Aeronautics and astronautics – Spacecraft – Attitude control
Patent
1994-02-10
1997-12-16
Barefoot, Galen L.
Aeronautics and astronautics
Spacecraft
Attitude control
B64G 124
Patent
active
056975821
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to a method for adjusting the orientation of satellites and for compensating interfering torques by means of solar pressure torques. More particularly, a method is provided for a satellite which is equipped with two solar generators moving on an orbit around the earth. The solar generators are arranged symmetrically on opposite sides of the main body of the satellite. The solar generators can be rotated by means of servomotors independently of one another about a first axis of rotation respectively which is orthogonal to the orbiting plane when the orientation of the satellite is correct. The desired orientation of such satellites, for example, with respect to the earth, is a function of the satellites mission. The orientation is disturbed by asymmetries in the construction of the satellite, by superstructures on the main body of the satellite (such as antenna reflectors), and also by constructional tolerances, the migration of the center of gravity during the mission, residual magnetism and the like in interaction with environmental conditions (such as gravitational forces, the earth's magnetic field and solar pressure). In the case of satellites having a large geometrical dimension which are situated at a high altitude above the earth surface, such as communication satellites on geostationary orbits, mainly of decisive significance are interference forces and interfering torques which are caused by the radiation pressure of the sun.
It has been attempted in various manners to at least partially compensate the occurring interfering torques by the utilization of the solar pressure forces which originally caused the torques. Thus, it has, for example, been known for a long time that, by means of the adjustment in opposite directions of the two solar generators, which are arranged symmetrically to the main body of the satellite, a so-called "wind-mill torque" may be generated which acts about an axis which coincides with the intersection line of the plane of incidence of the sunlight with the plane of the satellite orbit. The direction of a thus produced torque vector migrates in the terrestrial orbit system with the permanent sun orientation of the solar generators because of the movement of the earth around the sun approximately one degree per day. In the rotating axes system of earth-oriented satellites, which rotate around the earth by 360.degree. per orbit about an axis which is perpendicular to the orbiting plane, the line of effect of this wind-mill torque will rotate correspondingly in addition with the orbiting period of the satellite.
By means of a constant adjustment of the solar generators in opposite directions, those interfering influences whose creation is based on asymmetries of the solar generators themselves with respect to their axis of rotation and on deviations of their orientation from the desired direction, such as zero-point and positioning errors of the solar generator drives or torsions of the generator surfaces, caused by solar pressure, can be compensated in a simple manner. Interfering torques, which are caused by the solar pressure by way of deviations of the center of gravity, can be compensated by means of the above-mentioned adjusting measures only when the offset of the center of gravity is situated perpendicularly to the orbiting plane, that is, in the direction of the axis of rotation of the solar generators, and only in a time average by way of portions of the orbiting periods, but not when the displacement of the center of gravity is situated in the orbiting plane. This is normally the case because of the arrangement of the fuel tanks and their evacuation during the duration of the mission. For this reason and, in addition, because of the superposition of other interfering influences, the compensation is difficult and unreliable in practice.
For utilizing the solar pressure for producing adjusting torques for adjusting the position of satellites and for compensating interfering torques, devices and/or measure
REFERENCES:
patent: 4325124 (1982-04-01), Renner
patent: 4591116 (1986-05-01), Guenther et al.
patent: 4732354 (1988-03-01), Lievre
patent: 4747567 (1988-05-01), Johnson et al.
patent: 4949922 (1990-08-01), Rosen
patent: 5149022 (1992-09-01), Flament
Bittner Helmut
Surauer Michael
Barefoot Galen L.
Deutsche Aerospace AG
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