircraft;

transmitting from said aircraft at said predetermined time frame a return signal bearing data representative of said first relative time difference;

relaying said return signal by said first satellite to said fixed station;

receiving said return signal at said fixed station;

synchronizing at said fixed station a fixed station reception clock with said return signal as received at said fixed station, said fixed station reception clock offset in time with respect to said fixed station transmission clock as a function of said first forward signal propagation delay and a propagation delay of said return forward signal;

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

computing a first distance from said first satellite to said aircraft using said second relative time difference and a first known distance between said fixed station and said first satellite;

computing a second distance from said second satellite to said aircraft using said first and second relative time differences and a second known distance between said fixed station and said second satellite;

measuring an altitude of said aircraft with respect to a point on the surface of the earth;

combining said measured altitude with a third known distance corresponding to said point to a center of the earth to obtain a third distance; and

deriving a position of said aircraft within said ECEF coordinate frame from said first, second and third distances and said known positions of said first and second satellites.

16. The method of claim 15 wherein said first and second satellites have known Cartesian coordinates respectively (x.sub.S1, y.sub.S1, z.sub.S1) and (x.sub.S2, y.sub.S2, z.sub.S2) in said ECEF coordinate frame, said step of deriving said aircraft position comprises the step of solving a set of three independent nonlinear equations for said aircraft position Cartesian coordinates (x.sub.V, y.sub.V, y.sub.V) in said ECEF coordinate frame from said first (L1), second (L2) and third distances (L3) and said first and second satellite coordinates, wherein: ##EQU4##

17. The method of claim 16 wherein said step of measuring said altitude further comprises the steps of:

providing a baro altimeter upon said aircraft capable of measuring atmospheric pressure; and

converting said measured atmospheric pressure to an altitude measurement with respect to mean sea level.

18. The method of claim 17 wherein said third known distance is a distance from the center of the earth to mean sea level.

19. The method of claim 15 wherein said step of measuring said altitude further comprises the steps of:

providing a radar altimeter upon said aircraft; and

directly measuring distance from said aircraft to said point.

20. The method of claim 19 wherein said third known distance is determined from an earth shape model with a correction for local terrain differences from said earth shape model to said point.