Modeling, image processing and attitude estimation of high speed star sensors
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Attitude estimation and angular velocity estimation are the most critical components of a spacecraft's guidance, navigation and control. Usually, an array of tightlycoupled sensors (star trackers, gyroscopes, sun sensors, magnetometers) is used to estimate these quantities. The cost (financial, mass, power, time, human resources) for the integration of these separate sub-systems is a major deterrent towards realizing the goal of smaller, cheaper and faster to launch spacecrafts/satellites. In this work, we present a novel stellar imaging system that is capable of estimating attitude and angular velocities at true update rates of greater than 100Hz, thereby eliminating the need for a separate star tracker and gyroscope sub-systems. High image acquisition rates necessitate short integration times and large optical apertures, thereby adding mass and volume to the sensor. The proposed high speed sensor overcomes these difficulties by employing light amplification technologies coupled with fiber optics. To better understand the performance of the sensor, an electro-optical model of the sensor system is developed which is then used to design a high-fidelity night sky image simulator. Novel star position estimation algorithms based on a two-dimensional Gaussian fitting to the star pixel intensity profiles are then presented. These algorithms are non-iterative, perform local background estimation in the vicinity of the star and lead to significant improvements in the star centroid determination. Further, a new attitude determination algorithm is developed that uses the inter-star angles of the identified stars as constraints to recompute the body measured vectors and provide a higher accuracy estimate of the attitude as compared to existing methods. The spectral response of the sensor is then used to develop a star catalog generation method that results in a compact on-board star catalog. Finally, the use of a fiber optic faceplate is proposed as an additional means of stray light mitigation for the system. This dissertation serves to validate the conceptual design of the high update rate star sensor through analysis, hardware design, algorithm development and experimental testing.
Katake, Anup Bharat (2006). Modeling, image processing and attitude estimation of high speed star sensors. Doctoral dissertation, Texas A&M University. Available electronically from