Method of commanding an attitude control system and attitude control system of a space vehicle

1. A method for controlling an attitude control system ( 30 ) of a space vehicle ( 10 ) where the attitude control system has a maneuvering first subsystem that includes at least one reaction wheel, comprising:

controlling an attitude of the space vehicle ( 10 ) during at least one preparation phase; and

following said controlling, performing an observation phase during which an image capture is performed;

wherein the at least one preparation phase comprises the following sub-steps:

a preparation step ( 50 ), during which commands are issued to the maneuvering first subsystem in order to control the attitude of the space vehicle ( 10 ) such that the attitude of said space vehicle is tilted from one observation attitude to another, and

following said preparation step, a step ( 55 ) in which the at least one reaction wheel is stopped after the wheel has started moving, during which the speed of rotation of the at least one reaction wheel is set to a substantially zero value prior to each observation phase,

wherein, during the observation phase, commands are issued to a fine control second subsystem ( 310 ) that further controls the attitude of the space vehicle ( 10 ), said fine control second subsystem comprising at least one actuator and having a lower vibration signature than that of the maneuvering first subsystem, and

wherein the preparation phase and the observation phase are recurring phases.

2. The method according to claim 1 , wherein the step ( 55 ) in which the at least one reaction wheel is stopped comprises a sub-step ( 550 ) of closed-loop deceleration of said at least one reaction wheel.

3. The method according to claim 1 , wherein the step ( 55 ) in which a stopping of the at least one reaction wheel occurs comprises a sub-step ( 551 ) of open-loop braking the at least one reaction wheel.

4. The method according to claim 3 , wherein the at least one reaction wheel is driven in rotation by a polyphase electric motor, and the open-loop braking sub-step ( 551 ) consists in short-circuiting the phases of said polyphase electric motor.

5. The method according to claim 3 , wherein the open-loop braking sub-step ( 551 ) is executed when the speed of rotation of the at least one wheel is lower than a predefined threshold.

6. The method according to claim 1 , wherein the preparation step ( 50 ) comprises a maneuvering sub-step ( 501 ) during which commands are issued to the maneuvering subsystem to tilt the space vehicle's attitude from one observation attitude to another and a sub-step ( 502 ) during which commands are sent to the maneuvering subsystem to stabilize the attitude of the space vehicle ( 10 ) around the targeted observation attitude.

7. The method according to claim 1 , wherein the preparation step ( 50 ) comprises a sub-step ( 503 ) of offloading the maneuvering subsystem, during which all or part of the angular momenta caused by external disturbance torques are compensated for.

8. The method according to claim 7 , wherein the offloading is executed by sending commands to the fine control subsystem ( 310 ) and/or by sending commands to an offloading subsystem ( 320 ) of the attitude control system ( 30 ).

9. The method according to claim 1 ,

wherein, the maneuvering first subsystem comprises a plurality of reaction wheels configured such that there exists at least one vector of rotation speeds, not all equal to zero, for which a sum of elementary angular momenta generated by each of said reaction wheels is substantially zero, and

wherein the preparation step ( 50 ) comprises, at the start of the at least one preparation phase, a sub-step ( 500 ) of accelerating the reaction wheels around the maneuvering subsystem kernel.

10. The method according to claim 9 , wherein the preparation step ( 50 ) comprises a further sub-step ( 504 ) of decelerating the reaction wheels prior to the step ( 55 ) of stopping the reaction wheels.

11. The method according to claim 1 , further comprising:

during the observation phase, a step ( 60 ) of maintaining the at least one reaction wheel stopped, during which the speed of rotation of each reaction wheel is kept at a substantially zero value.

12. An attitude control system ( 30 ) for a space vehicle ( 10 ), comprising:

a maneuvering first subsystem comprised of at least one reaction wheel, that controls an attitude of the space vehicle during at least one preparation phase during which the attitude of said space vehicle is tilted from one observation attitude to another, said preparation phase being followed by at least one observation phase during which image captures are performed;

means adapted to stop the at least one reaction wheel after the reaction wheel has started moving, before said observation phase; and

a fine control second subsystem ( 310 ), used to control the space vehicle's ( 10 ) attitude during the at least one observation phase, said fine control second subsystem comprising at least one actuator and having a lower vibration signature than that of the maneuvering first subsystem,

wherein the preparation phase and the observation phase are recurring phases.

13. The system ( 30 ) according to claim 12 , further comprising:

an offloading subsystem ( 320 ) configured to compensate for all or part of the angular momenta caused by external disturbance torques.

14. The method according to claim 2 , wherein the step ( 55 ) in which the at least one reaction wheel is stopped comprises a sub-step ( 551 ) of open-loop braking the at least one reaction wheel.

15. The method according to claim 3 , wherein the open-loop braking sub-step ( 551 ) is executed when the speed of rotation of the at least one wheel is lower than a predefined threshold.