الفهرس 
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Abstract
Moderate –Accuracy attitude determination system for earth –pointing spacecraft typically use gyroscopes earth &sun sensors for Attitude sensing .the earth sensors can provide continuous roll and pitch data ,while the sun sensors can provide roll and yaw data However ,due to Spacecraft eclipse and limited fields of views ,these data are available for only part of each orbit .moreover from an economical perspective ,if a suitable observer is developed it can estimate the expected data from sun sensor in the Spacecraft eclipse periods therefore a substantial cost reduction can be achieved
To address this issue the thesis is concerned with an investigation and comparison of observers that can be used to estimate the yaw angle information
The Spacecraft ADCS exhibits several models we are concerned with the pre-stabilization mode where the variations in the Attitude angle are relatively small .thus our concern is to consider observers that are locally defined and are operating in the presence of the stabilizing controller which insures the small angle deviation .due to its simplicity and acceptable performance ,linear controllers are used in conjunction with the observers
Standard linear observers such as linear deterministic and Kalman filter are considered in addition nonlinear observers are investigated
EKF performance is compared to a developed sliding mode observers the stability analysis of the proposed sliding mode observers is provided