## 1 Introduction
Blue jets are relatively small TLEs compared to sprites and they occur in the stratosphere. right accession of the ascending node (RAAN) will be determined after VisionCube is deployed in orbit. The life span of the VisionCube is expected to be three months, carrying out the TLE observation mission. The mission requirement states that VisionCube should be able to detect TLEs at an arbitrary location, to reorient its camera towards the TLE, and to acquire the images of successive events. This is because that the repetition rate of TLEs is difficult to predict in advance. Nonetheless, it is known that TLEs occur in the upper atmosphere of the cloudy area [4], it is a possible scenario that another TLE would occur again near the initial location of occurrence. Figure 2 shows two possible scenarios of TLE detection process. When the observation system detects a TLE occurring along the nadir direction of the satellite shown in the left, the VisionCube starts locking-on to the detected TLE area by initiating attitude control. The lock-on lasts up to 40 s, waiting for another TLE occurrence in the detected area. In the second case, the detection process begins with scanning the large area by a \(\pm 50^{\circ}\) roll maneuver of the satellite. Similar to the previous case, the VisionCube locks-on to the detected TLE area near the limb of the Earth when the TLE observation system detects a TLE occurrence. During the lock-on period, the TLE observation system would acquire the images of successive events of TLEs. Since the onboard camera is equipped with a lens of 35\({}^{\circ}\) field of view (FOV), the geographical coverage of the acquired image is computed by 98,736 km\({}^{2}\) in the nadir direction and 301,920 km\({}^{2}\) in the limb direction, respectively. The lock-on duration 40 s is empirically determined in the consideration of the speed of the VisionCube and the minimum sensitivity of the TLE observation system. The onboard TLE observation system is designed and manufactured by taking advantage of system-on-chip (SoC) device so that the entire system fits on a small CubeSat platform. This TLE observation system allows detection of the occurrence of TLEs in orbit on its own and then captures and stores the digital images for later ground downlink. The technical significance of the presented paper lies in the development of miniaturized satellite, which is capable of detecting TLEs and obtaining digital images at considerably lower cost. In particular, both the dedicated electronic hardware and the software to implement the TLE observation algorithm were designed and built in the university laboratory. The paper focuses on three major aspects of the onboard TLE observation system: hardware development, TLE observation algorithm and software, and the experimental validation using the ground simulation facility, and is organized as follows: Section 2 describes the hardware design of the TLE observation system including electrical components and mechanical parts. Section 3 explains the basic algorithm for TLE observation with details about the software design of TLE observation system. In Section 4, the design of a ground experimental facility that simulates the mission environment and the validation results of the developed TLE observation system are presented. Finally, Section 5 summarizes the main results of this paper and possible future research directions will be presented. Figure 2: Two possible mission scenarios of the VisionCube to detect TLEs. The left figure shows the case when the VisionCube looks down towards the nadir direction. In contrast, the right figure shows the case when the VisionCube extends the observation area by scanning motion via the roll maneuver.