Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1997-06-25
2001-06-12
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06246501
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method and an arrangement for the employment of optical free space transmission systems for interruption-proof linkages between individual satellites of globally accessible low earth orbiting satellite communications systems.
BACKGROUND OF THE INVENTION
The cellular communication networks which have lately been established worldwide in many densely populated regions provide an unprecedented measure of individual accessibility in cooperation with already established land-line communications networks. The economic advantages for the consumers of corresponding services arising from this justify the high costs of the infrastructure connected with this. Under normal topographic conditions, a single fixed transmission station of such a network can cover mobile users at a distance of up to approximately 20 km, wherein the given maximum extent of a cell limited by this becomes considerably smaller in an unfavorable terrain. Based on positive experiences with communications satellites in geostationary orbits, the idea suggests itself to gain by similar means the considerable independence from the terrain achieved by this, as well as the large spatial extent of the covered area, also in connection with mobile communi-cation systems.
While, because of the development of very low noise preamplifiers and mixers, radio reception from geostatic satellites became possible even with relatively small antennas, in connection with bidirectional linkages the problem of transmitting acceptable data rates over very large distances (approximately 36,000 km) by means of antennas transmitting almost non-directionally and at very low transmission output to the geostationary satellite remains. A solution of this problem lies in the use of satellites circling relatively low above the earth's surface, whose limited range is compensated by the presence of a multitude of identical satellites, which exchange information with each other and pass it on. Several concrete proposals already exist, among these are IRIDIUM (P. Brunt, “Iridium: Overview and Status”, in Space Communications, vol. 14, No. 2, 1996, pp. 61 to 68), and M-STAR and TELEDESIC (System Description Excerpt, Mar. 21, 1994), wherein IRIDIUM will enter practical use before the end of the century.
A characteristic of all mentioned systems is the employment of a multitude of satellites moving in low orbits around the earth which, divided into sub-groups, are respectively evenly distributed over an orbit, which is distinguished in that it penetrates the plane containing the earth's equator at two points at an obtuse angle. The orbits of all satellites are arranged in respect to each other in such a way that an even coverage of the earth's surface by satellites is achieved. The orbits taken up by the satellites intercept each other in two points as a function of the inclination of the orbit. At the same altitude of all paths, a collision of satellites in different orbits is prevented by so-called inter-plane phasing and a selection of angles of the inclination of the individual orbital planes which is unlike 90°. At this time, the linkage of the individual satellites with each other takes place by means of appropriately aligned directional microwave antennas. This does not present a problem in connection with satellites in the same orbit, since the distance as well as the direction of the neighboring satellites are relatively stable. However, the situation becomes complicated with linkages with satellites of neighboring orbits.
In the course of one circumnavigation of the earth, a lateral change of the satellites flying along in neighboring orbits occurs at the intersections of all orbits. If there is a radio linkage with the satellites located laterally in respect to the direction of flight, it is necessary to perform tracking with the directional antenna over a larger angular spatial range as soon as the reception output falls below a minimum or, if this is not possible, a transfer to an antenna placed in a different direction must take place. The change of the directions of the neighboring satellites can take place almost instantaneously, if they are located nearly crosswise to the direction of flight. Therefore, under normal circumstances a transfer to another antenna takes place, because of which a contactless phase results because of the required acquisition time.
Also, rapid rotating movements of bodies of large mass and spatial expanse, which are attached to the satellite body, add to the destabilization of the latter. Finally, the limited extent of the directional antennas results in the transmission of energy over a comparatively large spatial angle, even when using microwaves, because of which as many different transmission channels as possible must be available in view of a situation of high density of satellites occurring on account of the closeness of the intersecting points of all orbits. This forces a limitation of the bandwidth of the individual channels because of the limited bandwidth of the directional microwave antennas employed. However, this is unacceptable for the linkage of the satellites with each other, since information from other satellites is also passed on via these linkages, so that the flow of information is considerably higher than in the traffic between the ground and the satellites.
OBJECT AND SUMMARY OF THE INVENTION
It is therefore the object of the invention described below, to prevent the disadvantages of the prior art and to assure communications links with arbitrary neighboring satellites of the above described systems which is free of interruptions and at high transmission rates by means of a device of small spatial size and low weight.
This object is attained by means of the method described in the characterizing portion of claims
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to
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and the arrangement described in the characterizing portion of claims
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and
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.
The basic concept of the invention lies in the replacement of certain transmitting devices based on microwaves on board the satellites utilized in the above mentioned systems by optical communications terminals. These are installed in an at least fourfold arrangement on each satellite and contain a transmission device intended for transmission and reception, which is designed to be rotatable in azimuth and elevation by means of an optical head. Terminals in accordance with the invention are applied to the exterior surfaces of the satellite respectively both in the direction of movement and opposite the direction of movement of the satellite in such a way that the zero azimuth alignment of their telescopes coincides with the tangent of the orbit described by the satellite.
Further configurations can contain any arbitrary number of terminals, in particular for the simultaneous maintenance of more than two linkages with neighboring satellites. Furthermore, in addition to other current systems, the homodyne method is provided for a data exchange between two satellites, which is unaffected by foreign light sources. It is possible in this way to also detect the signal of a satellite located directly in front of the sun. In the course of passing through the intersection points of all orbits, the contact made with a satellite which is in another, lateral orbit is maintained by tracking by means of the telescope, which is rotatable around two axes and is small and light. Tracking is also performed by means of coherently detected difference signals, and is therefore unaffected by interferences caused by the sun or other external light sources. Furthermore, the optical output of the light beam transmitted from an optical terminal is concentrated in a very narrow angular range in spite of the small aperture of the telescope, so that the required transmitting output is very low.
The very exact alignment of the light beam with the desired satellite required because of this also simultaneously contains as a critical factor the advantage of interference-free communication with satellites arranged at a short mutual distance, such as is the c
Dreischer Thomas
Kellermeier Hans
Browdy and Neimark
Contraves Space AG
Pascal Leslie
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