Position determination and message transfer system employing satellites and stored terrain map

Claims

What is claimed is:

1. A system for determining the position of a user located on the earth's surface, comprising:

a user-carried transceiver for receiving an interrogation signal and transmitting a reply signal in response to said interrogation signal;

two satellites at spaced orbital locations above the earth, each of said satellites carrying repeater means for receiving and retransmitting the reply signal transmitted by the user-carried transceiver; and

a ground station for periodically transmitting the interrogation signal and for receiving and processing the retransmitted reply signals from the two satellites, said ground station including means for calculating the user position based on the arrival times of the retransmitted reply signals at the ground station, the transmission time of the interrogation signal from the ground station, and the elevation of the user on the earth's surface, said elevation being derived form a stored terrain map providing local terrain elevations at a plurality of points on the earth's surface.

2. A system as claimed in claim 1, wherein the two satellites are in geostationary orbit about the earth, each of said satellites being located in a different position in said orbit.

3. A system as claimed in claim 2, wherein said terrain map provides terrain elevations directly above input reference points on a reference ellipsoid approximating the earth's surface.

4. A system as claimed in claim 2, wherein the interrogation signal is relayed from the ground station to the user-carried transceiver through one of said satellites.

5. A system as claimed in claim 1, wherein the reply signal transmitted by the user-carried transceiver is coded with information uniquely identifying the user, and wherein the ground station includes means for detecting the user identifying information in the received reply signals.

6. A system as claimed in claim 1, wherein the ground station further includes means for transmitting calculated position information to the user-carried transceiver, and wherein the user-carried transceiver further includes means for receiving and displaying said position information.

7. A method for determining the position of a user located on the earth's surface using two relay satellites at spaced orbital locations above the earth, said user carrying a transceiver for transmitting a reply signal in response to a received interrogation signal, said method comprising the steps of:

transmitting an interrogation signal to the user at a known or measured time;

receiving a reply signal from the user through each of the two satellites;

measuring the arrival times of the reply signals received through the two satellites;

calculating a line of position for the user based on the arrival times of the reply signal through the two satellites and the transmission time of the interrogation signal;

determining the intersection of the line of position with the earth's surface by reference to a stored terrain map providing local terrain elevations at a plurality of points on the earth's surface.

8. A method for providing approach guidance to a landing aircraft using two relay satellites at spaced orbital locations above the earth, said aircraft carrying a transceiver for transmitting a reply signal in response to a received interrogation signal, said method comprising the steps of:

transmitting an interrogation signal to the aircraft transceiver at a known or measured time;

receiving a reply signal from the aircraft transceiver through each of the two satellites;

calculating a line of position for the aircraft based on the arrival times of the reply signal through the two satellites and the transmission time of the interrogation signal;

estimating the aircraft location from the intersection between the line of position and a predetermined glide slope; and

providing the aircraft with altitude information representing desired altitude on the glide slope.

9. A system for determining the position of a user located on the earth's surface, comprising:

a user-carried transceiver for receiving an interrogation signal and transmitting a reply signal in response to said interrogation signal;

two satellites at spaced orbital locations above the earth, each of said satellites carrying repeater means for receiving and retransmitting the reply signal transmitted by the user-carried transceiver; and

a ground station for periodically transmitting the interrogation signal and for receiving and processing the retransmitted reply signals from the two satellites, said ground station comprising:

means for calculating the user position based on the arrival times of the retransmitted reply signals at the ground station, the transmission time of the interrogation signal from the ground station, and the elevation of the user on the earth's surface; and

a stored terrain map providing local terrain elevations at the plurality of points on the earth's surface, said user elevation being obtained from said stored terrain map by said calculating means in response to input information derived from the arrival times of said retransmitted reply signals.

10. A system claimed in claim 9, wherein the two satellites are in geostationary orbit about the earth, each of said satellites being located in a different position in said orbit.

11. A system as claimed in claim 10, wherein:

said terrain map provides terrain elevations directly above input reference points on a reference surface approximating the earth's surface;

said input information comprises an approximate user position defined by the intersection between said reference surface and a line of position established by said signal arrival times; and

said calculating means is effective to calculate a corrected user position based on the elevation obtained from said stored terrain map in response to said input information.

12. A system as claimed in claim 10, wherein the interrogation signal is relayed from the ground station to the user-carried transceiver through one of said satellites.

13. A system as claimed in claim 9, wherein the reply signal transmitted by the user-carried transceiver is coded with information uniquely identifying the user, and wherein the ground station includes means for detecting the user identifying information in the received reply signals.

14. A system as claimed in claim 9, wherein the ground station further includes means for transmitting calculated position information to the user-carried transceiver, and wherein the user-carried transceiver further includes means for receiving and displaying said position information.

15. A method for determining the position of a user located on the earth's surface using a ground station and two relay satellites at spaced orbital locations above the earth, said user carrying a transceiver for transmitting a reply signal in response to a received interrogation signal, said method comprising the steps of:

transmitting an interrogation signal from the ground station to the user at a known or measured time;

receiving a reply signal from the user at the ground station through each of the two satellites;

measuring the arrival times at the ground station of the reply signals received through the two satellites;

calculating at the ground station a line of position for the user based on the arrival times of the reply signal through the two satellites and the transmission time of the interrogation signal;

determining at the ground station the intersection point between the line of position and a reference surface approximating the earth's surface;

using said intersection point as an input to a stored terrain map at the ground station in order to obtain a local terrain elevation, said ground station providing local terrain elevations at a plurality of points on the earth's surface; and

using the local terrain elevation obtained from the stored terrain map and said line of position to determine at the ground station the position of the user.

16. A method as claimed in claim 15, wherein said intersection point corresponds to an approximate position of said user, and wherein the actual user position is determined by correcting said approximate position using said local terrain elevation.

17. A system for determining the position of a user located on the earth's surface, comprising:

a user-carried transmitter for transmitting a signal;

a plurality of satellites at spaced orbital locations above the earth, each of said satellites carrying repeater means for receiving and retransmitting the signal transmitted by the user-carried transmitter; and

a ground station for receiving and processing the retransmitted signals from the satellites, said ground station comprising:

means for calculating the user position based on the arrival times of the retransmitted reply signals at the ground station and the elevation of the user on the earth's surface; and

a stored terrain map providing local terrain elevations at a plurality of points on the earth's surface, said user elevation being obtained from said stored terrain map by said calculating means in response to input information derived from the arrival times of said retransmitted replay signals.

18. A system as claimed in claim 17, wherein said satellites are in geostationary orbit about the earth, each of said satellites being located in a different position in said orbit.

19. A system as claimed in claim 17, wherein the signal transmitted by the user-carried transmitter is coded with information uniquely identifying the user, and wherein the ground station includes means for detecting the user identifying information in the received signals.

20. A system as claimed in claim 17, wherein the ground station further includes means for transmitting calculated position information to the user-carried transmitter, and wherein the user-carried transmitter further includes means for receiving and displaying said position information.

21. A method for determining the position of a user located on the earth's surface using a ground station and a plurality of satellites at spaced orbital locations above the earth, said user carrying a transmitter for transmitting a signal to the ground station through said satellites, said method comprising the steps of:

receiving a signal from the user at the ground station through each of said satellites;

measuring the arrival times at the ground station of the signals received from said satellites; and

calculating at the ground station the position of the user based on the arrival times of the signals received from the satellites and the elevation of the user on the earth's surface, said elevation be