Telecommunications – Carrier wave repeater or relay system – Portable or mobile repeater
Reexamination Certificate
2000-11-13
2004-03-02
Maung, Nay (Department: 2684)
Telecommunications
Carrier wave repeater or relay system
Portable or mobile repeater
C455S012100, C455S013200, C455S427000
Reexamination Certificate
active
06701126
ABSTRACT:
FIELD OF THE INVENTION
The present invention is generally related to satellite communications systems and, more particularly, to a constellation of non-geostationary satellites that may be deployed and utilized in a manner that substantially increases global communications satellite capacity and does not interfere with the existing geostationary satellite ring.
BACKGROUND OF THE INVENTION
Geostationary (“geo”) satellites for telecommunications applications were first proposed many years ago by the author Arthur C. Clark. Today, there are numerous communications systems employing geo satellites for such diverse applications as telephone and data trunking, television distribution, direct-to-home broadcasting, and mobile communications. Geo satellites operate on the physical principle that a satellite, in circular orbit at the proper altitude above the equator, will orbit the earth at the same angular velocity as the earth's rotation. These satellites therefore, appear to be fixed relative to a point on the earth. This characteristic of geo satellites facilitates their use for communications applications by allowing communications terminals on the earth to simply point their antennas at one position in the sky.
There are however, a number of distinct drawbacks associated with geostationary satellite systems. One major drawback is the high cost of raising a satellite into geo orbit. Geostationary orbits have a radius from the earth center of approximately 36,000 kilometers. Typically, a geo satellite is launched first into an elliptical transfer orbit having an apogee at geostationary altitude, and then its orbit is circularized by using a kick motor to impart the necessary addition momentum to the satellite at apogee. The apogee kick motor, before it is fired, typically weighs as much as the satellite itself, meaning that the launch vehicle must initially launch a payload twice as heavy as the satellite in final orbit. Accordingly, the cost of putting a satellite into the high circular orbit required for geostationary operation is significantly greater than for non-geostationary satellites. The cost associated with deployment of satellites must generally be amortized over the lifetime of the satellite, making use of geo satellites more expensive.
Another problem associated with the altitude at which geo satellites orbit is the delay in the round trip transmission to and from the satellite. For a pair of diverse communications terminals located within the coverage area of a geo satellite, the path length from terminal-to-satellite-to terminal is at least 70,000 kilometers. For the average satellite “hop” the associated transmission delay is approximately one-quarter of a second. For voice communications by satellite, the delay may not be noticeable to most users, but does make it necessary to use special circuitry for echo control. For data communications, the delay complicates the use of protocols that are predicated on the characteristics of terrestrial circuits.
Other problems arise from the geometry of coverage of geo satellite systems. A geostationary satellite system intended to provide “global” services would include three geo satellites spaced equal along the equatorial arc at 120-degree intervals. The coverage of each of these satellites describes a circle on the surface of the earth with its center on the equator. At the equator, the coverage areas of two adjacent geo satellites overlap approximately 40 degrees in longitude. However, the overlap decreases as latitude increases, and there are points on the earth, north and south of the coverage areas, from which none of the geo satellites is visible. The lack of coverage is most pronounced at points where the coverage areas intersect, mid-way between satellite orbital locations.
For a geo system, in which the satellites are in orbit above the equator, earth stations in the equatorial regions generally “see” the satellites at high elevation angles above the horizon. However, as the latitude of an earth station location increases, the elevation angle to geo satellites from the earth station decreases. For example, elevation angles from ground stations in the United States to geostationary satellites range from 20 to 50 degrees. Low elevation angles can degrade the satellite communications link in several ways. The significant increase in path length through the atmosphere at low elevation angles exacerbates such effects as rain fading, atmospheric absorption and scintillation. For mobile communications systems in particular, low elevation angles increase link degradation due to blockage and multi-path effects.
Because each of the geo satellites only covers one part of the world, some communications links may require more than one satellites hop, or some combined use of satellite and terrestrial transmission facilities to reach their destination. The problem with multiple satellite hops is that for satellites in geostationary orbit, there is a corresponding significant increase in total circuit delay. Of course, multiple satellite hops require an earth station located in view of both satellites that can relay the transmission from one satellite to another.
Direct, inter-satellite links have been proposed as a means for extending the coverage of Geo satellites without the need for such an intermediate earth station. Although the inter-satellite link eliminates the earth station and one round-trip path to the satellite, the benefit is largely offset by the delay incurred in the path between the two orbit satellites. For geo satellites spaced at 120 degrees, the path between satellites is approximately 50,000 kilometers. Moreover, the equipment needed on-board the satellites to implement the inter-satellite link, whether microwave or optical, is complex and expensive. As a result, inter-satellite links have not found extensive application in geo stationary satellites.
Another, and perhaps more significant, problem resulting from the specific geometry of the geo orbit, is the limited availability of orbital positions (or “slots”) along the geostationary orbital arc. The ring of geostationary satellites that has grown up over time generally occupies multiple slots spaced two degrees apart and identified by their longitudinal positions. This arrangement has been adopted internationally to allow for satellite communications with a minimum of interference between adjacent satellites operating in the same frequency bands. The two-degree spacing is achieved by using high gain, directional antennas at the ground stations accessing the satellites. The geo ring around the equator thus provides a total of 180 slots (360 degrees/two degrees per slot). Most of the Geo slots are now occupied, making it difficult to find positions for more geo satellites. Frequency, polarization and beam diversity have been used to multiply capacity, but capacity in the geostationary arc remains limited. Moreover, not all geo orbital positions are equally useful or attractive for various applications.
Various non-geostationary satellite systems have been implemented in the past to overcome some of the drawbacks of geo satellites. An example is the Russian Molniya system, which employed satellites in elliptical 12-hour orbits to provide coverage to the northern latitudes in the Soviet Union. The Iridium and Globalstar systems use satellites in low circular orbits to significantly reduce transmission delay. Generally, non-geostationary systems operate in inclined orbits, and pose a potential for interference with satellites operating at the same frequencies as they cross the geo ring.
In January 1999, an application was filed before the Federal Communications Commission (FCC) by Virtual Geosatellite LLC for the construction of a global broadband satellite communications system based on the teachings of U.S. Pat. Nos. 5,845,206 and 5,957,409, issued to the inventor of the present invention and two other individuals on Dec. 21, 1998 and Sep. 28, 1999, respectively. The system proposed in the FCC application employs three arrays of satellites in elliptical o
Avruch Philip G.
Orgad Edan
Rabin & Berdo P.C.
Space Resource International Ltd.
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