Low-earth orbit satellite acquisition and synchronization system using a beacon signal

• US 6,127,967 A
• Filed: 07/19/1999
• Issued: 10/03/2000
• Est. Priority Date: 03/05/1998
• Status: Expired due to Term

• Current Assignee: Hanger Solutions, LLC (IP Investments Group LLC)
• Original Assignee: Teledesic, LLC (Michael J. Larson, MBA)
• Inventors: Ghazvinian, Farzad, Shah, Biren N., Sarrafan, Shiva S., Sturza, Mark A., Hinedi, Sami M.
• Primary Examiner(s): Tarcza, Thomas H.
• Assistant Examiner(s): PHAN, DAO LINDA
• Application Number:
  US09/356,061
• Time in Patent Office: 442 Days
• Field of Search: 342/352, 342/354, 342/356, 342/359, 455/12.1, 455/13.2, 455/427
• US Class Current: 342/354
• CPC Class Codes: H04B 7/18532 Arrangements for managing transmission, i.e. for transporting data or a signalling message
• CPC Class Codes: H04B 7/2041 Spot beam multiple access
• CPC Class Codes: H04B 7/2125 Synchronisation

Abstract

A system for acquiring the beacon of a satellite and synchronizing data transmission between the satellite and a ground terminal is disclosed. The ground terminal conducts a search for a satellite to be acquired. The search may be based on previously developed information from which the location of the satellite can be predicted and, thus, limited to a small area of the sky, or cover a large area of the sky in accordance with a search routine. After the beacon of a satellite is acquired, the geographic area served by the satellite is determined. If the satellite does not serve the cell within which the ground terminal is located, a further satellite search is conducted, which may be based in part on information contained in the beacon of the acquired satellite. After the satellite serving the cell containing the ground terminal is acquired, a test is made to determine how long the satellite will continue to cover the cell. If the time period is short, communication waits until the next satellite to cover the cell is acquired. If the time period is long, communication is allowed to begin. The beacon is also used by the ground terminal to: (i) accurately time uplink data transmissions; (ii) estimate uplink Doppler, Doppler rate, and Doppler rate derivative and use this information to pre-compensate the carrier frequency of the uplink data transmissions for Doppler variations; and (iii) estimate the carrier frequency of downlink data transmission(s) by continuously tracking the beacon carrier frequency and scaling the result by a suitable scaling factor.

Claims

1. A ground terminal acquisition and synchronization system for a satellite communication network comprising a plurality of orbiting satellites and a plurality of ground terminals that communicate with one another via uplink and downlink data channels to and from the orbiting satellites, each of said orbiting satellites producing a beacon signal, said ground terminal acquisition and synchronization system comprising:

(a) a satellite acquisition system for locating a satellite, acquiring the beacon signal produced by the satellite and determining if the satellite is a desired satellite serving the geographic area in which the ground terminal is located; and

(b) a satellite synchronization system for synchronizing the ground terminal to the satellite serving the geographic area in which the ground terminal lies based on satellite position and velocity information contained in the beacon signal produced by the satellite, if the satellite is the desired satellite serving the geographic area in which the ground terminal is located.

2. A ground terminal acquisition and synchronization system as claimed in claim 1, wherein said beacon signal includes a unique word and wherein said unique word is used by said satellite synchronization system to identify a beacon frame rate.

3. A ground terminal acquisition and synchronization system as claimed in claim 2, wherein the satellite synchronization system also utilizes said unique word to determine when the satellite position and velocity is updated.

4. A ground terminal acquisition and synchronization system as claimed in claim 1, wherein the satellite synchronization system uses the position and velocity information to determine the range between the ground terminal and the satellite and wherein the satellite synchronization system uses said range to determine the downlink propagation time.

5. A ground terminal acquisition and synchronization system as claimed in claim 4, wherein the downlink propagation time is used by the satellite synchronization system to accurately time uplink data transmissions.

6. A ground terminal acquisition and synchronization system as claimed in claim 1, wherein the satellite synchronization system uses information contained in the beacon signal to pre-compensate the carrier frequency of the uplink data channel for Doppler effects.

7. A ground terminal acquisition and synchronization system as claimed in claim 6, wherein:

the satellite synchronization system uses the position and velocity information in the beacon signal to determine the range between the ground terminal and the satellite serving the geographic area in which the ground terminal lies; and

the range is used to estimate the uplink Doppler, Doppler rate, and Doppler rate derivative for purposes of compensating for Doppler effects.

8. A ground terminal acquisition and synchronization system as claimed in claim 1, wherein the satellite synchronization system uses information contained in the beacon signal to acquire a downlink carrier frequency by continuously tracking the beacon carrier frequency, scaling the beacon carrier frequency by a scaling factor, and using the scaled beacon carrier frequency to estimate and acquire the downlink carrier frequency.

9. A ground terminal acquisition and synchronization system as claimed in claim 8, wherein the scaling factor is equal to a ratio of a first scaling factor applied to generate the beacon carrier frequency by the satellite serving the geographic area in which the ground terminal lies and a second scaling factor applied to generate the downlink carrier frequency by the satellite serving the geographic area in which the ground terminal lies.

10. A ground terminal synchronization system for a satellite communication network comprising a plurality of orbiting satellites and a plurality of ground terminals that communicate with one another via uplink and downlink data channels to and from the orbiting satellites, the data communicated via the uplink and downlink data channels separated into frames, each of said orbiting satellites producing a beacon signal, said ground terminal synchronization system comprising:

an uplink data frame synchronization system for synchronizing the data frame rate of the ground terminal to the data frame rate of the satellite serving the geographic area in which the ground terminal lies based on information contained in the beacon signal produced by the satellite;

wherein the information contained in the beacon signal includes a unique word and wherein said uplink data frame synchronization system uses said unique word to identify a beacon frame rate, said beacon frame rate being utilized by said uplink data frame synchronization system to determine the data frame rate of the ground terminal; and

a phase synchronization system for synchronizing the phase of the data frames of the ground terminal with the phase of the data frames of the satellite serving the geographic area in which the ground terminal lies based on satellite position and velocity information contained in the beacon signal produced by the satellite.

11. A ground terminal synchronization system as claimed in claim 10, wherein the unique word is used to determine when the satellite position and velocity is updated.

12. A ground terminal synchronization system as claimed in claim 10, wherein the phase synchronization system uses the satellite position and velocity information to determine the range between the ground terminal and the satellite serving the geographic area in which the ground terminal lies and wherein the phase synchronization system uses the range to determine the downlink propagation time.

13. A ground terminal synchronization system as claimed in claim 12, wherein the phase synchronization system uses said downlink propagation time to delay a ground terminal clock to accurately time uplink data transmissions.

14. A ground terminal synchronization system for a satellite communication network comprising a plurality of orbiting satellites and a plurality of ground terminals that communicate with one another via uplink and downlink data channels to and from the orbiting satellites, each of said orbiting satellites producing a beacon signal, said ground terminal synchronization system comprising:

an uplink frequency compensation system for pre-compensating the carrier frequency of the uplink data channel for Doppler effects based on information contained in the beacon signal produced by a satellite, said information including the satellite position and velocity.

15. A ground terminal synchronization system as claimed in claim 14, wherein said uplink frequency compensation system uses the position and velocity information to determine the range between the ground terminal and the satellite serving the geographic area in which the ground terminal lies.

16. A ground terminal synchronization system as claimed in claim 15, wherein said uplink frequency compensation system uses said range to estimate the uplink Doppler, Doppler rate and Doppler rate derivative and wherein said uplink frequency compensation system uses said estimate to pre-compensate the carrier frequency of the uplink data channel for Doppler effects.

17. A ground terminal synchronization system for a satellite communication network comprising a plurality of orbiting satellites and a plurality of ground terminals that communicate with one another via uplink and downlink data channels to and from the orbiting satellites, each of said orbiting satellites producing a beacon signal and communicating via said downlink data channels in bursts and at irregular intervals at downlink carrier frequencies, said ground terminal synchronization system comprising:

a downlink carrier frequency acquisition system for acquiring a downlink carrier frequency based on information contained in the beacon signal produced by a satellite by continuously tracking the beacon carrier frequency, scaling the beacon carrier frequency by a scaling factor, and using the scaled frequency to estimate and acquire the downlink carrier frequency.

18. A ground terminal synchronization system as claimed in claim 17, wherein the scaling factor is equal to a ratio of a first scaling factor applied to generate the beacon carrier frequency by the satellite and a second scaling factor applied to generate the downlink carrier frequency by the satellite.

19. A ground terminal synchronization method for synchronizing a ground terminal to a satellite serving a geographic area in which the ground terminal is located, said ground terminal and said satellite forming part of a satellite communication network comprising a plurality of orbiting satellites and a plurality of ground terminals that communicate with one another via uplink and downlink data channels to and from the orbiting satellites, the data communicated via the uplink and downlink data channels separated into frames, each of said orbiting satellites producing a beacon signal, said ground terminal synchronization method comprising:

synchronizing the data frame rate of a ground terminal with the data frame rate of the satellite serving the geographic area in which the ground terminal is located based on information contained in the beacon signal produced by the satellite, said information including a unique word; and

synchronizing the phase of the data frames of a ground terminal with the phase of the data frames of the satellite serving the geographic area in which the ground terminal is located based on information contained in the beacon signal produced by the satellite, said information including the satellite position and velocity.

20. A ground terminal synchronization method as claimed in claim 19, wherein synchronizing the data frame rate of the ground terminal comprises:

identifying a unique word in the information contained in the beacon signal;

correlating the identified unique word against a unique word generated by the ground terminal to determine a beacon frame rate; and

calculating the data frame rate as a function of the beacon frame rate.

21. A ground terminal synchronization method as claimed in claim 20, wherein synchronizing the phase of the data frames of the ground terminal comprises:

using the satellite position and velocity information in the beacon signal to determine a range between the ground terminal and the satellite;

calculating a propagation time as a function of the range; and

utilizing the propagation time to synchronize the phase of the data frames of the ground terminal with the phase of the data frames of the satellite serving the geographic area in which the ground terminal lies.

22. A ground terminal synchronization method as claimed in claim 21, wherein utilizing the propagation time to synchronize the phase of the data frames of the ground terminal comprises delaying uplink data transmissions from the ground terminal to the satellite serving the geographic area in which the ground terminal lies based on the propagation time.

23. A method of compensating for Doppler effects in the carrier frequency of an uplink data signal transmitted from a ground terminal to a satellite serving the geographic area in which the ground terminal is located, said ground terminal and said satellite forming part of a satellite communication network comprising a plurality of orbiting satellites and a plurality of ground terminals that communicate with one another via uplink and downlink data channels to and from the orbiting satellites, each of said orbiting satellites producing a beacon signal, said method comprising:

acquiring position and velocity information about the satellite serving the geographic area in which the ground terminal is located, said satellite position and velocity information contained in the beacon signal produced by the satellite;

determining the range between the ground terminal and the satellite serving the geographic area in which the ground terminal lies based upon the satellite position and velocity information;

using the range to estimate Doppler effects on the uplink data signal; and

using the estimated Doppler effects to pre-compensate the carrier frequency of the uplink data signal for Doppler effects.

24. A method of acquiring the downlink carrier frequency of downlink data transmissions received in bursts at a ground terminal from a satellite serving the geographic area in which the ground terminal is located, said ground terminal and said satellite forming part of a satellite communication network comprising a plurality of orbiting satellites and a plurality of ground terminals that communicate with one another via uplink and downlink data channels to and from the orbiting satellites, each of said orbiting satellites producing a beacon signal that is transmitted continuously on a beacon carrier frequency, said method comprising:

continuously tracking the beacon carrier frequency;

scaling the beacon carrier frequency by a scaling factor; and

using the scaled beacon carrier frequency to estimate and acquire the downlink carrier frequency when a burst of downlink data is received by the ground terminal.

25. A method as claimed in claim 24, wherein the scaling factor is equal to a ratio of a first scaling factor applied to generate the beacon carrier frequency by the satellite serving the geographic area in which the ground terminal lies and a second scaling factor applied to generate the downlink carrier frequency by the satellite serving the geographic area in which the ground terminal lies.

26. In a satellite communication system comprising a plurality of orbiting satellites and a plurality of ground terminals that communicate with one another via uplink and downlink data channels to and from the orbiting satellites, each of said orbiting satellites producing a beacon signal, the improvement comprising:

an uplink data frame synchronization system for synchronizing the ground terminal to the satellite serving the geographic area in which the ground terminal lies based on information contained in the beacon signal produced by the satellite, said uplink data frame synchronization system using satellite position and velocity information in the beacon signal to determine a range and propagation time between the ground terminal and the satellite serving the geographic area in which the ground terminal lies in order to accurately time uplink data communications.

27. In a satellite communication system comprising a plurality of orbiting satellites and a plurality of ground terminals that communicate with one another via uplink and downlink data channels to and from the orbiting satellites, each of said orbiting satellites producing a beacon signal, the improvement comprising:

an uplink frequency compensation system in a ground terminal for pre-compensating the carrier frequency of the uplink data channel for Doppler effects by calculating a range between the ground terminal and the satellite serving the geographic area in which the ground terminal lies based on satellite position and velocity information contained in the beacon signal produced by the satellite serving the geographic area in which the ground terminal lies.

28. The improvement claimed in claim 27, wherein said uplink frequency compensation system uses said range to estimate the uplink Doppler, Doppler rate, and Doppler rate derivative and wherein said uplink frequency compensation system uses said estimates to pre-compensate the carrier frequency of the uplink data channel for Doppler effects.

29. In a satellite communication system comprising a plurality of orbiting satellites and a plurality of ground terminals that communicate with one another via uplink and downlink data channels to and from the orbiting satellites, each of said orbiting satellites producing a continuously transmitted beacon signal, each of said beacon signals having a beacon carrier frequency, downlink data communicated via said downlink data channels occurring in bursts and at irregular intervals on downlink carrier frequencies, the improvement comprising:

a downlink carrier frequency acquisition system for acquiring a downlink carrier frequency based on information contained in the beacon signal produced by a satellite by continuously tracking the beacon carrier frequency, scaling the beacon carrier frequency by a scaling factor, and using the scaled frequency to estimate and acquire the downlink carrier frequency when a burst of downlink data is received.

30. The improvement claimed in claim 29, wherein the scaling factor is equal to a ratio of a first scaling factor applied to generate the beacon carrier frequency by the satellite serving the geographic area in which the ground terminal lies and a second scaling factor applied to generate the downlink carrier frequency by the satellite serving the geographic area in which the ground terminal lies.