Spacecraft momentum management using solar array

1. A method comprising:

determining a target profile of stored momentum as a function of time for a spacecraft, the spacecraft including at least one solar array and a propulsion subsystem;

measuring a momentum storage error (MSE), said MSE comprising a difference between a momentum value actually stored on the spacecraft and a desired momentum value, said desired momentum value conforming to the target profile at a particular time;

reducing the MSE by producing a torque on the spacecraft, said torque resulting from selectively controlling at least one solar array position offset angle, said offset angle being an angle of the at least one solar array with respect to a nominal sun pointing direction.

2. The method of claim 1 , wherein the spacecraft comprises two solar arrays, and reducing the MSE comprises selectively controlling the solar array position offset of only one of the two solar arrays.

3. The method of claim 1 , wherein the spacecraft comprises two solar arrays, and reducing the MSE comprises selectively controlling the solar array position offset of each of the two solar arrays such that a first offset of a first solar array is substantially equal in magnitude and opposite in sign to a second offset of a second solar array.

4. The method of claim 1 , wherein the target profile has a periodic component and the spacecraft comprises momentum storage devices having a capacity sufficient to store the periodic component.

5. The method of claim 4 , wherein the periodic component of the target momentum profile is determined using a harmonic torque estimator (HTE).

6. The method of claim 5 , wherein the HTE is configured to (i) receive respective input values for an angular velocity of the spacecraft relative to an inertial frame, a wheel speed of at least one reaction wheel, and a predicted external torque, and (ii) estimate observable periodic torque based on the received respective input values.

7. The method of claim 1 , wherein the measuring step and the reducing step are performed as part of a substantially continuous feedback control loop.

8. The method of claim 1 , wherein the measuring step is performed episodically at a first discrete interval and the reducing step is performed episodically at a second discrete interval.

9. The method of claim 8 , wherein the first discrete interval is less than one hour and the second discrete interval is in the range of four to forty eight hours.

10. The method of claim 1 , wherein each of the target profile and the actually stored momentum value are determined separately with respect to each of a yaw axis and a roll axis of the spacecraft, and the produced torque is one of (i) a substantially pure inertial yaw torque, and (ii) a combined inertial roll/yaw torque.

11. The method of claim 10 , wherein the produced torque is the combined inertial roll/yaw torque selected from among (i) positive roll/positive yaw, (ii) positive roll/negative yaw, (iii) negative roll/positive yaw, and (iv) negative roll/negative yaw, such that the produced torque reduces a magnitude of at least one of a roll MSE and a yaw MSE.

12. The method of claim 11 , wherein the produced torque reduces the magnitude of both roll MSE and yaw MSE.

13. The method of claim 1 , wherein the MSE comprises a roll MSE, a yaw MSE, and a pitch MSE, and at least the roll MSE and the yaw MSE are reduced without recourse to the propulsion subsystem.

14. The method of claim 13 , wherein the pitch MSE is reduced by the propulsion subsystem.

15. The method of claim 13 , wherein the pitch MSE is periodically reduced by the propulsion subsystem only during low thrust north south stationkeeping maneuvers.

16. The method of claim 13 , wherein the pitch MSE is periodically reduced by the propulsion subsystem only during east west stationkeeping maneuvers.

17. The method of claim 13 , wherein the pitch MSE is reduced without recourse to the propulsion subsystem.

18. The method of claim 17 , wherein the pitch MSE is reduced by producing a pitch torque on the spacecraft, said pitch torque resulting from at least one of adjusting an angle of at least one solar array with respect to a pitch axis of the spacecraft and actuating a solar array trim tab.

19. The method of claim 1 , wherein the offset angle is not greater than 15 degrees.

20. The method of claim 1 , wherein selectively controlling at least one solar array position offset excludes reversing direction with respect to a nominal solar array rotation.

21. The method of claim 1 , wherein a low thrust north south stationkeeping thruster is employed to remove a residual roll/yaw momentum error.

22. The method of claim 1 , wherein reducing the MSE by producing a torque on the spacecraft comprises:

determining a solar array position offset to be controlled, an associated resulting torque, and a corresponding profile of wheel speed of a momentum storage device with respect to time; and

feeding forward the corresponding profile of wheel speed with respect to time to the momentum storage device.

23. A spacecraft comprising:

a satellite control electronics, a propulsion subsystem and at least one solar array, wherein:

the satellite control electronics is configured to manage momentum of the spacecraft by: (i) determining a target profile of stored momentum as a function of time for the spacecraft; (ii) measuring a momentum storage error (MSE), said MSE comprising a difference between a momentum value actually stored on the spacecraft and a desired momentum value, said desired momentum value conforming to the target profile at a particular time; and (iii) reducing the MSE by producing a torque on the spacecraft, said torque resulting from selectively controlling at least one solar array position offset angle, said offset angle being an offset of at least one solar array of the spacecraft from a nominal sun pointing direction.

24. The spacecraft of claim 23 , wherein the target profile has a periodic component and the spacecraft comprises momentum storage devices having a capacity sufficient to store said periodic component.

25. The spacecraft of claim 24 , wherein the satellite control electronics determines the periodic component of the target profile using a harmonic torque estimator (HTE).

26. The spacecraft of claim 25 , wherein the HTE is configured to (i) receive respective input values for an angular velocity of the spacecraft relative to an inertial frame, a wheel speed of at least one reaction wheel, a predicted external torque, and (ii) estimate observable periodic torque based on the received respective input values.

27. The spacecraft of claim 23 , wherein the MSE comprises a roll MSE, a yaw MSE, and a pitch MSE, and at least the roll MSE and the yaw MSE are reduced without recourse to the propulsion subsystem.

28. The spacecraft of claim 27 , wherein the pitch MSE is periodically reduced by the propulsion subsystem only during low thrust north south stationkeeping maneuvers.

29. The spacecraft of claim 27 , wherein the pitch MSE is reduced without recourse to the propulsion subsystem.

30. The spacecraft of claim 27 , wherein the offset angle is not greater than 15 degrees.

31. The spacecraft of claim 23 , wherein selectively controlling at least one solar array position offset excludes reversing direction with respect to a nominal solar array rotation.

32. The spacecraft of claim 23 , wherein a low thrust north south stationkeeping thruster is employed to remove a residual roll/yaw momentum error.

33. The spacecraft of claim 23 , wherein reducing the MSE by producing a torque on the spacecraft comprises:

determining a solar array position offset to be controlled, an associated resulting torque, and a corresponding profile of wheel speed of a momentum storage device with respect to time; and

feeding forward the corresponding profile of wheel speed with respect to time to the momentum storage device.