Development of a pico-satellite : sun sensors for attitude determination

Abstract

Following the completion of the micro-satellite XSAT program, and currently waiting for the launch, NTU has started another program to develop a pico- and nano-satellite pair VELOX-I. This program includes many FYP projects working on the various sub-systems of a satellite.

For a satellite to determine and control its location and orientation in space, the Attitude Determination and Control System (ADCS) is an essential sub-system for the task. This project focuses on the sun sensor in the ADCS which determines the sun direction with respect to the vertical axis of the satellite. The sun direction will allow the satellite to orientate itself perpendicular to the sun direction for maximum solar power collection and also to shelter sensitive equipments away from the strong solar radiation.

In this project, hardware prototype has been designed and implemented on PCB. It has been tested using a sun simulator to simulate real parallel sunrays and a 2-axis turn motor to rotate the sensor. The hardware will provide four voltage analog outputs dependent on the sun spot position.

The voltage outputs is first processed by the microcontroller. Subsequently, the digital data is transferred to PC for noise removal using Kalman filter, computation of sunray angle and error calibration based on the analysis of errors caused by mechanical and electrical design.

The accuracy for the sensor is -1.3º to 0.7º for a field-of-view of -30º to 30º, and ±0.7º for a field of view of -25º to 25º. This performance is comparable to commercial products that have accuracies less than 1º. The prototype has also been tested in outdoors under the real sun and proven to function satisfactorily.