# In-Orbit Results and Attitude Analysis of the SNUGLITE CubeSatellite
## 1 Introduction
The SNUGLITE CubeSat was successfully launched and settled in orbit on December 03, 2018, and it has broadcast its status data continuously for more than a year, with signals being received worldwide, both at the SNUGLITE ground station and at amateur ground stations [13]. The received data include information on the status of the SNUGLITE, estimated attitude information, and the calculated GPS results. The attitude system for the SNUGLITE CubeSat consists of low-cost sensors and actuators: coarse sun-sensors (five-photodiodes), a three-axis magnetometer, and a three-axis gyroscope, as attitude sensors, and three-axis magnetorquers as actuators. One of the contributions of this study is that the LQG-based attitude system for the Cubesat platform was developed for an optimal solution operable for the in-orbit environment. The LQG controller is an optimal controller where a quadratic cost function is minimized when the plant has random initial conditions, white noise disturbance input, and white measurement noise [14]. The solution of the LQG problem combines the solutions of the deterministic linear quadratic regulator (LQR) problem and the optimal state estimation problem using the linear-quadratic state estimator, the Kalman filter. In the SNUGLITE CubeSat, the filter and controller are implemented in an onboard computer that performs in real-time. Previous researchers have derived the solution of the LQG attitude system for the CubeSat platform and performed simulation-based analyses [15; 16], while others have performed hardware-based, single-axis verification experiments using wire and a Helmholtz cage [17]. This paper describes and analyzes the in-orbit real-time performance of the attitude estimation using the LQG system. Though LQG was considered for the attitude system, only estimation results could be analyzed properly using the received status data. Therefore, the focus of this study was the performance analysis of the attitude estimation. Because previous studies have limitations in representing the generalized attitude estimation performance, another contribution is the proposed evaluation method for the in-orbit estimation performance for the CubeSat platform. The attitude systems of the RAX-1 and RAX-2 CubeSats were quantified by the state error covariance of the estimation filter without reference attitude [8]. Camera images were utilized to generate the reference attitude [6; 9]. However, the number of image samples (less than 15 samples) was not sufficient to evaluate the attitude estimation performance. In this study, the estimation performance was investigated by comparing two sun-pointing vectors: the calculated sun-pointing vector by estimated attitude and the reference sun-pointing vector by the generated power levels of the solar panel on each side. Therefore, a sufficient number of samples of reference attitude were considered for the estimation analysis. To generate the sun-pointing vector in the body-frame using the estimated attitude, we utilized the filtered results of the magnetometers, gyroscopes, and photodiodes, as well as the positions of the satellite and the sun. For the reference vector, power levels, which are independent measurements from the photodiodes--although both measure the strength of the sunlight--were normalized to point to the sun in the body-frame. Error vectors between two sun-pointing vectors and their difference in angle were mainly investigated for the performance analysis of the attitude estimation. To calculate the sun-pointing vector utilizing estimated attitude, the position of the CubeSat was determined from the results of the GPS receiver. Because the GPS receiver was newly developed for the SNUGLITE CubeSat, its validation was also required. Given that the fundamental operability of GPS receivers has already been presented [18], this paper describes newly found characteristics, such as altitude variations by perturbations. This paper describes the results of the technical mission of the SNUGLITE CubeSat.