Method of controlling the attitude and stabilization of a satellite in low orbit

#The present invention relates to methods of controlling the attitude of a satellite placed in an orbit that is low enough for the intensity of the earth's magnetic field to enable attitude to be measured by means of a three-axis magnetometer and to enable attitude to be changed by causing magneto-couplers carried by the satellite to interact with the earth's magnetic field.

#In practice, this condition is satisfied when at least part of the satellite's orbit is at an altitude of less than 2000 kilometers (km).

#An attitude control method is already known (FR-A-2 742 243 or U.S. Pat. No. 5,788,188) making it possible to reduce the speed of rotation of a satellite, in particular on being released from its launcher, and to orient an axis bound with the satellite so that it is normal to the plane of the orbit. In that method, using a so-called “B dot” relationship because it makes use of the derivative of the earth's magnetic field B, the earth's magnetic field is measured along three axes of a frame of reference associated with the satellite, the measurements are differentiated with respect to time, the derivatives are multiplied by a gain, and a current representative of the result is passed through magneto-couplers to create magnetic moments tending to keep the satellite stationary relative to the lines of force of the earth's magnetic field.

#FIG. 1 is a diagram showing the configuration of the lines of the earth's magnetic field through which a satellite traveling in polar orbit passes.

#FIG. 2 shows one possible structure for a satellite to which the invention is applicable.

#FIG. 3 shows successive orientations of a satellite placed on a polar orbit or on an orbit that is highly inclined relative to the equator, from injection until speeds have been reduced in application of the “B spin” relationship.

#It is recalled that the lines of the magnetic field of the earth T have the general appearance shown by the lines B in FIG. 1 in a plane that contains the poles. The use of a B dot relationship makes it possible to absorb the rotational energy presented by the satellite, after separating from the launcher or in the event of an incident, until the satellite becomes fixed relative to the lines of the earth's magnetic field, which means that the satellite is then rotating at twice the angular frequency ω0 of its orbit about the normal to the plane of its orbit.

#In order to avoid interactions between the magneto-couplers and the magnetometer, it is possible to take the measurements and to drive the magneto-couplers in alternation, for example.

#By way of example, the description below relates to a satellite whose general configuration is as shown in FIG. 2. The satellite comprises a casing 34 having solar generators 36 mounted thereon in a fixed orientation. An axis in the plane of the solar generators is written Ys and the axis normal to the plane of the solar generators is written Zs. The casing carries a three-axis magnetometer 38 and magneto-couplers (not shown) enabling moments to be applied to the satellite by interacting with the earth's magnetic field. The satellite also carries a sun sensor 40. It will be assumed that the satellite is to be placed on a low polar orbit that is heliosynchronous and that local time for its ascending node is close to noon-midnight, with the spin axis of the satellite being oriented towards the sun so as to obtain maximum illumination of its solar generators.