Data processing: vehicles – navigation – and relative location – Navigation – Space orbits or paths
Reexamination Certificate
2001-11-10
2003-07-22
Arthur, Gertrude (Department: 3661)
Data processing: vehicles, navigation, and relative location
Navigation
Space orbits or paths
C244S158700, C342S352000
Reexamination Certificate
active
06597989
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to a multi-satellite constellation in low earth orbit which provides continuous coverage over specifically designated regions. More specifically, the system provides continuous coverage to the populated land masses of the earth and wherein such coverage is tailored to the population density of the land masses therein.
BACKGROUND AND SUMMARY OF THE INVENTION
This application discloses a new, improved class of satellite constellation that provides continuous satellite coverage of the earth's surface above 25° of north latitude, and by extension global coverage in proportion to the population densities by latitude. The constellation consists of satellites in low altitude (below 4,000 kilometers) inclined elliptical orbits with northern apogees, and in its extension with a subconstellation of satellites with equatorial orbits, these being either circular or elliptical.
The basic constellation provides maximum satellite coverage above 25° of north latitude, that is, over the earth's northern temperate zones. It is optimized to provide service to the United States, Canada, Europe, the Middle East, the former Soviet Union, China, Korea and Japan. In its extension the constellation provides maximum coverage and satellite resources above 25° of north latitude, where the bulk of the world's population lies; it further provides gradually reduced satellite resources but continuous coverage below 25° north, down to 50° of south latitude with minimum horizon look angles of 15° at all times.
The amount of coverage and deployed satellite resources can be increased by overlaying arrays of satellites with suitable offsets or by varying the number of satellites in each subconstellation. The invention applies to any constellation of satellites of the earth where at least twelve satellites in the constellation are in inclined elliptical orbits with apogees in the northern hemisphere below 4,000 kilometers in altitude, and at least six satellites in the constellation are in one equatorial orbit.
While communications satellites, by virtue of their altitudes which can range from hundreds to thousands of kilometers above the earth, can see and serve large areas of the earth at once, satellites are very limited in how they move about the earth by the well known laws of planetary motion. For example, all satellites must revolve about the earth following an elliptical path (including circular paths, which are ellipses of zero eccentricity), and their orbits always lie in a plane that contains the center of the earth. Those seeking to provide satellite communications services have for many years sought ways to construct orbits or constellations of orbits that, while obeying the laws of planetary motion, tailored the amount of satellite coverage and of satellite resources made available to conform to the demand of services in specific geographic areas.
Early satellite orbits were relatively circular and low in altitude (only several hundred kilometers). In such low orbits, the satellites rotate around the earth at speeds higher than earth's own rotation about its axis; consequently a single satellite is in view of a specific region of earth for only a limited time interval. Many satellites are thus required to provide continuous coverage to a particular service area.
It was quickly realized that the inclination, relative to the equator, of the orbital plane of these satellites could be adjusted to optimize the coverage that the satellite constellation provided to the preferred service areas. The Echo satellites were an example of such an approach. This approach tailored the maximum extent in latitude north and south to which satellites would provide service. However coverage was symmetrical about the equator, with an unavoidable fixed distribution of coverage and other satellite resources having a slight preference in coverage for latitudes near the latitude limits over those near the equator.
Later, equatorial geosynchronous satellites became feasible and the preferred approach to provide optimum satellite coverage to a specific service area. Satellites launched into this circular orbit, having a period of 24 hours and zero inclination, revolve about the earth in step with the rotation of the earth. Consequently the satellite appears to remain stationary above a fixed point on the earth's surface. This orbit permits adjusting the satellite longitudinal coverage on the earth to a preferred longitude, but the satellite latitudinal coverage is fixed in a pattern that is symmetrical about the equator, and that favors areas nearest the equator. Moreover, satellites in these orbits do not provide coverage beyond about 70° latitude north or south. These orbits are also very high (22,300 miles), resulting in more expensive satellite and launch costs. Signal delays to and from these satellites can be objectionable, and signal path losses are high, requiring more powerful and sensitive communications equipment.
In recent times another difficulty with geosynchronous satellites has arisen. The geosynchronous equatorial orbit used by these satellites is becoming very crowded. Available positions for future satellites—orbital “slots”—are dwindling.
In order to compensate for poor coverage at high latitudes, as provided by geosynchronous satellites, orbits inclined relative to the equator were considered. For example the United States Milstar satellite system was designed to use inclined circular orbits to obtain better coverage at higher latitudes. However, even in this case, the satellites spend as much time over the southern hemisphere as over the northern.
To provide better and selective coverage at high latitudes, the Soviet Union developed the Molniya satellite system using highly elliptical orbits with apogees in the northern hemisphere. This constellation provides better coverage at higher northern latitudes. The relatively high orbit, (about 40,000 km of apogee by 426 km of perigee with a 12 hour orbital period), combined with the high inclination (63.4°), of the Molniya satellites gives some longitude preference to the Soviet Union and the United States over other longitudes. This inclination prevents precession of the line of apsides (line containing the semi-major axis of the ellipse) around the orbit with an argument of perigee of 270°, i.e. apogee at the northernmost point of the orbit. Since in inclined elliptical orbits satellites appear to linger at the apogee and race past the point of perigee, these orbits provide very good coverage of the northern latitudes of the former Soviet Union while spending relatively little time in the southern latitudes where they are not needed. Molniya satellites in orbits with a northern apogee in fact do not, and are not intended to, provide continuous coverage of southern latitudes. The Washington/Moscow Hot line used Molniya satellites.
A combination of Molniya orbits, some with northern apogees and some with southern apogees, could provide global coverage. However, coverage would be symmetrical about the equator.
U.S. Pat. No. 4,854,527 describes a constellation of elliptical orbits that provides continuous coverage at all altitudes (above those which correspond to a 27-hour period) with a minimum of four satellites, two having apogees in the northern hemisphere and two in the southern hemisphere. The satellites define the points of a continually rotating tetrahedron whose sides never or almost never intersect the earth.
Several observations can be made concerning the Draim constellation. First, continuous coverage using this constellation requires satellite periods of at least 27 hours and concomitant altitudes even higher than those of geosynchronous altitudes. As a result, satellites for these orbits would be expensive to build and launch, and voice communications would suffer from unacceptable delays. Second, minimum elevation angles are very low, 4° or less, even for constellations with periods of as much as 100 hours. For many satellite systems, and especially so in mobile applicat
Arthur Gertrude
Mobile Communications Holdings, Inc.
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