Telecommunications – Carrier wave repeater or relay system – Portable or mobile repeater
Reexamination Certificate
1997-11-24
2001-05-22
Vo, Nguyen (Department: 2745)
Telecommunications
Carrier wave repeater or relay system
Portable or mobile repeater
C455S013200, C455S427000, C455S063300
Reexamination Certificate
active
06236834
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to satellite communication systems, and particularly to methods and systems for limiting signal interference between satellite communication systems.
BACKGROUND ART
The recent proliferation of satellite communication systems has increased the likelihood of interference between signals associated with neighboring satellites. Such interference can take place, for example, when a non-geostationary satellite comes within the field of view of a geostationary satellite. As is well known, geostationary satellites remain fixed in equatorial orbits over particular locations on the surface of the earth. Since geostationary satellites ordinarily exhibit some minor variation in latitude relative to the equatorial arc, there exists a narrow “geostationary band” centered about the equatorial arc corresponding to the set of orbital locations potentially occupied by geostationary satellites . Unlike geostationary satellites, the orbits of non-geostationary satellites continuously vary with respect to the earth's surface. Non-geostationary satellites typically traverse low and medium altitude orbits below the geostationary band.
Signal interference between geostationary and non-geostationary communication systems can result when non-geostationary satellites move into the field of view of ground stations oriented toward a particular satellite within the geostationary band. The potential for such interference arises whenever a non-geostationary satellite becomes located proximate the feeder link path between a geostationary satellite and one of its ground stations, hereinafter referred to as GSY ground stations. Such interference can occur because non-geostationary satellite systems are generally allocated, on a secondary basis, the same feeder link frequency bands primarily earmarked to geostationary systems. Consequently, it is incumbent upon the operators of non-geostationary systems. Consequently, it is incumbent upon the operators of non-geostationary systems to avoid disrupting communication within geostationary systems. Although it is conceivable that the feeder link band could be shared by geostationary and non-geostationary systems, the frequency separation required between the channels allocated to each system in order to ensure acceptable interference levels would make this approach unfeasible under most circumstances.
Since geostationary satellites are distributed throughout the geostationary band above the surface of the equator, the points on the surface of the earth in approximate alignment with the geostationary band and a non-geostationary satellite form a range of “in-line” latitudes across the earth's surface. The position of this terrestrial in-line latitude range will vary with changes in the latitude of the non-geostationary satellite. Yet non-geostationary satellites may interfere with geostationary systems even when not so aligned between a geostationary satellite and a GSY ground station, since the antenna of the GSY ground station projects a radiation pattern across a finite discrimination angle relative to its beam axis. Accordingly, it has generally been necessary for non-geosynchronous satellites to cease signal transmission when in orbit above GSY ground stations in the vicinity of this in-line latitude range. This restriction on transmission range has hindered the performance of non-geostationary satellite systems coordinated in frequency with geostationary systems.
One way of minimizing interference between satellite systems would be simply to operate one system over frequency bands not already allocated to the other systems. Unfortunately, the limited frequency spectrum available for satellite communication systems renders this solution untenable. Moveover, well-established technology is available for implementing communications equipment designed to process signals over the frequency bands primarily allocated to geosynchronous systems.
While most satellite systems have in the past used geostationary or geosynchronous satellites, the future development of systems using low and medium earth orbits is likely to increase the problem of interference, not only between geostationary and non-geostationary systems, but also between two or more non-geostationary systems.
U.S. Pat. No. 5,227,802 discloses a method of controlling the amount of overlap between cells projected by different satellites, by turning off cells from one satellite when they overlap by more than a predetermined amount with cells from another satellite.
DISCLOSURE OF THE INVENTION
According to one aspect of the present invention, there is provided a method of reducing interference between transmissions from a first satellite and from one or more second satellites located within one or more orbital locations, comprising: determining a forbidden area of the surface of the earth within which said first satellite and each of said orbital locations are separated by less than a predetermined minimum discrimination angle, and inhibiting transmission by the first satellite to said forbidden area.
According to another aspect of the present invention, there is provided a method of reducing interference in a link via a first satellite from transmissions from one or more ground stations to one or more second satellites located within one or more orbital locations, comprising: determining a forbidden area of the surface of the earth within which said first satellite and each of said orbital locations are separated by less then a predetermined minimum discrimination angle, and inhibiting reception via the first satellite from said forbidden area.
According to another aspect of the present invention, there is provided apparatus for reducing interference between transmissions from a first satellite and from one or more second satellites located within one or more orbital locations, comprising: means operable to determine a forbidden area of the surface of the earth within which said first satellite and each of said orbital locations are separated by less than a predetermined minimum discrimination angle, and means operable to inhibit transmission by the first satellite to said forbidden area.
According to another aspect of the present invention, there is provided apparatus for reducing interference in a link via a first satellite from transmissions from one or more ground stations to one or more second satellites located within one or more orbital locations, comprising: means operable to determine a forbidden area of the surface of the earth within which said first satellite at said orbital locations are separated by less than a predetermined minimum discrimination angle and means operable to inhibit reception via the first satellite from said forbidden area.
An advantage of the present invention is that the first satellite is only inhibited from transmitting to or receiving from those areas in which unacceptable interference will occur, instead of inhibiting transmission/reception whenever the beams of the first satellite and the second satellite or satellites overlap. In this way, disruption of the service provided by the first satellite is reduced.
Communications with ground stations in the forbidden area may be handed over to a third satellite which is able to communicate with the ground stations without interference.
Alternatively, communications between the first satellite and ground stations within the forbidden area may be handed over to ground stations outside the forbidden area. Where the ground stations provide alternative gateways into a ground network, communications with the ground network may thereby be maintained.
Communication with ground stations in the forbidden area may be inhibited by adjusting the antenna beam pattern of the first satellite, preferably by inhibiting spot beams incident on the forbidden area. This provides an advantageous method of reducing interference, since the carrier frequencies and/or reference signals of the antenna beam pattern are also inhibited in the forbidden area.
The predetermined discrimination angl
Gosselin Stephane
Mullins Dennis
Nomoto Shinichi
Poskett Peter
Banner & Witcoff , Ltd.
International Mobile Satellite Organization
Vo Nguyen
LandOfFree
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