Modeling, image processing and attitude estimation of high speed star sensors

Abstract

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