SPACECRAFT ATTITUDE DETERMINATION USING GYROS AND QUATERNION MEASUREMENTS

Five separate Kalman filter formulations are developed in this paper f or determining spacecraft attitude using standard rare gyros and senso rs which output three-axis attitude measurements. All five formulation s estimate a six-state vector composed of three error quaternion state s and three gyro bias states. Two of the filters presented are derivat ives of previous formulations which were originally developed using st andard star tracker vector measurements. These two filters are coupled through the state equations, requiring the solution of Is Kalman gain s. The other three filters utilize assumptions on body rates to uncoup le the state equations, requiring the solution of only 6 Kalman gains. Additionally, four of the filters lend themselves to simple steady-st ate solutions. The last filter represents a novel hybrid formulation w hich is designed to take advantage of the computational ease of the st eady-state filters while maintaining the convergence speed of the time -varying filters. A nadir-pointing spacecraft in an elliptical orbit i s used as an example to exercise each of the filters and compare their responses. Each filter results in essentially identical estimates eve n though the level of computational effort is quite different.