Satellite communications system and handover processing method

• US 6,512,920 B1
• Filed: 05/04/2000
• Issued: 01/28/2003
• Est. Priority Date: 05/06/1999
• Status: Active Grant

• Current Assignee: NEC Corporation
• Original Assignee: NEC Corporation
• Inventors: Yaoya, Naoko
• Primary Examiner(s): Hunter, Daniel
• Assistant Examiner(s): TRAN, TUAN A
• Application Number:
  US09/564,775
• Time in Patent Office: 999 Days
• Field of Search: 455/427, 455/428, 455/430, 455/436-439, 455/12.1, 455/13-- 1
• US Class Current: 455/428
• CPC Class Codes: H04B 7/18541 for handover of resources
• CPC Class Codes: H04W 36/18 for allowing seamless reselection, e.g. soft reselection
• CPC Class Codes: H04W 76/10 Connection setup
• CPC Class Codes: H04W 84/06 Airborne or Satellite Networks space-based or airborne stations H04B7/185
• CPC Class Codes: H04W 92/16 Interfaces between hierarchically similar devices

Abstract

A handover processing method in a satellite communications system allowing stable and reliable handover process is disclosed. Schedule information is previously registered in each of the satellites. The schedule information of each satellite includes a radius of a coverage area thereof, a point of time provided thereto, and physical locations of a center point of the coverage area thereof according to the point of time. A handover start time is calculated based on the schedule information and the physical location of a mobile station. A control signal regarding the handover start time is transmitted to an adjacent satellite. Since the handover start time is known in advance at both the first satellite and the adjacent satellite, the handover can be performed and completed between them without sending handover request signal.

Claims

1. A satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite using an established communication link,

each of the satellites comprising:

a ground communication section for communicating with stations located in the coverage areas on the earth;

an intersatellite communication section for communicating with other satellites through intersatellite communications links; and

a controller controlling the ground and intersatellite communication sections to perform a handover process,

wherein

in a first satellite forming a coverage area in which the station is located, a first controller calculates a handover start time at which the handover process should be started for the station, and controls the intersatellite communication section so as to transmit a control signal having a handover completion time added thereto to an adjacent satellite that is to be a handover destination satellite, wherein the handover completion time is calculated from the handover start time, and

in a second satellite receiving the control signal having the handover completion time added thereto from the first satellite, a second controller performs the handover process by establishing a new communication link to the station before the handover completion time.

2. The satellite communications system according to claim 1, wherein, in the first satellite, the first controller releases the communication link used to communicate with the station at the handover completion time calculated from the handover start time.

3. The satellite communications system according to claim 1, wherein, in the first satellite, the first controller registers a size of a coverage area formed by each of the satellites, first schedule information indicating physical locations immediately below the first satellite according to a point of time provided to the first satellite, and second schedule information indicating physical locations immediately below the adjacent satellite according to a point of time provided to the adjacent satellite,

wherein the first controller calculates the handover start time from a leaving time required for the physical location of the station to leave the coverage area of the first satellite and an entering time required for the physical location of the station to enter the coverage area of the adjacent satellite, wherein the leaving time and the entering time are calculated based on sizes of first and second coverage areas formed respectively by the first and adjacent satellites, the first and second schedule information, and the physical location of the station covered by the first satellite.

4. The satellite communications system according to claim 3, wherein, when the adjacent satellite is the second satellite following the first satellite on the non-stationary orbit,

the leaving time is calculated by measuring a distance the station moves on the first coverage area of the first satellite which is a length of a straight line joining the physical location of the station with an intersection between an edge of the first coverage area and a straight line passing through the physical location of the station and parallel with a movement direction of the first coverage area with respect to the ground, and dividing the distance the station moves by a movement speed of the first coverage area, and

the entering time is calculated by measuring a distance the station moves toward the second coverage area of the second satellite which is a length of a straight line joining the physical location of the station with an intersection between an edge of the second coverage area and a straight line passing through the physical location of the station and parallel with a movement direction of the first coverage area with respect to the ground, and dividing the distance the station moves by a movement speed of the second coverage area.

5. A satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite,

each of the satellites comprising:

a ground communication section for communicating with stations on the earth;

an intersatellite communication section for communicating with other satellites through intersatellite communications links; and

a controller controlling the ground and intersatellite communication sections to perform a handover process,

wherein

in a first satellite forming a coverage area in which the station is located, a first controller calculates a handover start time at which the handover process should be started for the station, and controls a first intersatellite communication section so as to transmit a control signal having a handover completion time added thereto and communication data received from the station at the handover start time and having next handover-related data added thereto to an adjacent satellite that is to be a handover destination satellite, wherein the handover completion time is calculated from the handover start time, and

in a second satellite receiving from the first satellite the control signal having the handover completion time added thereto and the communication data having the next handover-related data added thereto, a second controller performs the handover process by establishing a new communication link to the station before the handover completion time, compares communication data received from the station using the new communication link with the next handover-related data to determine whether the communication data and the next handover-related data are in sequence, and controls second ground and intersatellite communication sections such that a sequence of communication data is received from the station and is transmitted toward an opposite-side station communicating with the station.

6. The satellite communications system according to claim 5, wherein, in the first satellite, the first controller releases the communication link used to communicate with the station at the handover completion time calculated from the handover start time.

7. The satellite communications system according to claim 5, wherein,

in the second satellite, the second controller controls the second intersatellite communication section such that the next handover-related data received from the first satellite is transmitted to the first satellite so that it is received by the first satellite at the handover completion time, and

in the first satellite, the first controller releases the communication link used to communicate with the station when having received the next handover-related data from the second satellite.

8. A satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite,

each of the satellites comprising:

a ground communication section for communicating with stations on the earth;

an intersatellite communication section for communicating with other satellites through intersatellite communications links;

a storage device for storing data received from other satellites through the intersatellite communications links;

a selector for selecting one of data received by the ground communication section and data stored in the storage device; and

a controller controlling the ground and intersatellite communication sections and the selector to perform a handover process,

wherein in a first satellite forming a coverage area in which the station is located,

a first controller calculates a handover start time T−

t at which the handover process should be started for the station, wherein T is a handover completion time and t is a time required for the handover process, and controls a first intersatellite communication section so as to transmit a control signal having the handover completion time T added thereto and communication data received from the station at the handover start time T−

t and having next handover-related data added thereto to an adjacent satellite that is to be a handover destination satellite, wherein the handover completion time T is calculated from the handover start time T−

t, and

in a second satellite receiving from the first satellite the control signal having the handover completion time T added thereto and the communication data having the next handover-related data added thereto,

a second controller performs the handover process by establishing a new communication link to the station before a lapse of the handover completion time T, inputing data from the second ground communication section using the new communication link, inputting data from a second storage device when the selector is switched by the second intersatellite communication section at the handover completion time T, comparing communication data received from the station using the new communication link with the next handover-related data to determine whether the communication data and the next handover-related data are in sequence, and controls second ground and intersatellite communication sections such that a sequence of communication data is received from the station and is transmitted toward an opposite-side station communicating with the station.

9. The satellite communications system according to claim 8, wherein, in the first satellite, the first controller releases the communication link used to communicate with the station at the handover completion time T calculated from the handover start time.

10. The satellite communications system according to claim 8, wherein,

in the second satellite, the second controller controls the second intersatellite communication section such that the next handover-related data received from the first satellite is transmitted to the first satellite so that it is received by the first satellite at the handover completion time T, and

in the first satellite, the first controller controls the first ground communication section so as to transmit last communication data to the station before releasing the communication link used to communicate with the station when having received the next handover-related data from the second satellite.

11. A satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite using an established communication link,

each of the satellites comprising:

a ground communication section for communicating with stations located in the coverage areas on the earth;

an intersatellite communication section for communicating with other satellites through intersatellite communications links; and

a controller controlling the ground and intersatellite communication sections to perform a handover process,

wherein

in a first satellite forming a coverage area in which a station is located, a first controller calculates a handover start time at which the handover process should be started for the station, and controls the intersatellite communication section so as to transmit a control signal having a handover completion time added thereto to a second satellite that is to be a handover destination satellite adjacent to the first satellite using a first intersatellite communication link and a third satellite which is communicating with the first satellite using a second intersatellite communication link, wherein the handover completion time is calculated from the handover start time, and

in the second satellite receiving the control signal having the handover completion time added thereto from the first satellite, a second controller performs the handover process by establishing a new communication link to the station before the handover completion time, receiving communication data from the station using the new communication link, and transmitting the received communication data to the third satellite using a third intersatellite communication link.

12. The satellite communications system according to claim 11, wherein, in the first satellite, the first controller releases the communication link used to communicate with the station at the handover completion time calculated from the handover start time.

13. The satellite communications system according to claim 11, wherein,

in the first satellite, the first controller adds next handover-related data to communication data received from the station at the handover start time and controls the first intersatellite communication section so as to transmit the next handover-related data to the second satellite using the first intersatellite communication link,

in the second satellite, the second controller controls the second intersatellite communication section such that the next handover-related data received from the first satellite is transmitted to the first satellite so that it is received by the first satellite at the handover completion time, and

in the first satellite, the first controller releases the communication link used to communicate with the station when having received the next handover-related data from the second satellite.

14. The satellite communications system according to claim 11, wherein,

in the third satellite, a third controller controls a third ground communication section so as to transmit communication data received from an opposite-side station communicating with the station to the second satellite at the handover start time,

in the second satellite, the second controller controls the second ground communication section such that a new communication link to the station is established before the handover completion time and the communication data received from the third satellite is transmitted to the station.

15. The satellite communications system according to claim 11, wherein,

in the first satellite, the first controller adds next handover-related data to communication data received from the station at the handover start time and controls the first intersatellite communication section so as to transmit the next handover-related data to the third satellite using the second intersatellite communication link,

in the third satellite, the third controller compares communication data having the next handover-related data received from the first satellite station with the communication data received from the second satellite to determine whether these received communication data are in sequence, and controls third ground and intersatellite communication sections such that a sequence of communication data is received from the station and is transmitted toward the opposite-side station communicating with the station.

16. The satellite communications system according to claim 11, wherein,

in the first satellite, the first controller adds next handover-related data to communication data received from the station at the handover start time and controls the first intersatellite communication section so as to transmit the next handover-related data to the second satellite using the first intersatellite communication link,

in the second satellite, the second controller controls the second intersatellite communication section such that the next handover-related data received from the first satellite is transmitted to the third satellite so that it is received by the third satellite at the handover completion time, and

in the third satellite, the third controller compares next handover-related data received from the first satellite station with the next handover-related data received from the second satellite to determine whether these received next handover-related data are identical to each other, and, if these received next handover-related data are identical, then the third controller determines that the second satellite is the handover destination satellite, and controls third ground and intersatellite communication sections such that a forwarding destination of communication data received from an opposite-side station communicating with the station is switched from the first satellite to the second satellite.

17. The satellite communications system according to claim 11, wherein the first controller calculates the handover start time by subtracting t from TA on condition that there are TA and TB satisfying a first expression:

18. The satellite communications system according to claim 17, wherein the first controller calculates the handover start time for the station by equally dividing an area satisfying the first expression (1) and covered by the first satellite in a direction perpendicular to a movement direction of the first coverage area, and determining which of divisional areas the station is located in.

19. The satellite communications system according to claim 11, wherein the first controller calculates the handover start time for a handover destination satellite by subtracting t from Ta, wherein the hanover destination satellite is determined to be a satellite satisfying a second expression:

20. The satellite communications system according to claim 19, wherein, when a plurality of adjacent satellites satisfy the second expression (2), an adjacent satellite having the largest value of Ti+t is selected as the handover destination satellite.

21. A satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite using an established communication link,

each of the satellites comprising:

a ground communication section f or communicating with stations located in the coverage areas on the earth;

an intersatellite communication section for communicating with other satellites through intersatellite communications links; and

a controller controlling the ground and intersatellite communication sections to perform a handover process,

wherein in a first satellite forming a coverage area in which the station is located,

a first controller calculates a handover start time T−

t at which the handover process should be started for the station, wherein T is a handover completion time and t is a time required for the handover process, controls a first intersatellite communication section so as to transmit a control signal having the handover completion time T added thereto to a second satellite that is to be a handover destination satellite adjacent to the first satellite using a first intersatellite communication link and a third satellite which is communicating with the first satellite using a second intersatellite communication link, and controls the first intersatellite communication section so as to transmit first communication data having next handover-related data added thereto to the third satellite at the handover star time T−

t, wherein the handover completion time is calculated from the handover start time, and

in the second satellite receiving the control signal having the handover completion time T added thereto from the first satellite, a second controller performs the handover process by establishing a new communication link to the station before the handover start time T−

t, and transmitting second communication data to the third satellite using a third intersatellite communication link, wherein the second communication data is received from the station using the new communication link,

in the third satellite receiving the first communication data from the first satellite and the second communication data from the second satellite, a third controller compares the first communication data having the next handover-related data added thereto with the second communication data to determine whether these received communication data are in sequence, and controls third ground and intersatellite communication sections such that a sequence of communication data received from the station is transmitted toward an opposite-side station communicating with the station.

22. The satellite communications system according to claim 21, wherein, in the first satellite, the first controller releases the communication link used to communicate with the station at the handover completion time T calculated from the handover start time T−

t.

23. The satellite communications system according to claim 21, wherein,

in the second satellite, the second controller controls the second intersatellite communication section such that the next handover-related data received from the first satellite is transmitted to the first satellite so that it is received by the first satellite at the handover completion time T, and

in the first satellite, the first controller controls the first ground communication section so as to transmit last communication data to the station before releasing the communication link used to communicate with the station when having received the next handover-related data from the second satellite.

24. The satellite communications system according to claim 21, wherein,

in the first satellite, the first controller adds next handover-related data to communication data received from the station at the handover start time T−

t and controls the first intersatellite communication section so as to transmit the communication data having the next handover-related data added thereto to the second satellite using the first intersatellite communication link,

in the second satellite, the second controller controls the second intersatellite communication section such that the next handover-related data received from the first satellite is transmitted to the third satellite so that it is received by the third satellite at the handover completion time T, and

in the third satellite, the third controller compares next handover-related data received from the first satellite station with the next handover-related data received from the second satellite to determine whether these received next handover-related data are identical to each other, and, if these received next handover-related data are identical, then the third controller determines that the second satellite is the handover destination satellite, and controls third ground and intersatellite communication sections such that a forwarding destination of communication data received from an opposite-side station communicating with the station is switched from the first satellite to the second satellite.

25. A handover processing method in a satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite using an established communication link,

each of the satellites including:

a ground communication section for communicating with stations located in the coverage areas on the earth;

an intersatellite communication section for communicating with other satellites through intersatellite communications links; and

a controller controlling the ground and intersatellite communication sections to perform a handover process,

the method comprising the steps of;

at a first satellite forming a coverage area in which the station is located,

a) calculating a handover start time at which the handover process should be started for the station;

b) transmitting a control signal having a handover completion time added thereto to an adjacent satellite that is to be a handover destination satellite, wherein the handover completion time is calculated from the handover start time; and

at a second satellite receiving the control signal having the handover completion time added thereto from the first satellite,

c) performing the handover process by establishing a new communication link to the station before the handover completion time.

26. The handover processing method according to claim 25, further comprising the steps of:

at the first satellite,

releasing the communication link used to communicate with the station at the handover completion time calculated from the handover start time.

27. The handover processing method according to claim 25, further comprising the step of:

at the first satellite,

registering a size of a coverage area formed by each of the satellites, first schedule information indicating physical locations immediately below the first satellite according to a point of time provided to the first satellite, and second schedule information indicating physical locations immediately below the adjacent satellite according to a point of time provided to the adjacent satellite,

wherein, in the step (a), the handover start time is calculated from a leaving time required for the physical location of the station to leave the coverage area of the first satellite and an entering time required for the physical location of the station to enter the coverage area of the adjacent satellite, wherein the leaving time and the entering time are calculated based on sizes of first and second coverage areas formed respectively by the first and adjacent satellites, the first and second schedule information, and the physical location of the station covered by the first satellite.

28. The handover processing method according to claim 27, wherein, when the adjacent satellite is the second satellite following the first satellite on the non-stationary orbit,

the leaving time is calculated by measuring a distance the station moves on the first coverage area of the first satellite which is a length of a straight line joining the physical location of the station with an intersection between an edge of the first coverage area and a straight line passing through the physical location of the station and parallel with a movement direction of the first coverage area with respect to the ground, and dividing the distance the station moves by a movement speed of the first coverage area, and

the entering time is calculated by measuring a distance the station moves toward the second coverage area of the second satellite which is a length of a straight line joining the physical location of the station with an intersection between an edge of the second coverage area and a straight line passing through the physical location of the station and parallel with a movement direction of the first coverage area with respect to the ground, and dividing the distance the station moves by a movement speed of the second coverage area.

29. The handover processing method according to claim 27, wherein in the step (a), the handover start time is calculated by subtracting t from TA on condition that there are TA and TB satisfying a first expression:

30. The handover processing method according to claim 29, wherein in the step (a), the handover start time is calculated for the station by equally dividing an area satisfying the first expression (1) and covered by the first satellite in a direction perpendicular to a movement direction of the first coverage area, and determining which of divisional areas the station is located in.

31. The handover processing method according to claim 29, wherein in the step (a), the handover start time is calculated for a handover destination satellite by subtracting t from Ta, wherein the hanover destination satellite is determined to be a satellite satisfying a second expression:

32. The handover processing method according to claim 31, wherein, when a plurality of adjacent satellites satisfy the second expression (2), an adjacent satellite having the largest value of Ti+t is selected as the handover destination satellite.

33. A handover processing method in a satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite,

each of the satellites comprising:

a ground communication section for communicating with stations on the earth;

an intersatellite communication section for communicating with other satellites through intersatellite communications links; and

a controller controlling the ground and intersatellite communication sections to perform a handover process,

the method comprising the steps of;

at a first satellite forming a coverage area in which the station is located,

a) calculating a handover start time at which the handover process should be started for the station;

b) transmitting a control signal having a handover completion time added thereto and communication data received from the station at the handover start time and having next handover-related data added thereto to an adjacent satellite that is to be a handover destination satellite, wherein the handover completion time is calculated from the handover start time;

at a second satellite receiving from the first satellite the control signal having the handover completion time added thereto and the communication data having the next handover-related data added thereto,

c) establishing a new communication link to the station before the handover completion time;

d) comparing communication data received from the station using the new communication link with the next handover-related data to determine whether the communication data and the next handover-related data are in sequence;

e) receiving a sequence of communication data from the station; and

f) transmitting the sequence of communication data toward an opposite-side station communicating with the station.

34. The handover processing method according to claim 33, further comprising the steps of:

at the first satellite,

releasing the communication link used to communicate with the station at the handover completion time calculated from the handover start time.

35. The handover processing method according to claim 33, further comprising the steps of:

at the second satellite,

transmitting the next handover-related data received from the first satellite to the first satellite so that it is received by the first satellite at the handover completion time; and

at the first satellite,

releasing the communication link used to communicate with the station when having received the next handover-related data from the second satellite.

36. A handover processing method in a satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite,

each of the satellites comprising:

a ground communication section for communicating with stations on the earth;

an intersatellite communication section for communicating with other satellites through intersatellite communications links;

a storage device for storing data received from other satellites through the intersatellite communications links;

a selector for selecting one of data received by the ground communication section and data stored in the storage device; and

a controller controlling the ground and intersatellite communication sections and the selector to perform a handover process,

at a first satellite forming a coverage area in which the station is located,

a) calculating a handover start time T−

t at which the handover process should be started for the station, wherein T is a handover completion time and t is a time required for the handover process;

b) transmitting a control signal having the handover completion time T added thereto and communication data received from the station at the handover start time T−

t and having next handover-related data added thereto to an adjacent satellite that is to be a handover destination satellite, wherein the handover completion time T is calculated from the handover start time T−

t;

at a second satellite receiving from the first satellite the control signal having the handover completion time T added thereto and the communication data having the next handover-related data added thereto,

c) establishing a new communication link to the station before a lapse of the handover completion time T,

d) inputing data from the second ground communication section using the new communication link;

e) inputting data from a second storage device when the selector is switched by the second intersatellite communication section at the handover completion time T;

f) comparing communication data received from the station using the new communication link with the next handover-related data to determine whether the communication data and the next handover-related data are in sequence;

g) receiving a sequence of communication data from the station; and

h) transmitting the sequence of communication data toward an opposite-side station communicating with the station.

37. The handover processing method according to claim 36, further comprising the step of:

at the first satellite,

releasing the communication link used to communicate with the station at the handover completion time T calculated from the handover start time.

38. The handover processing method according to claim 36, further comprising the steps of:

at the second satellite,

transmitting the next handover-related data received from the first satellite to the first satellite so that it is received by the first satellite at the handover completion time T;

at the first satellite,

transmitting last communication data to the station before releasing the communication link used to communicate with the station, when having received the next handover-related data from the second satellite.

39. A handover processing method in a satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite using an established communication link,

each of the satellites comprising:

a ground communication section for communicating with stations located in the coverage areas on the earth;

an intersatellite communication section for communicating with other satellites through intersatellite communications links; and

a controller controlling the ground and intersatellite communication sections to perform a handover process,

the method comprising the steps of;

at a first satellite forming a coverage area in which a station is located,

a) calculating a handover start time at which the handover process should be started for the station;

b) transmitting a control signal having a handover completion time added thereto to a second satellite that is to be a handover destination satellite adjacent to the first satellite using a first intersatellite communication link and a third satellite which is communicating with the first satellite using a second intersatellite communication link, wherein the handover completion time is calculated from the handover start time;

at the second satellite receiving the control signal having the handover completion time added thereto from the first satellite,

c) performing the handover process by establishing a new communication link to the station before the handover completion time;

d) receiving communication data from the station using the new communication link; and

e) transmitting the received communication data to the third satellite using a third intersatellite communication link.

40. The handover processing method according to claim 39, further comprising the step of:

at the first satellite,

releasing the communication link used to communicate with the station at the handover completion time calculated from the handover start time.

41. The handover processing method according to claim 39, further comprising the steps of:

at the first satellite,

adding next handover-related data to communication data received from the station at the handover start time; and

transmitting the next handover-related data to the second satellite using the first intersatellite communication link;

at the second satellite,

transmitting the next handover-related data received from the first satellite to the first satellite so that it is received by the first satellite at the handover completion time; and

at the first satellite,

releasing the communication link used to communicate with the station when having received the next handover-related data from the second satellite.

42. The handover processing method according to claim 39, further comprising the steps of:

at the third satellite,

transmitting communication data received from an opposite-side station communicating with the station to the second satellite at the handover start time;

at the second satellite,

establishing a new communication link to the station before the handover completion time; and

transmitting the communication data received from the third satellite to the station.

43. The handover processing method according to claim 39, further comprising the steps of:

at the first satellite,

adding next handover-related data to communication data received from the station at the handover start time; and

transmitting the next handover-related data to the third satellite using the second intersatellite communication link;

at the third satellite,

comparing communication data having the next handover-related data received from the first satellite station with the communication data received from the second satellite to determine whether these received communication data are in sequence;

receiving a sequence of communication data from the station; and

transmitting the sequence of communication data toward the opposite-side station communicating with the station.

44. The handover processing method according to claim 39, further comprising the steps of:

at the first satellite,

adding next handover-related data to communication data received from the station at the handover start time; and

transmitting the next handover-related data to the second satellite using the first intersatellite communication link;

at the second satellite,

transmitting the next handover-related data received from the first satellite to the third satellite so that it is received by the third satellite at the handover completion time;

at the third satellite,

comparing next handover-related data received from the first satellite station with the next handover-related data received from the second satellite to determine whether these received next handover-related data are identical to each other;

when these received next handover-related data are identical, determining that the second satellite is the handover destination satellite; and

switching a forwarding destination of communication data received from an opposite-side station communicating with the station from the first satellite to the second satellite.

45. A handover processing method in a satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite using an established communication link,

each of the satellites comprising:

a ground communication section for communicating with stations located in the coverage areas on the earth;

an intersatellite communication section for communicating with other satellites through intersatellite communications links; and

a controller controlling the ground and intersatellite communication sections to perform a handover process,

the method comprising the steps of;

at a first satellite forming a coverage area in which the station is located,

a) calculating a handover start time T−

t at which the handover process should be started for the station, wherein T is a handover completion time and t is a time required for the handover process;

b) transmitting a control signal having the handover completion time T added thereto to a second satellite that is to be a handover destination satellite adjacent to the first satellite using a first intersatellite communication link and a third satellite which is communicating with the first satellite using a second intersatellite communication link;

c) transmitting first communication data having next handover-related data added thereto to the third satellite at the handover star time T−

t, wherein the handover completion time is calculated from the handover start time;

at the second satellite receiving the control signal having the handover completion time T added thereto from the first satellite,

d) establishing a new communication link to the station before the handover start time T−

t;

e) transmitting second communication data to the third satellite using a third intersatellite communication link, wherein the second communication data is received from the station using the new communication link;

at the third satellite receiving the first communication data from the first satellite and the second communication data from the second satellite,

f) comparing the first communication data having the next handover-related data added thereto with the second communication data to determine whether these received communication data are in sequence; and

g) transmitting a sequence of communication data received from the station toward an opposite-side station communicating with the station.

46. The handover processing method according to claim 45, further comprising the step of:

at the first satellite,

releasing the communication link used to communicate with the station at the handover completion time T calculated from the handover start time T−

t.

47. The handover processing method according to claim 45, further comprising the step of:

at the second satellite,

transmitting the next handover-related data received from the first satellite to the first satellite so that it is received by the first satellite at the handover completion time T;

at the first satellite,

transmitting last communication data to the station before releasing the communication link used to communicate with the station when having received the next handover-related data from the second satellite.

48. The handover processing method according to claim 45, further comprising the step of:

at the first satellite,

adding next handover-related data to communication data received from the station at the handover start time T−

t;

transmitting the communication data having the next handover-related data added thereto to the second satellite using the first intersatellite communication link;

at the second satellite,

transmitting the next handover-related data received from the first satellite to the third satellite so that it is received by the third satellite at the handover completion time T;

at the third satellite,

comparing next handover-related data received from the first satellite station with the next handover-related data received from the second satellite to determine whether these received next handover-related data are identical to each other;

when these received next handover-related data are identical, determining that the second satellite is the handover destination satellite; and

switching a forwarding destination of communication data received from an opposite-side station communicating with the station from the first satellite to the second satellite.

49. A handover processing method in a satellite communications system using a plurality of satellites which are arranged on at least one non-stationary orbit, wherein a plurality of satellites on a non-stationary orbit emit beams to form a plurality of coverage areas covering a circumference of the earth and a station located in a coverage area formed by a satellite communicates with the satellite using an established communication link, the method comprising the steps of:

previously registering schedule information in each of the satellites, the schedule information including a radius of a coverage area thereof, a point of time provided thereto, and physical locations of a center point of the coverage area thereof according to the point of time;

determining a physical location of a first station which is located in a first coverage area of a first satellite;

calculating a leaving time required for the first satellite to leave the first coverage area from the physical location of the first station, the radius and a physical location of the center point of the first coverage area at a current point of time;

calculating a handover start time based on a difference between the leaving time and a predetermined handover time required for handover process; and

transmitting a control signal including the leaving time to an adjacent satellite to perform the handover process.