Method and apparatus for providing satellite service using satellites in inclined geosynchronous orbits

Abstract

Disclosed methods and apparatuses involve the coordinated use of two or more satellites operating in inclined orbits, with a defined phase offset in terms of their latitudinal oscillations above and below the geosynchronous arc.

Description

METHODS AND APPARATUSES FOR PROVIDING SATELLITE SERVICE USING SATELLITES IN INCLINED GEOSYNCHRONOUS ORBITSTECHNICAL FIELD

[0001] Embodiments disclosed herein relate to satellite communications systems, and particularly relate to providing satellite-based communication services using satellites in inclined geosynchronous orbits.BACKGROUND

[0002] A geosynchronous equatorial orbit is a circular orbit that follows the rotation of the Earth at an altitude of 22,236 miles above the equator. A satellite in a geosynchronous equatorial orbit remains at a fixed location relative to the surface of the Earth. Such orbits are therefore referred to as geostationary orbits, with the term “geostationary arc” referring to the corresponding path in space above the equator where geostationary satellites orbit. Satellite operators operate geostationary satellites in assigned orbital slots along the geostationary arc, with a two-degree spacing along the arc being a customary separation.

[0003] If the orbital plane is inclined relative to the geostationary arc (geo arc), the satellite may be referred to as following an inclined geosynchronous orbit and herein the term is shortened to “inclined orbit.” A satellite following an inclined orbit is stationary relative to the surface of the Earth in terms of its longitudinal position, but it moves latitudinally.

[0004] Specifically, a satellite in an inclined orbit maintains a nominally fixed longitudinal position but has a latitudinal position that oscillates above and below the geostationary arc according to an oscillation cycle that repeats each orbital period of the satellite. From the perspective of a ground observer at the corresponding longitudinal position on the surface of the Earth, the satellite traces a figure-eight shape in the sky each day. For circular inclined orbits, the figure-eight is symmetrical above and below the geosynchronous arc, and asymmetrical for elliptical inclined orbits.

[0005] One example use of inclined orbits relates to end-of-life operations. A satellite assigned for geostationary operation in a given orbital slot performs station keeping to maintain its assigned longitudinal position in the geosynchronous arc and to avoid drifting latitudinally. Relaxing the station keeping requirements by allowing the satellite to take on an inclined orbit reduces fuel consumption and thereby increases the remaining service life of the satellite. Current regulations limit the maximum inclination to plus or minus fifteen degrees.SUMMARY

[0006] Disclosed methods and apparatuses involve the coordinated use of two or more satellites operating in inclined orbits, with a defined phase offset in terms of their latitudinal oscillations above and below the geosynchronous arc. The arrangement puts one or more of the satellites in an advantaged position at least for a portion of the day, with respect to serving terrestrial terminals in a particular satellite service area. “Advantaged” position refers to separation from the geosynchronous arc or angle of elevation relative to the satellite service area. The arrangement allows the involved satellite communications system to serve types of terminals in the service area that do not support lower elevational angles otherwise allowed for in the system or terminals that are more sensitive to inter-satellite interference. Of course, one or more of these satellites may serve other terminals in the same or another service area, e.g., at different times and / or frequencies.

[0007] One embodiment comprises a method of operating a satellite communications system. The method includes maintaining a set of two or more candidate satellites in respective inclined orbits relative to the geosynchronous arc, such that each candidate satellite maintains a nominally fixed longitudinal position but has a latitudinal position that oscillates above and below the geosynchronous arc according to an oscillation cycle that repeats each day. The oscillation cycles of the two or more candidate satellites are offset in phase, and the method further includes dynamically selecting a serving satellite from among the two or more candidate satellites for serving one or more terrestrial terminals operating within a first satellite service area, based on a combined consideration of separation from the geostationary arc and elevation angle relative to the first satellite service area.

[0008] Another embodiment comprises a satellite communications system (SCS). The SCS includes a set of two or more candidate satellites operating in respective inclined orbits relative to the geosynchronous arc, such that each candidate satellite maintains a nominally fixed longitudinal position but has a latitudinal position that oscillates above and below the geosynchronous arc according to an oscillation cycle that repeats each day. The oscillation cycles of the two or more candidate satellites are offset in phase and the SCS further includes a communications processing system (CPS) in a ground network of the SCS. The CPS comprises one or more computer servers configured to dynamically select a serving satellite from among the two or more candidate satellites for serving one or more terrestrial terminals operating within a first satellite service area, based on a combined consideration of separation from the geostationary arc and elevation angle relative to the first satellite service area.

[0009] Of course, the present invention is not limited to the above features and advantages. Indeed, those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a block diagram of a satellite communications system (SCS), according to an example embodiment.

[0011] Figure 2 is a plot of an example ground track of a satellite operating in an inclined geostationary orbit.

[0012] Figure 3 is a logic flow diagram of a method of operation by a SCS, according to an example embodiment.

[0013] Figure 4 is a block diagram of details for a ground network of a SCS, according to an example embodiment.

[0014] Figure 5 is a block diagram of details for a communications processing system (CPS) of a ground network, according to an example embodiment.

[0015] Figure 6 is a diagram of example satellite service areas being served using inclined- orbit satellites, according to an example embodiment.

[0016] Figure 7 is a plot of relative phasing for a set of three satellites operating as a coordinated set of candidate satellites, according to an example embodiment.

[0017] Figure 8 is a table illustrating example satellite occupancy within ranges of elevation angle above and below the geosynchronous arc, according to an example embodiment.

[0018] Figure 9 is a diagram illustrating relative positioning of a set of candidate satellites operating with inclined orbits over successive intervals within a twenty-four span.DETAILED DESCRIPTION

[0019] Figure 1 illustrates a satellite communications system (SCS) 10 according to an example embodiment, where the SCS 10 includes a ground network 12 and a set of two or more satellites 14 that are maintained in respective inclined orbits 16 relative to the geosynchronous arc, such that each candidate satellite 14 maintains a nominally fixed longitudinal position as shown in Figure 2, but has a latitudinal position that oscillates above and below the geosynchronous arc according to an oscillation cycle that repeats each day. To avoid clutter, Figure 1 depicts only two candidate satellites 14, labeled as 14-1 and 14-2 for differentiation. The candidate satellite 14-1 has a corresponding inclined orbit 16-1 and the candidate satellite14-2 has an inclined orbit 16-2, and the oscillation cycles of the two or more candidate satellites 14 are offset in phase.

[0020] All candidate satellites 14 in the set of two or more candidate satellites 14 may have the same nominal longitudinal position, or they may have longitudinal positions spaced within a relatively small angular range, e.g., within ten degrees of longitude along the geosynchronous arc. In any case, saying that the respective candidate satellites 14 have offset phases — offset orbital phases — means that the respective candidate satellites 14 occupy different latitudinal positions in the sky at different times, relative to terrestrial terminals 20 operating in a first satellite service area 22. In Figure 2, the illustration of times t0-t4 refers to the figure-8 path effectively formed in the sky by each candidate satellite 14 over the course of each orbital period, with the phase offsetting resulting in respective ones of the two or more candidate satellites 14 being at different positions along the figure-8 path at any given time of the day.

[0021] According to this arrangement, at any given time of day, one of the candidate satellites 14 occupies an advantaged position in comparison to one or more other ones of the candidate satellites 14 with respect to serving terrestrial terminals 20 in the first satellite service area 22. “Advantaged” in this context connotes greater separation from the geosynchronous arc and / or a favorable elevation angle relative to the first satellite service area 22. Hence, the term “candidate” emphasizes that the set of two or more candidate satellites 14 are candidates for providing communication services to terrestrial terminals 20 in the first satellite service area 22, with different ones of the candidate satellites 14 being the favored choice for providing such service at different times of the day, as a consequence of the inclined orbits 16 and the corresponding orbital phase offsets.

[0022] Figure 3 illustrates a corresponding method 300 of operating the SCS 10. The method 300 includes maintaining (Block 302) a set of two or more candidate satellites 14 in respective inclined orbits 16 relative to the geosynchronous arc, such that each candidate satellite 14 maintains a nominally fixed longitudinal position but has a latitudinal position that oscillates above and below the geosynchronous arc according to an oscillation cycle that repeats each day. The oscillation cycles of the two or more candidate satellites 14 are offset in phase and the method 300 further includes dynamically selecting (Block 304) a serving satellite from among the two or more candidate satellites 14 for serving one or more terrestrial terminals 20 operating within a first satellite service area 22, based on a combined consideration of separation from the geostationary arc and elevation angle relative to the first satellite service area 22.

[0023] In one or more embodiments, the offset in phase is 180 degrees or less. As a particular example, the offset in phase is 120 degrees. In at least one embodiment, the set ofcandidate satellites 14 includes three candidate satellites 14, with their respective oscillation cycles offset from one another by 120 degrees.

[0024] The set of two or more candidate satellites 14 is associated with a same assigned orbital slot on the geosynchronous arc. That is, all candidate satellites 14 in the set maintain a longitudinal position corresponding to one assigned orbital slot; however, given the phase offsets among their respective latitudinal oscillations, the respective candidate satellites 14 each oscillate latitudinally within the same orbital slot. In one or more other embodiments, the set of two or more candidate satellites 14 is associated with two or more assigned orbital slots on the geosynchronous arc, where the two or more assigned orbital slots are within a maximum longitudinal angle. For example, each candidate satellite 14 in a set of three has its own assigned orbital slot in the geosynchronous arc, but the three assigned slots are all within ten degrees of one another along the geosynchronous arc. Here, ten degrees is an example, and the allowed range may be lesser or greater, with the understanding that the benefits of time-coordinated use of inclined-orbit satellites for advantaged service to a given satellite service area are lost if the satellites in question are spaced too far apart along the geosynchronous arc.

[0025] In one or more embodiments of the SCS 10, the one or more terrestrial terminals 20 are of a first type, and the method 300 further comprises using a fixedly selected one among the two or more candidate satellites 14 for serving one or more further terrestrial terminals 24 that are of a second type and are operating in the first satellite service area 22. In at least one case, the first and second types of terrestrial terminals 20 and 24 are distinguished by the second type being operable with a lower minimum elevation angle in comparison to the first type, or the second type being operable with higher inter-satellite interference than the first type.

[0026] In an example configuration of the SCS 10, the set of two or more candidate satellites 14 comprises a set of three candidate satellites 14, and the offset in phase between the respective candidate satellites 14 is 120 degrees. As such, at any given time of day, at least one candidate satellite 14 in the set satisfies at least one of: a separation from the geosynchronous arc in excess of a defined minimum separation; or an elevation angle relative to a reference location within the first satellite service area in excess of a defined minimum elevation angle.

[0027] Dynamically selecting (Block 304) the serving satellite comprises, for example, performing service handover between respective pairs of candidate satellites 14. Here, service handover occurs in conjunction with one candidate satellite 14 in each respective pair experiencing increasing separation from the geosynchronous arc while the other candidate satellite 14 in the respective pair experiences decreasing separation from the geosynchronous arc.

[0028] With reference to Figure 4, in one or more embodiments, the method 300 includes a communications processing system (CPS) 30 in the ground network 12 of the SCS routing user traffic associated with the one or more terrestrial terminals 20 in the first satellite service area 22 to or from respective gateway terminals 32 in coordination with dynamic selection of the serving satellite from the set of candidate satellites 14. In other words, to the extent that different gateway terminals of the SCS 10 are involved in handing service over from one candidate satellite 14 to another candidate satellite 14 in the dynamic selection process, the CPS 30 provides routing control in the ground network 12.

[0029] Of course, the ground network 12 may include additional nodes or network (NW) elements 34, e.g., for overall control and management of the SCS 10. The user traffic routed through the SCS 10 may originate or terminate in one or more external networks 36, such as the Internet and / or one or more other external communications networks.

[0030] As shown in Figure 5, the CPS 30 in one or more embodiments comprises at least one computer server 40 comprising one or more processors 42 and associated memory 44, along with I / O circuitry 46, such as Ethernet or other communications interfaces for communicatively coupling with other nodes within or external to the ground network. For example, the I / O circuitry 46 is configured for use by the one or more processors 42 in sending / receiving user traffic and related timing and control signaling.

[0031] In at least one embodiment, the one or more processors 42 are specially adapted to carry out the ground-based aspects of the method 300 based on the execution of computer program instructions (CPI) 50 stored in the memory 44. The memory 44 in one or more embodiments further includes configuration data 52 comprising, for example, scheduling information for the dynamic selection of a serving satellite from among the two or more candidate satellites 14. The configuration data 52 comprises, for example, a timetable indicating which candidate satellite 14 is to be selected as the serving satellite at particular times or time periods.

[0032] As shown in Figure 6, in one or more embodiments, the SCS 10 provides communication services to a second satellite service area 62, in addition to providing communication services to the first satellite service area 22. In particular, the SCS in one or more embodiments serves one or more terrestrial terminals 20 in a first satellite service area 22 using a set of two or more candidate satellites 14, wherein a serving one among the set is dynamically selected based on joint consideration of satellite separation from the geosynchronous arc and satellite elevation angle relative to the first satellite service area 22. Further, the SCS 10 uses atleast one of those same candidate satellites 14 to serve one or more terrestrial terminals 60 in a second satellite service area 62.

[0033] In the example case, the first satellite service area 22 is above the Equator and the second satellite service area 62 is below the Equator, but other absolute and / or relative locations of the first and second satellite service areas 22 and 62 are possible. In any case, the method 300 in one or more embodiments includes dynamically selecting a further serving satellite 14 from among the set of two or more candidate satellites 14 for serving a second plurality of terrestrial terminals in a second satellite service area. That is, dynamically selected ones among the same set of candidate satellites 14 are used for serving the first and second satellite service areas 22 and 62.

[0034] As noted, the first satellite service area 22 may lie within one hemisphere while the second satellite service area 62 lies within the other hemisphere. Regardless, in at least one embodiment, the dynamic selections of which candidate satellites 14 serve the first satellite service area 22 and the second satellite service area 62 at any given time or during any given interval is based on consideration of separation from the geosynchronous arc and elevation angle.

[0035] Depending upon how many candidate satellites 14 are operated in the described inclined orbital arrangements, at any given time of day, one of them will be in an advantaged position or an advantaged range of positions relative to the first satellite service area 22, and another one of them will be in an advantaged position or an advantaged range of positions relative to the second satellite service area 62. That is, which one among the set of two or more candidate satellites 14 is dynamically selected as the serving satellite for the first satellite service area 22 and which one is dynamically selected as the further serving satellite for the second satellite service area 62 depends upon oscillations of the candidate satellites 14 above and below the geosynchronous arc.

[0036] Thus, in at least one embodiment, the method 300 includes using at least one candidate satellite 14 among the set of two or more candidate satellites 14 to serve one or more further terrestrial terminals 60 in a second satellite service area 62 that is disjoint from the first satellite service area 22, during times when the at least one candidate satellite is not dynamically selected as the serving satellite for the one or more terrestrial terminals 20 in the first satellite service area 22. The terrestrial terminals 20 and / or 24 in the first satellite service area 22 are, for example, user terminals associated with subscribers or other authorized users of the SCS 10. Similarly, in at least one embodiment, the terrestrial terminals 60 in the second satellite service area 62 are user terminals. The user terminals may be a mix of different terminal types withdifferent capabilities. For example, with respect to the first satellite service area 22, the terrestrial terminal(s) 20 may be less capable than the terrestrial terminal(s) 24 in terms of minimum supported elevation angle and / or maximum tolerable inter-satellite interference.

[0037] For carrying out the method 300 or extensions or variations of the method 300, the SCS 10 includes a set of two or more candidate satellites 14 operating in respective inclined orbits 16 relative to the geosynchronous arc, such that each candidate satellite 14 maintains a nominally fixed longitudinal position but has a latitudinal position that oscillates above and below the geosynchronous arc according to an oscillation cycle that repeats each day, and wherein the oscillation cycles of the two or more candidate satellites are offset in phase. The SCS 10 further includes a CPS 30 in a ground network 12 of the SCS 10. In one or more embodiments, the CPS 30 comprising one or more computer servers 40 is configured to dynamically select a serving satellite 14 from among the two or more candidate satellites 14 in the set, for serving one or more terrestrial terminals 20 operating within a first satellite service area 22, based on a combined consideration of separation from the geostationary arc and elevation angle relative to the first satellite service area 22.

[0038] Another embodiment comprises a method of operating a SCS 10, where the method includes: maintaining a satellite in an inclined geosynchronous orbit around the Earth, such that the satellite maintains a nominally fixed longitudinal position but has a latitudinal position that oscillates above and below the equator according to an oscillation cycle that repeats each orbital period of the satellite; and, with respect to each orbital period, (a) providing service coverage via the satellite to a first coverage area that is above the equator at one or more first times during which the latitudinal position of the satellite is above the equator, and (b) providing service coverage via the satellite to a second coverage area that is below the equator at one or more second times during which the latitudinal position of the satellite is below the equator.

[0039] According to the above method, providing service coverage via the satellite to the first coverage area comprises using the satellite to exchange user traffic with one or more first terrestrial terminals located in the first coverage area, and providing service coverage via the satellite to the second coverage area comprises using the satellite to exchange user traffic with one or more second terrestrial terminals located in the second coverage area. The one or more first and second terrestrial terminals are of a certain type that supports a first range of elevation angles for satellite communications, wherein one or both of the first and second coverage areas include one or more third terrestrial terminals of a certain other type that supports a larger, second range of elevation angles for satellite communications, and wherein the method further includes providing service coverage for the one or more third terminals over a range oflatitudinal positions greater than a range of latitudinal positions over which service coverage is provided via the satellite for the one or more first or second terrestrial terminals.

[0040] The method may be extended, in which the method includes providing service coverage via the satellite to a third coverage area, irrespective of the latitudinal position of the satellite. In at least one example of this extended scenario, the third coverage area is closer to the equator than one or both the first and second coverage areas. As one possibility, the third coverage area coincides with one of the first and second coverage areas, and wherein the other one of the first and second coverage areas is further from the equator than the third coverage area.

[0041] As explained earlier herein, the satellite may be one of three or more satellites, with the method correspondingly including maintaining the three or more satellites in respective inclined geosynchronous orbits, where the respective nominally fixed longitudinal locations of the three or more satellites are the same or constrained to be within a defined range of longitudinal angles, and where the respective oscillatory cycles are offset. The three or more satellites comprise, for example, three satellites having a same nominally fixed longitudinal position but having their respective oscillation cycles offset from one another by 120 degrees, such that there is at least one of the three satellites above the equator at any given time, for providing service coverage to the first coverage area, and at least one of three satellites below the equator at any given time, for providing service coverage to the second coverage area.

[0042] Figure 7 illustrates an example arrangement of phase offset for three satellites (SAT1, SAT2, and SAT3). In the context of the diagram, the three satellites operate in inclined orbits with their respective latitudinal oscillation cycles offset in phase by 120 degrees.

[0043] In one or more embodiments, the SCS 10 is configured to utilize the full 15 degree range of inclinations allowed for a geostationary satellite. In at least one embodiment, the SCS 10 includes a set of three satellites 14 that are operated in inclined orbits with offset phase, such that each one of the three satellites 14 is a candidate for dynamic selection as a serving satellite with respect to a particular satellite service area. In more detail, each of the three candidate satellites 14 in the set operates eight hours out of phase with the other two. This arrangement results in a changing ground elevation angle for each of the three candidate satellites 14 over the course of 24 hours.

[0044] Figure 8 illustrates the results of the foregoing 3-sat / 120-deg offset arrangement, where each one of the three candidate satellites 14 in the set spends eight hours in each of the three inclination regions above, below and near the geosynchronous (GEO) arc. If the three candidate satellites 14 are phased 8 hours apart (which means being phased 120 degrees apart inground track latitude), then there is always one candidate satellite 14 in each of the depicted regions. Although these details refer to a set of three candidate satellites 14, the set may have more than three candidate satellites 14 for use in a similar way. For example, the set in one or more embodiments includes four candidate satellites 14 that are phased ninety degrees apart in ground track latitude, which results in four elevation regions that each always have one candidate satellite 14 spending six hours within it.

[0045] Figure 9 provides additional illustration of the three-satellite case. In particular, Figure 9 illustrates details for an embodiment where a set of three candidate satellites 14, shown as Satellites 1-3 in the diagram, are used for serving terrestrial terminals in a satellite service area, based on dynamically selecting a serving one among them, in dependence on the relative positions of the three candidate satellites 14. Figure 9 assumes that the three candidate satellites 14 all operate within the same orbital slot, with all three rising above and falling below the geosynchronous arc according to their respective oscillation cycles and where those cycles are offset in phase by 120 degrees.

[0046] Example advantages of the purposeful, coordinated use of two or more candidate satellites 14 operating in respective inclined orbits include lower adjacent satellite interference (ASI) when operating with larger angular separation from neighboring satellites. For example, assume an example case where inter-satellite interference relates to geostationary satellites in neighboring orbital slots along the geosynchronous arc. By operating a satellite in an inclined orbit rather than in a geostationary orbit, the satellite oscillates above and below the geostationary arc and thus gains increased separation from the neighboring geostationary satellites. In an example case where a set of three candidate satellites 14 is operated with phaseoffset inclined orbits as described herein, there will always be one candidate satellite 14 with at least 7.7 degree separation between neighboring geostationary satellites on the geosynchronous arc.

[0047] As a further advantage, the described use of inclined orbits lowers station keeping fuel usage for the candidate satellites 14, thus resulting in a longer service life.

[0048] Still further, the described use of inclined orbits offers the ability for operation using user terminal apertures that are as much as ten times smaller than conventional apertures, due to the larger angular separation from neighboring satellites without ASI issues. With the at least 7.7 degrees of separation described above, as compared to the nominal two degrees of separation in normal geostationary operation, the terminal aperture area required to avoid ASI is much smaller and in the case of the preferred embodiment is over ten times smaller.

[0049] Spatial diversity for frequency reuse is yet another advantage, assuming that multiple inclined satellites are operated in the same orbital slot but using different phasing — i.e., different inclination time phasing. Again assuming a set of three candidate satellites 14 operating with inclined orbits in the same orbital slot and with 120 degrees of phase offset between them, two of the three candidate satellites 14 sharing that same orbital slot will always have an angular separation of more than fifteen degrees and thus can share the same frequency spectrum and coverage area as the other satellites without interference. That is, although the set of candidate satellites 14 periodically pass each other according to their oscillation cycles, in a set of three with offset cycles, at least two of the three candidate satellites 14 always have angular separation relative to one another, meaning that they can reuse the frequency spectrum in the same geography. Also, whenever the individual candidate satellites 14 are not crossing the GEO arc, they have angular separation from neighboring satellites stationed on the GEO arc. Frequency reuse across multiple satellites latitudinally oscillating out of phase within the same orbital slot significantly increases the commercial value of that slot.

[0050] Another advantage mentioned earlier is that the disclosed techniques provide higher elevation angles for terrestrial terminals. For example, assuming three candidate satellites 14 operating as a set in the same orbital slot with 120 degrees of phase offset, the average elevation angle is twelve degrees higher at the edge of coverage than it would be with a “standard” GEO satellite operating without inclination, and this arrangement is especially beneficial for electronically scanned phased array antennas where scan loss is high.

[0051] Further, conventional non-geosynchronous orbit (NGSO) systems with fixed ground antennas need to scan in both azimuth and elevation over a large range, whereas an inclined GEO system needs to only scan in elevation, and only a limited amount of elevation scan (e.g., approximately ten degrees or so). Thus, another advantage for systems configured according to the techniques disclosed herein is single axis scanning for fixed terminals versus the need for two-axis scanning.

[0052] A further advantage is that the disclosed techniques allow for the use of asymmetric (rectangular) mobile terminal apertures at locations near the Equator without ASI. Terminals of this type may suffer skew problem that cause higher ASI when operating near the Equator. Such terminals are often used in aircraft because they reduce the required radome height, but they have the problem of creating asymmetrically shaped beams towards their serving satellite, which means that when operating directly north or south of the serving satellite, there is little or no adjacent satellite interference because the large dimension of the antenna corresponding to the smallest dimension of the beam is pointed towards the satellite. However when operating eastor west of the serving satellite near the Equator, the narrow antenna dimension corresponding to the wide beam dimension is oriented towards the serving satellite and causes much higher ASI in that orientation.

[0053] Even with a single inclined satellite, there would be times of the day during which at least some of the foregoing advantages are realized. These times could be synchronized with peak demand times. Thus, even a single satellite flying in an inclined orbit provides advantages if properly phased corresponding to times of day with high demand or times of day for serving user terminals that have a restricted range of elevational angles or less tolerance of ASI.

[0054] Notably, modifications and other embodiments of the disclosed invention(s) will come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention(s) is / are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

CLAIMSWhat is claimed is:

1. A method of operating a satellite communications system, the method comprising: maintaining a set of two or more candidate satellites in respective inclined orbits relative to the geosynchronous arc, such that each candidate satellite maintains a nominally fixed longitudinal position but has a latitudinal position that oscillates above and below the geosynchronous arc according to an oscillation cycle that repeats each day, and wherein the oscillation cycles of the two or more candidate satellites are offset in phase; and dynamically selecting a serving satellite from among the two or more candidate satellites for serving one or more terrestrial terminals operating within a first satellite service area, based on a combined consideration of separation from the geostationary arc and elevation angle relative to the first satellite service area.

2. The method according to claim 1, wherein the offset in phase is 180 degrees or less.

3. The method according to claim 1 , wherein the offset in phase is 120 degrees.

4. The method according to any one of claims 1-3, wherein the set of two or more candidate satellites is associated with a same assigned orbital slot on the geosynchronous arc.

5. The method according to any one of claims 1-3, wherein the set of two or more candidate satellites is associated with two or more assigned orbital slots on the geosynchronous arc, wherein the two or more assigned orbital slots are within a maximum longitudinal angle.

6. The method according to any one of claims 1-5, wherein the one or more terrestrial terminals are of a first type, and wherein the method further comprises using a fixedly selected one among the two or more candidate satellites for serving one or more further terrestrial terminals that are of a second type and are operating in the first satellite service area.

7. The method according to claim 6, wherein the first and second types of terrestrial terminals are distinguished by the second type being operable with a lower minimum elevation angle in comparison to the first type, or the second type being operable with higher inter-satellite interference than the first type.

8. The method according to any one of claims 1 -7, wherein the set of two or more candidate satellites comprises a set of three candidate satellites, and wherein the offset in phase is 120 degrees, such that at any given time of day, at least one candidate satellite in the set satisfies at least one of: a separation from the geosynchronous arc in excess of a defined minimum separation; or an elevation angle relative to a reference location within the first satellite service area in excess of a defined minimum elevation angle.

9. The method according to any one of claims 1-8, wherein dynamically selecting the serving satellite comprises performing service handover between respective pairs of candidate satellites, wherein service handover occurs in conjunction with one candidate satellite in each respective pair experiencing increasing separation from the geosynchronous arc while the other candidate satellite in the respective pair experiences decreasing separation from the geosynchronous arc.

10. The method according to any one of claims 1-9, wherein the method includes a communications processing system in a ground network of the satellite communications system routing user traffic associated with the one or more terrestrial terminals to or from respective gateway terminals in coordination with dynamic selection of the serving satellite.

11. The method according to any one of claims 1-10, wherein the one or more terrestrial terminals comprise a first plurality of terrestrial terminals operating within the first satellite service area, and wherein the method further comprises dynamically selecting a further serving satellite from among the set of two or more candidate satellites for serving a second plurality of terrestrial terminals in a second satellite service area.

12. The method according to claim 11, wherein the first satellite service area lies within one hemisphere and the second satellite service lies within the other hemisphere, and wherein which ones among the set of two or more candidate satellites are dynamically selected as the serving satellite and the further serving satellite depends upon oscillation of each candidate satellite above and below the geosynchronous arc.

13. The method according to any one of claims 1-12, further comprising using at least one candidate satellite among the set of two or more candidate satellites to serve one or more further terrestrial terminals in a second satellite service area that is disjoint from the first satellite servicearea, during times when the at least one candidate satellite is not dynamically selected as the serving satellite for the one or more terrestrial terminals in the first satellite service area.

14. A satellite communications system comprising: a set of two or more candidate satellites operating in respective inclined orbits relative to the geosynchronous arc, such that each candidate satellite maintains a nominally fixed longitudinal position but has a latitudinal position that oscillates above and below the geosynchronous arc according to an oscillation cycle that repeats each day, and wherein the oscillation cycles of the two or more candidate satellites are offset in phase; and a communications processing system (CPS) in a ground network of the satellite communications system, the CPS comprising one or more computer servers configured to dynamically select a serving satellite from among the two or more candidate satellites for serving one or more terrestrial terminals operating within a first satellite service area, based on a combined consideration of separation from the geostationary arc and elevation angle relative to the first satellite service area.

15. The satellite communications system according to claim 14, wherein the offset in phase is 180 degrees or less.

16. The satellite communications system according to claim 14, wherein the offset in phase is 120 degrees.

17. The satellite communications system according to any one of claims 14-16, wherein the set of two or more candidate satellites is associated with a same assigned orbital slot on the geosynchronous arc.

18. The satellite communications system according to any one of claims 14-16, wherein the set of two or more candidate satellites is associated with two or more assigned orbital slots on the geosynchronous arc, wherein the two or more assigned orbital slots are within a maximum longitudinal angle.

19. The satellite communications system according to any one of claims 14-18, wherein the one or more terrestrial terminals are of a first type, and wherein the CPS is configured to use afixedly selected one among the two or more candidate satellites for serving one or more further terrestrial terminals operating in the first satellite service area, the one or more further terrestrial terminals being of a second type.

20. The satellite communications system according to claim 19, wherein the first and second types of terrestrial terminals are distinguished by the second type being operable with a lower minimum elevation angle in comparison to the first type, or the second type being operable with higher inter-satellite interference than the first type.

21. The satellite communications system according to any one of claims 14-20, wherein the set of two or more candidate satellites comprises a set of three candidate satellites, and wherein the offset in phase is 120 degrees, such that at any given time of day, at least one candidate satellite satisfies at least one of: a separation from the geosynchronous arc that exceeds a defined minimum separation; or an elevation angle relative to a reference location within the satellite service area that exceeds a defined minimum elevation angle.

22. The satellite communications system according to any one of claims 14-21, wherein, with respect to dynamically selecting the serving satellite, the CPS is configured to perform service handover between respective pairs of candidate satellites, and wherein service handover occurs in conjunction with one candidate satellite in each respective pair experiencing increasing separation from the geosynchronous arc while the other candidate satellite in the respective pair experiences decreasing separation from the geosynchronous arc.

23. The satellite communications system according to any one of claims 14-22, wherein the CPS is configured to route user traffic associated with the one or more terrestrial terminals to or from respective gateway terminals of the satellite communications system in coordination with dynamic selection of the serving satellite.

24. The satellite communications system according to any one of claims 14-23, wherein the one or more terrestrial terminals comprise a first plurality of terrestrial terminals operating within the first satellite service area, and wherein the CPS is configured to dynamically select a further serving satellite from among the set of two or more candidate satellites for serving a second plurality of terrestrial terminals in a second satellite service area.

25. The satellite communications system according to claim 24, wherein the first satellite service area lies within one hemisphere and the second satellite service lies within the other hemisphere, and wherein which ones among the set of two or more candidate satellites are dynamically selected as the serving satellite and the further serving satellite depends upon oscillation of each candidate satellite above and below the geosynchronous arc.

26. The satellite communications system according to any one of claims 14-25, wherein the CPS is further configured to use at least one candidate satellite among the set of two or more candidate satellites to serve one or more further terrestrial terminals in a second satellite service area that is disjoint from the first satellite service area, during times when the at least one candidate satellite is not dynamically selected as the serving satellite for the one or more terrestrial terminals in the first satellite service area.

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