Aeronautics and astronautics – Spacecraft – Attitude control
Reexamination Certificate
1999-04-29
2001-09-04
Barefoot, Galen L. (Department: 3664)
Aeronautics and astronautics
Spacecraft
Attitude control
C244S165000, C342S354000
Reexamination Certificate
active
06283415
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates generally to a method for steering a satellite antenna beam, and more particularly, to a method for simplifying the steering of an antenna beam 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.
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 fairly simple 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 yawing the antenna (preferably by yawing the entire satellite, for example, by using a reaction wheel system) by an appropriate angle, which varies throughout the orbit. The yaw steering method in accordance with the present invention, which is easy to implement, results in a considerable simplification of the antenna beam steering system.
The yaw steering method of the present invention uses a yaw angle &phgr;, which is a function of the time from the ascending node of the orbit of the satellite, the period of the orbit and the inclination of the orbit. Assuming a circular orbit the desired yaw angle &phgr;, in accordance with the present invention, is given by the expression:
tan(&phgr;))=[sin(i)cos(2&pgr;/P)]/[(D/P)-cos(i)]
where:
&phgr;is the desired yaw angle;
i is the inclination of the orbit;
t is the time in the orbit of the satellite from the ascending node of the orbit;
P is the period of the orbit; and
D is the period of the rotation of the Earth.
For polar orbits, the inclination, i, is 90° and the expression for the desired yaw angle, &phgr;, reduces to:
tan(&phgr;)=(P/D) cos(2&pgr;/P).
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 the inclination of the orbit; determining the time in the orbit from the ascending node; determining the period of the orbit; determining the period of the rotation of the object; and steering the antenna about the yaw axis by an angle, &phgr;, wherein &phgr; 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 accordance with a further aspect of the present invention, the steering step includes a step of calculating the angle &phgr; 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.
In accordance with yet another 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 the Earth, the orbit having an inclination and an ascending node. The method comprises the steps of: determining the inclination of the orbit; determining the time in the orbit from the ascending node; determining the period of the orbit; and steering the antenna about the yaw axis by an angle, &phgr;, wherein &phgr; 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 Earth.
The use of the present invention for an antenna mounted to a satellite in a low-earth orbit inclined at about 85 degrees at an altitude of about 1400 km (about 870 miles) is predicted to reduce the cross-track path of a ground target relative to the antenna from about 4.5 degrees to about 0.04 degrees.
The invention itself, together with further objects and attendant advantages, will be best understood by reference to the following detailed description, taken in conjunction with the accompanying drawing.
REFERENCES:
patent: 5184139 (1993-02-01), Hirako et al.
patent: 5463400 (1995-10-01), Tayloe
patent: 6023242 (2000-02-01), Dixon
Barefoot Galen L.
Gudmestad T.
Hughes Electronics Corporation
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