Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
1999-12-23
2001-09-11
Issing, Gregory C. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
Reexamination Certificate
active
06288670
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates generally to a method for steering a satellite antenna beam or array of beams, and more particularly, to a method for simplifying the steering of an antenna beam or array of beams on a satellite in an inclined earth orbit in order to compensate for cross-track motion of earth-based terminals that is caused by rotation of the Earth.
(b) Description of Related Art
Antenna systems for communication satellites that are in non-geostationary orbits may require continuous adjustment of beam steering directions relative to the satellite to maintain coverage of users located within an earth-fixed cell during the pass of the satellite over the cell. The direction from the satellite to the users in satellite coordinates is affected by the rotation of the Earth as well as by the orbital motion of the satellite. The surface speed of the Earth due to rotation is proportional to the cosine of the latitude of the satellite, which varies throughout the orbit for all but equatorial (zero-inclination) orbits. This variation of relative velocity as a function of latitude normally requires beam steering in the cross-track direction (i.e., orthogonal to the velocity vector of the satellite) as well as in the along-track direction (i.e., along the velocity vector of the satellite), which in turn results in excessively complicated and cumbersome beam steering systems.
LEO satellite systems have the disadvantage that the satellite is in motion relative to the stationary or slowly moving user on or near the earth's surface. Users are usually grouped into cells depending on the user's geographic location. In the communications system, each cell is associated with a satellite antenna beam that transmits signals to or receives signals from the users located in a particular cell.
In prior art satellite systems, the cell-beam relationship can be described as either earth-fixed cells or satellite-fixed beams. In satellite-fixed beam systems, the beams point in fixed directions relative to the satellite body and thus sweep over the cells as the satellite moves through its orbit. As a result, the users must be reassigned to different beams frequently. There must be rapid reassignment calculations and frequent messages exchanged between the satellite and the user to coordinate the reassignment, leading to a significant overhead load being used for control (i.e., messages instead of for voice, data, video, etc.) on the system.
In earth-fixed cell systems, the satellite must continuously repoint the antenna beams to follow the motion of the cells as seen from the moving satellite. Implementing earth-fixed cells requires a very complex antenna that can steer many beams in two angular dimensions. Rapid reassignment calculations and overhead load are reduced at the expense of a vastly more complex antenna.
If the satellite antenna system is an electronically steered, high gain, low side lobe multibeam array, antenna steering may involve the control of the phase and amplitude of many elements. The number of active control elements required is substantially increased when beam steering is required in the cross-track direction as well as the along-track direction. This is normally the case, since for an antenna array aligned with the satellite geometric axes, cross-track motion results from the rotation of the Earth.
SUMMARY OF THE INVENTION
By using a combined roll-yaw steering method for the satellite, cross-track beam steering can be avoided, thereby greatly simplifying the antenna beam control steering system. In accordance with the present invention, cross-track motion of ground targets resulting from the rotation of the Earth can be dramatically reduced in antenna coordinates by rolling and/or yawing the antenna by an appropriate angle, which varies throughout the orbit. Preferably, the entire satellite is rolled and/or yawed, for example, by using a reaction wheel system. The roll-yaw steering method in accordance with the present invention results in a considerable simplification of the antenna beam steering system.
In accordance with one aspect of the present invention, a method is provided for steering a satellite antenna mounted to a satellite. The satellite has a pitch axis, a roll axis, and a yaw axis and travels in an orbit around a rotating object. The orbit has an inclination and an ascending node. The method comprises the steps of: determining a curvature of trajectories induced by the antenna; determining an angular distance of the satellite from the ascending node; determining an inclination of the orbit; determining a time in the orbit from the ascending node; determining a period of the orbit;
determining the period of the rotation of the object; steering the antenna about the roll axis by a first angle, wherein the first angle is a function of the curvature of trajectories induced by the antenna and the angular distance of the satellite from the ascending node; and steering the antenna about the yaw axis by a second angle, wherein the second angle is a function of the inclination of the orbit, the time in the orbit from the ascending node, the period of the orbit, and the period of the rotation of the object.
In one embodiment, the steering step includes a step of calculating the first angle using the formula R=Croll*sin(U), where Croll is a constant which depends on the curvature of trajectories induced by the antenna and U is the angular distance of the satellite from the ascending node.
In some embodiments, the steering step includes a step of calculating the first angle using the formula &phgr;= arctan [[sin(i)cos(2&pgr;t/P)]/[(D/P)-cos(i)]], where i is the inclination of the orbit, t is the time in the orbit from the ascending node, P is the period of the orbit, and D is the period of the rotation of the object.
REFERENCES:
patent: 6154692 (2000-11-01), Cielaszyk et al.
Rosen Harold A.
Villani Daniel D.
Gudmestad T.
Hughes Electronics Corporation
Issing Gregory C.
Mull Fred H
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