Satellite communications systems using satellites in a zero-drift constellation

• US 6,198,907 B1
• Filed: 02/02/1998
• Issued: 03/06/2001
• Est. Priority Date: 02/02/1998
• Status: Expired due to Term

• Current Assignee: CDC Propriete Intellectuelle
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Torkington, Richard S., Emmons, Thomas P. Jr., Warble, Keith
• Primary Examiner(s): TROST IV, WILLIAM GEORGE
• Application Number:
  US09/016,976
• Time in Patent Office: 1,128 Days
• Field of Search: 455/12.1, 455/13.1, 455/13.2, 455/427-429, 342/352, 342/357.16, 342/422, 342/428, 370/316, 370/323-324
• US Class Current: 455/12.1
• CPC Class Codes: H04B 7/18541 for handover of resources
• CPC Class Codes: H04B 7/195 Non-synchronous stations

Abstract

A zero-drift constellation (200 FIG. 2) is used to simplify the tracking and hand-off requirements of terrestrial-based user terminals (110 FIG. 1). Each satellite (120 FIG. 1) traces out a common ground track which has a number of southbound segments and an equal number of adjacent northbound segments. This allows user terminals (110) to employ antennas with only one degree of freedom to track satellites (120) in zero-drift constellation (200). User terminals (110) perform hand-offs with satellites (120) that are within a limited field of view with respect to user terminal (110). User terminal (110) tracks a first satellite until a crossover point is reached and then performs a hand-off to a second satellite traveling in the opposite direction along an adjacent segment. User terminal (110) tracks the second satellite until another crossover point is reached and then performs a hand-off to a third satellite traveling in the same direction as the first satellite along an adjacent segment.

Claims

1. A satellite communications system for providing communications services, said satellite communications system comprising:

N satellites configured in a zero-drift constellation for providing said communications services, where N is a integer greater than two, said N satellites being located in N orbital planes which are equally spaced around the earth at the equator, wherein said N satellites orbit the earth in separate circular orbits, said N satellites are interconnected using crosslinks, and all of said N satellites trace a common ground track which repeats after n revolutions, where n is an odd integer which is in a range from seven to fifteen, and each satellite is phased with respect to satellites in adjacent orbital planes so that said N satellites are distributed at varying points along said common ground track, said common ground track comprising n northbound segments and n southbound segments, a portion of each northbound segment being substantially adjacent to a portion of a southbound segment; and

a plurality of user terminals for obtaining said communications services from said N satellites in said zero-drift constellation, wherein a user terminal includes means for establishing a first link with a first satellite traveling in a first direction along said common ground track, means for using said first link to establish a communications channel for obtaining said communications services until a first crossover point is reached, means for establishing, at said first crossover point, a second link with a second satellite traveling along a southbound segment of said common ground track, means for handing-off said communications channel from said first link to said second link, and means for using said second link for obtaining said communications services until a second crossover point is reached.

2. The satellite communications system as claimed in claim 1, wherein said N satellites have inclinations between 85 and 95 degrees.

3. The satellite communications system as claimed in claim 1, wherein said N satellites have Right Ascension of the Ascending Node (RAAN) offsets defined by (360-360/N) degrees.

4. The satellite communications system as claimed in claim 1, wherein satellites in adjacent orbital planes are separated from each other by a phasing angle substantially equal to 360*n/N degrees.

5. The satellite communications system as claimed in claim 1, wherein northbound segments are associated with northbound satellites, said northbound satellites moving across the equator at a first point from a southern hemisphere to a northern hemisphere;

and southbound segments are associated with southbound satellites, said southbound satellites moving across the equator at a second point from said northern hemisphere to said southern hemisphere, said first point and said second point occurring at substantially equal points of longitude.

6. The satellite communications system as claimed in claim 5, wherein said crosslinks are established between a northbound satellite and a southbound satellite that are tracing adjacent segments of the common ground track and between satellites which are tracing non-adjacent segments of the common ground track.

7. The satellite communications system as claimed in claim 1, wherein said first crossover point and said second crossover point occur when a southbound satellite'"'"'s ground track and a northbound satellite'"'"'s ground track cross the same latitude.

8. The satellite communications system as claimed in claim 1, wherein said first crossover point and said second crossover point occur when a southbound satellite'"'"'s elevation angle and a northbound satellite'"'"'s elevation angle are substantially equal with respect to a user terminal'"'"'s field of view.

9. The satellite communications system as claimed in claim 1, wherein said user terminal has a limited tracking range which is defined by a single degree of steering freedom.

10. The satellite communications system as claimed in claim 1, wherein said user terminal includes means for establishing, at said second crossover point another link with another satellite traveling in said first direction along said common ground track, means for handing-off said communications channel from said second link to said another link, and means for using said another link to obtain said communications services until said first crossover point is reached.

11. The satellite communications system as claimed in claim 10, wherein said N satellites include means for establishing a communication link with a control center.

12. A user terminal for obtaining communications services from N satellites in a zero-drift constellation, said N satellites being located in N orbital planes which are equally spaced around the earth at the equator, where N is an integer greater than two, wherein said N satellites are interconnected using crosslinks, all of said N satellites trace a common ground track which repeats after n revolutions, where n is an odd integer which is in a range from seven to fifteen, and each satellite is phased with respect to satellites in adjacent orbital planes so that said N satellites are distributed at varying points along said common ground track, said common ground track comprising n northbound segments and n southbound segments, a portion of each northbound segment being substantially adjacent to a portion of a southbound segment, wherein said user terminal comprises:

an antenna subsystem comprising at least one controllable antenna and controller coupled to said at least one controllable antenna;

transceiver coupled to said antenna subsystem; and

processor coupled to said transceiver, said user terminal including means for establishing a communications channel with a first satellite in said zero-drift constellation as said first satellite travels along a northbound segment of said common ground track, said antenna subsystem comprising tracking means for tracking said first satellite until a first crossover point is reached, said user terminal further including means for handing-off said communications channel to a second satellite in said zero-drift constellation as said second satellite travels along an adjacent southbound segment of said common ground track, said tracking means comprising means for tracking said second satellite until a second crossover point is reached, wherein said at least one controllable antenna has a limited tracking range, said limited tracking range being defined by said two crossover points determined by satellite positions in said zero-drift constellation, and said controller comprising means for controlling a pointing direction within said limited tracking range.

13. The user terminal as claimed in claim 12, wherein said user terminal further comprises a user interface.

14. The user terminal as claimed in claim 12, wherein said tracking means moves said at least one controllable antenna in one dimension over a restricted range of elevation angles, said restricted range being determined by said crossover points.

15. The user terminal as claimed in claim 12, wherein said tracking means moves said at least one controllable antenna within a limited angular range, wherein said limited angular range is determined by two limits which are defined by elevation angles.

16. The user terminal as claimed in claim 12, wherein said tracking means includes means for tracking said first satellite from a first elevation angle to a second elevation angle and means for tracking said second satellite from said second elevation angle to a third elevation angle, wherein said first elevation angle and said third elevation angle are substantially equal.

17. A method of operating a satellite communications system for providing communications services, said method comprising the steps of:

a) establishing a zero-drift constellation having N crosslinked satellites, where N is a integer greater than two, the satellites being located in N orbital planes, orbiting the earth in circular orbits, and producing a common ground track which repeats after n revolutions, where n is odd integer in a range from seven to fifteen, said common ground track comprising n northbound segments and n southbound segments, a portion of each northbound segment being substantially adjacent to a portion of a southbound segment;

b) establishing a communications channel to provide said communications services using a link between a user terminal and a first satellite moving along a northbound segment, said user terminal having an antenna which tracks said satellites over a restricted range of elevation angles;

c) maintaining said communications channel as said first satellite moves in a first direction through a field of view of said user terminal;

d) establishing a crosslink between said first satellite and a second satellite moving in a second direction alone a southbound segment;

e) establishing a new link between said second satellite and said user terminal;

f) handing-off said communications channel to said second satellite at a first crossover point;

g) maintaining said communications channel as said second satellite moves in a second direction through said field of view of said user terminal;

h) establishing a new crosslink between said second satellite and a third satellite moving along said northbound segment;

i) establishing another new link between said third satellite and said user terminal;

j) handing-off said communications channel to said third satellite at a second crossover point; and

k) repeating steps c) through i) as long as communications services are being provided to said user terminal.

18. The method as claimed in claim 17, wherein step a) further comprises the step of:

inserting one of said N crosslinked satellites into each of said N orbital planes, said N orbital planes having inclinations between 85 and 95 degrees and wherein said N orbital planes have equatorial offsets defined by (360-360/N) degrees, wherein satellites in adjacent orbital planes being phased using a phasing angle substantially equal to 360*n/N degrees.

19. A method of operating a satellite in a zero-drift constellation to establish a communications channel with a user terminal, said zero-drift constellation having N crosslinked satellites, where N is a integer greater than two, the satellites being located in N orbital planes, orbiting the earth in circular orbits, and producing a common ground track which repeats after n revolutions, where n is odd integer in a range from seven to fifteen, said common ground track comprising n northbound segments and n southbound segments, a portion of each northbound segment being substantially adjacent to a portion of a southbound segment, said method comprising the steps of:

a) establishing said communications channel by providing a link with said user terminal when said satellite is moving in a first direction along a northbound segment;

b) retaining said communications channel by maintaining said link with said user terminal as said satellite moves in said first direction along said northbound segment;

c) determining when a second satellite moving in a second direction along an adjacent southbound segment is visible to said satellite;

d) establishing a crosslink with said second satellite moving in said second direction along said adjacent southbound segment; and

e) handing-off said communications channel to said second satellite at a crossover point.

20. A method of operating a user terminal to establish a communications channel using at least one satellite in a zero-drift constellation, said zero-drift constellation having N crosslinked satellites, where N is a integer greater than two, the satellites being located in N orbital planes, orbiting the earth in circular orbits, and producing a common ground track which repeats after n revolutions where n is odd integer in a range from seven to fifteen, said common ground track comprising n northbound segments and n southbound segments, a portion of each northbound segment being substantially adjacent to a portion of a southbound segment, said method comprising the steps of:

a) creating said communications channel by establishing a first link with a first satellite moving along a northbound segment;

b) retaining said communications channel by maintaining said first link, wherein said user terminal tracks said first satellite as it moves along said northbound segment in a first direction relative to said user terminal;

c) determining when a second satellite moving along an adjacent southbound segment is visible to said user terminal;

d) establishing a second link with said second satellite;

e) handing-off said communications channel to said second satellite at a first crossover point;

f) retaining said communications channel by maintaining said second link, wherein said user terminal tracks said second satellite as it moves along said southbound segment in a second direction relative to said user terminal;

g) determining when a third satellite moving along said northbound segment is visible to said user terminal;

h) establishing a new first link with said third satellite;

i) handing-off said communications channel to said third satellite; and

j) repeating steps b) through i) as long as said communications channel is required.