High latitude geostationary satellite system

Details High latitude geostationary satellite system High latitude geostationary satellite system

2000-02-11

2001-12-25

Chin, Wellington (Department: 2664)

Multiplex communications

Communication over free space

Having a plurality of contiguous regions served by...

C342S356000, C342S357490, C342S357490, C455S013100, C455S436000

Reexamination Certificate

active

06333924

ABSTRACT:

BACKGROUND OF THE INVENTION
There is an increasing demand for the provision of fixed point-to-point, or point-to-multi-point satellite communications services. The use of the orbital frequency spectrum that meets the needs for satellite communication services has increased significantly, resulting in scarcity or unavailability of geostationary orbital slots, or satellite positions, for certain prime frequency bands, and orbit locations. The frequency spectrum used by satellites in the geostationary satellite orbit is finite and its reuse by those satellites is limited by the level of interference incurred between the different satellite systems in that orbit sharing the frequency bands. With this present level of increasing congestion (both spectrum and orbital), a need exists to establish alternative systems, where satellites have essentially the same user characteristics as satellites in the geostationary satellite orbit, in that the satellites remain almost stationary relatively to the ground, and provide, without any ground intervention, continuous service to the users.
An object of the present invention is to provide such an alternative. The orbit of this invention allows the spectrum used by satellites in the geostationary satellite orbit to be completely reused by satellites of this invention without causing any interference. Communication ground stations operating with satellites of this invention will be the same as the ground stations operating with satellites in the geostationary satellite orbit. The only difference is that, by virtue of an angular separation of 45 degrees or more between satellites of this invention and the geostationary satellites, as seen from any ground location, the ground terminal of the systems of this invention will be pointed to a different place in the sky.
Another object of the present invention is to place each satellite in an orbit that appears almost stationary relatively to a fixed point on the earth during an operational portion of the orbit, along which the satellite defines to a ground-based user a small loop, hence its characterization as “geostationary”. In fact, to a ground-based observer, at a single longitudinal location, a pair of loops are defined, one being a minor loop at the highest latitude with its apogee extending to 63.4 degrees, and the other being a major loop, the pair of loops being disposed on one side of the equator. Since the operational portion of the orbit (the minor loop) is located at high latitudes (above 45 degrees N. or S. and at or below 63.4 degrees N. or S., respectively), this invention is defined as a High Latitude Geostationary Satellite System. A distinction that needs to be made between satellites operating in the orbits of this invention and more popular “Geostationary Satellites” is that satellites of this invention are geostationary during a part of their orbit, namely their operational part, whereas the popular geostationary satellites are located in a 24 hour orbit which is geostationary, and therefore the satellites are stationary for 24 hours per day.
Another object of the present invention is to achieve a switching of active communications between satellites by use of precise timing signals from the Global Positioning System (GPS) and the use of inter-satellite links between the satellites in order to ensure that a seamless transition of communication services is provided to users on the ground without the need of any ground control intervention.
SUMMARY OF THE INVENTION
The present invention comprises a satellite system, related apparatus and methods of operation for the provision of satellite broadcast and communications services, in the same manner as presently provided to users of the geostationary satellite orbit while allowing complete frequency reuse of the radio frequency spectrum that is used by satellites in the geostationary satellite orbit providing similar services. Although the existing satellite systems and the present invention use the same frequency bands, no interference is experienced and no mitigating factors are required by either system to avoid interference.
The satellites related to the present invention are placed in a constellation of either 3 or 4 satellites in highly elliptical orbits at an operational altitude ranging from
−
30,000 to
−
40,000 km. A constellation of such satellites can be implemented to serve either the northern hemisphere or the southern hemisphere. In each hemisphere, two regions of coverage are obtained separated from each other by 180 degrees in longitude. The satellites are in a nearly 12 hour orbit. With a 3-satellite constellation, two satellites are simultaneously operating, each one of them over one of the two coverage regions, while the third satellite transitions between the coverage regions. In a 4-satellite constellation, two satellites are simultaneously operating, each one over one of the two coverage regions, while the other two satellites transition between coverage regions. In the case of the 3-satellite constellation, each satellite spends 8 hours of service on each of the two coverage regions every 24 hours. In the case of the 4-satellite constellation, each satellite spends 6 hours of service on each of the two coverage regions every 24 hours. During the operational phase, each satellite serving its designated region appears approximately fixed relatively to a user on the ground. When the operating satellite is about to be replaced by the next satellite entering the same region, both satellites, which are providing identical coverage, remain co-located for a period of time. This period is adequate to permit hand-over with no service interruption to the ground network, using highly directional antennas having narrow beams and being operational without the need for a dynamic pointing capability, or at most pointing in a single plane.
This seamless hand-over is accomplished between the two satellites independently of any ground control network operating with the satellites and uses the precise timing obtained from the Global Positioning System (GPS) to ensure that transmissions terminating on one satellite are synchronized with the transmissions commencing on the new satellite for any ground stations in the coverage region. The use of GPS may be supplemented with the use of real-time timing, but not control, signals from earth stations.
The orbits of satellites used in connection with this invention are such that coverage can be provided to selected geographical areas, where population density is high and demand for these types of satellite services exists. Primarily, these highly populated areas occur in the northern hemisphere and to a certain extent in the southern hemisphere, but generally not around the equator. The satellites of this invention, operating along small geostationary loops at a high latitude in either the northern or southern hemisphere, can therefore provide service to users in heavily populated areas, with the benefits of high elevation angles of view to the satellites, as compared with low elevation angles with which satellites in the geostationary satellite orbit can service the same areas. Therefore, the present invention results in the capability of extending service to many high-latitude users who would otherwise receive poor service, or no service at all, for their inability to advantageously use the equatorial geostationary satellites, as a result of their lower elevation angles and consequent effects of terrain blockage and atmospheric attenuation. High elevation angles at high latitudes, as provided by satellites of this invention, are also advantageous in areas subject to rain fading.
The total number of satellites for each constellation is either 3 or 4. The weight and therefore the launch costs of these satellites are considerably less than those of the corresponding equatorial geostationary satellites. For equal capability communication satellite payloads, two satellites of this invention could be launched on the same launch vehicle that would only be capable of handling one eq

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