Spacecraft three-axis attitude acquisition from sun direction measurement

1. A method for orienting a spacecraft orbiting about a celestial body, said method comprising the steps of:

determining a set of actual conditions of the spacecraft, said set of actual conditions comprising (i) a position of the spacecraft in inertial space as a function of time, (ii) a set of angular rotation rates of the spacecraft, said set of angular rotation rates being described with respect to a coordinate frame of the spacecraft, and (iii) an actual instantaneous direction of the sun, said actual instantaneous direction being described with respect to the coordinate frame;

propagating an estimated sun direction, described with respect to the coordinate frame, said estimated sun direction being an estimate of the actual sun direction propagated as a function of time based on the previously determined set of angular rotation rates and the actual instantaneous direction of the sun;

determining a desired sun direction, said desired sun direction varying as a function of time and being described with respect to the coordinate frame using orbital ephemeris data and actual time with respect to a known epoch;

rotating the spacecraft and adjusting angular rotation rates of the spacecraft so that (i) an actual angle between the spacecraft coordinate frame and the sun substantially coincides with the desired sun direction with respect to the coordinate frame, and (ii) said actual angle and said desired sun direction vary as a function of time in a substantially similar manner; and,

rotating the spacecraft around an axis defined by a line between the sun and the spacecraft until the celestial body is observed by a celestial body sensor of the spacecraft.

2. The method of claim 1 , wherein prior to the step of determining a set of actual conditions, at least one element of the set of actual conditions is arbitrary and unknown.

3. The method of claim 1 , wherein prior to the step of determining a set of actual conditions, every element of the set of actual conditions is arbitrary and unknown.

4. The method of claim 1 , wherein the step of determining a set of actual conditions includes determining an output of a set of spacecraft rate sensors.

5. The method of claim 4 , wherein the set of spacecraft rate sensors is actuated autonomously by logic embedded in a control electronics module of the spacecraft.

6. The method of claim 4 , wherein the set of rate sensors is actuated by a ground controller command.

7. The method of claim 1 , wherein the step of propagating an estimated actual sun direction is accomplished using quaternion algebra.

8. The method of claim 1 , wherein the step of rotating the spacecraft and adjusting rotation rates of the spacecraft is accomplished in accordance with a quaternion based calculation.

9. The method of claim 1 , wherein the step of rotating the spacecraft and adjusting rotation rates of the spacecraft is accomplished in a single maneuver.

10. The method of claim 1 , wherein the step of rotating the spacecraft and adjusting rotation rates of the spacecraft is accomplished autonomously.

11. A method for orienting a spacecraft orbiting about a celestial body, said method comprising the steps of:

determining a set of actual angular rotation rates of the spacecraft, said set of actual angular rotation rates being described with respect to a coordinate frame of the spacecraft while said spacecraft is in a substantially nominal 3-axis attitude with respect to a celestial body and the sun;

determining a position of the spacecraft in inertial space as a function of time;

determining an actual instantaneous direction of the sun, said actual instantaneous direction being described with respect to the coordinate frame;

propagating an estimated sun direction, described with respect to the coordinate frame, said estimated sun direction being an estimate of the actual sun direction propagated as a function of time based on the previously determined actual angular rotation rates and the actual instantaneous direction of the sun;

performing a spacecraft procedure;

monitoring an actual attitude of the spacecraft with respect to the celestial body and the sun; and

when said monitoring detects an abnormal departure from the nominal 3-axis attitude with respect to the celestial body and the sun:

determining a desired sun direction, said desired sun direction varying as a function of time and being described with respect to the coordinate frame, using orbital ephemeris data and actual time with respect to a known epoch;

rotating the spacecraft and adjusting angular rotation rates of the spacecraft so that (i) an actual angle between the spacecraft coordinate frame and the sun substantially coincides with the desired sun direction with respect to the coordinate frame, and (ii) said actual angle and said desired sun direction vary as a function of time in a substantially similar manner; and,

rotating the spacecraft around an axis defined by a line between the sun and the spacecraft until the celestial body is observed by a celestial body sensor of the spacecraft.

12. The method of claim 11 , wherein the step of propagating an estimated actual sun direction is accomplished using quaternion algebra.

13. The method of claim 11 , wherein the step of rotating the spacecraft and adjusting angular rotation rates of the spacecraft is accomplished in accordance with a quaternion based calculation.

14. The method of claim 11 , wherein the step of rotating the spacecraft and adjusting angular rotation rates of the spacecraft is accomplished in a single maneuver.

15. The method of claim 11 , wherein the step of rotating the spacecraft and adjusting angular rotation rates of the spacecraft is accomplished autonomously.