Multiplex communications – Communication over free space – Repeater
Reexamination Certificate
2000-12-29
2004-03-30
Pham, Chi (Department: 2662)
Multiplex communications
Communication over free space
Repeater
C455S427000, C455S431000
Reexamination Certificate
active
06714521
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 can be deployed and utilized in a manner that materially increases global communications satellite capacity, does not interfere with the existing geostationary satellite ring, and provides simplified satellite tracking.
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 essentially 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 additional 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.
The high altitude of the geostationary orbit also adds to the size and weight of geo satellites. Path loss, the attenuation suffered by radio signals traveling in free space, is proportional to the square of the distance between the source and the receiver. This means that the antenna size and transmitted power of a geo satellite must be greater than those of a satellite in lower orbit in order to achieve the same communications link performance. This is particularly true in mobile and other direct-to-user applications where the size and power of the user terminal are constrained by practical considerations and the burden of providing acceptable link performance falls largely on the satellite. The generally larger size and weight of geo satellites adds further to the cost of launch as compared to satellites intended to operate in lower orbits.
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 gee 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 is noticeable to some users, and may require the 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 area 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. For example, many points in Alaska, Canada and Scandinavia cannot even see the geo satellites, these satellites being below their visible horizon.
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.
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 early 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 and allow acceptable link performance with very small user terminals. However, non-geostationary systems operate in inclined orbits, and thus pose a potential for interference with geo satellites operating at the same frequencies as they cross the geostationary 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 on Dec. 21, 1998 and Sep. 28, 1999, respectively, to the inventor of the present invention and two other individuals. The system proposed in the FCC application employs three arrays of satellites in elliptical orbits, two arrays covering the northern hemisphere and one covering the southern hemisphere, each array having five 8-hour satellites emulating many of the characteristics of geo satellites. The satellites appear to “hang” in the sky because their angular velocity at or near
Avruch Phillip G.
Ly Anh-Vu H
Pham Chi
Rabin & Berdo P.C.
Space Resources International Ltd.
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