Development of an advanced nano-satellite (VELOX-I) – MEMS based attitude sensing system

Abstract

The satellite research centre (SaRC) of Nanyang Technological University is developing the first nano-satellite in Singapore. This project focuses on the Attitude Determination System of the VELOX-I, more specifically, the Coarse Sun Sensor. As the first part of the project, the photodiodes that was used to implement Coarse Sun Sensor was investigated. The photodiode cosine characteristics were found to have deviations from the true cosine property by up to 5%. The maximum output current from the photodiode at a solar constant, AM0, was found to be 1.3mA. The Coarse Sun Sensor was proven to have a pointing accuracy of 8 degrees which is comparable to other satellites as reported in the literature. An orbit path was also simulated and results reflected a similar result of close to 8 degrees. A significant finding was that maximum error occurs when light shines on only 1 photodiode. Software calibration was proposed to improve it to an estimated accuracy of less than 2 degree. The second part of the project focuses on the study of the effects of Earth Albedo. Using the Reflected Shortwave Radiation data from National Aeronautics and Space Administration (NASA), a Matlab simulation model of Earth Albedo was formed. It provided the necessary albedo information to be used in the orbital simulation. The effect of this Earth Albedo on the Sun pointing accuracy can be as high as 11 degrees and the attitude determination error was found to be as high as 12 degrees in the roll, pitch and yaw. Compensation for this albedo effect was proposed. Last but not least, fault detection for the vector sensors was achieved through the use of the residuals present in the current design of the Kalman Filter.