Attitude Determination and Control System for Low Earth Orbit CubeSat Considering Operation Scenario

Abstract

SNUGLITE (Seoul National University GNSS Laboratory satellITE), a cubesat scheduling for launch in 2017, must maintain nadir pointing attitude control in order to obtain mission data successfully and send them to ground station using S-band antenna. This thesis will examine the ADCS (Attitude Determination and Control System) algorithm and how the proposed algorithm satisfies the ADCS requirements for this particular operation scenario. The ADCS is composed of 3-axis MEMs gyroscope sensor, 2-axis coarse photodiode type sun sensors, 3-axis MEMs magnetometer, dual frequency GPS receivers, and 3-axis magnetic torquers. In the simulation environment, studies were done on the eclipse and various disturbance torques, including gravity gradient torque, solar radiation torque, and aerodynamic torque. The LQG (Linear Quadratic Gaussian) controller has been chosen for ADCS algorithm for SNUGLITE. Furthermore, this thesis will provide SILS (Simulation In the Loop System)—which takes low earth orbit environments into account—to verify the proposed ADCS algorithm. Due to the fact that the precise launch schedule of the cubesat has not yet been decided, the orbit elements have been chosen for the worst case scenario: it will provide a maximum eclipse time in altitude of 600[km] from the circular suns synchronous orbit. In order to monitor the low earth orbit environment which has a direct effect on the attitude dynamics, we developed a low earth orbit simulator that is comprised of attitude dynamics and orbit dynamics which can influence each other. We used two computers, one for the ADCS algorithm and one for the orbit environment, and transmitted data using serial communication. Thus, the team of scholars after us could use this SILS we developed to further the research on PILS (Processor In the Loop System). The operation scenario consists of two parts. The first step starts from deployment of cubesat from P-POD (Poly Picosatellite Orbital Deployer), and detumbling using B-dot control. Second, the use of LQG controller for nadir pointing control. However, the second part of the operation scenario is also divided into two segments. In the first phase, only GPS is used as a payload but magnetic boom is not deployed, while in the second phase, magnetic boom is used for the earthquake mission. After evaluating the simulation results, we have come to a conclusion that all the ADCS requirements were met. For instance, the attitude estimation errors were less than 5 [deg] in eclipse and less than 2 [deg] per day. In addition, the attitude control errors were less than 10 [deg] in eclipse and less than 5[deg] per day. Finally, the ADCS algorithm enabled the cubesat to turn over even in an up-side-down position. In summary, this thesis developed and verified the ADCS algorithm which was based on the Matlab by using the LQG controller

moreover, it offers the space environment simulator which could be used for the PILS (Processor In the Loop System) study in the future. We expect these results will contribute to making SNUGLTIEs mission a success.