# Design of Novel Laser Crosslink Systems Using Nanosatellites in Formation Flying: The VISION
###### Abstract
For a laser crosslink, a pointing, acquisition, and tracking (PAT) system is required. The proposed systems are applied to a monostatic architecture that shares the beam path for communication and PAT with a single aperture. This scheme can mitigate a steady-state pointing error with closed-loop feedback based on a fast-steering mechanism (FSM), reducing the residual line-of-sight (LOS) jitter to less than 1 \(\upmu\)rad. The LCT is integrated with a 6U nanosatellite bus, which has a precise pointing system for LOS alignment during laser crosslinking. To generate an accurate target LOS vector, a relative navigation algorithm using both GPS L1 and GPS L2 signals has been designed to mitigate ionospheric delay effects for a long baseline. The two satellites are equipped with a propulsion system for rapid and accurate orbit maneuvers to adjust inter-satellite distances. Figure 1: Conceptual illustration of the VISION mission. The proposed system can transfer large-sized image data and will be applied to construct a high-speed data relay system in future. The contributions of this study are as follows. First, a novel system architecture for future laser crosslinks is proposed. In the last ten years, the key technologies of laser communication systems have been developed and validated by on-orbit missions, adopting large-sized satellite platforms with additional beam-pointing and tracking systems such as gimbal mechanisms [10]. The proposed systems share beam paths for communication and PAT with a single aperture. As the PAT is only assisted by the nanosatellite bus, the systems require precise feedback control combined with bus attitude and FSM operations. This challenging architecture can significantly reduce the steady-state errors of beam paths between communication and tracking, not only by reducing the system size but also by enhancing communication performance. Second, the design processes of a practical laser crosslink mission and related systems are outlined. The system budgets were allocated from the optical link budget design and analysis, including on-orbit noise estimations such as incident sunlight and thermal effects on the detectors. The handover process of the crosslink, applying a monostatic architecture, was designed for link access and maintenance. In addition, considering new technologies for space optics, such as FEO, it is important to evaluate the communication performance based on the optical link budget. The on-orbit operation scenario includes laser crosslink tests with various inter-satellite distances [11]. Unlike other systems utilizing nanosatellite platforms, the proposed systems include a propulsion system for rapidly and accurately changing the distances. Additionally, considering the long baseline, a relative navigation algorithm handling ionospheric delay was designed and analyzed. We adopted a standardized and commercial-off-the-shelf (COTS)-based platform, which would contribute to the agile construction and maintenance of LEO mega-constellations for global networks in the future. Finally, fundamental technologies for advanced space-optical systems are proposed. In particular, a deployable space telescope (DST) is applied to the FEO to improve the optical link performance with a large aperture, which is up to 10 times wider than that of other systems. The optics are composed of three segmented reflectors for the primary mirror and a boom mechanism for the secondary mirror. This deployable optics technology can be applied to Earth observation missions for super-high-resolution images using Cube-/nanosatellite platforms, significantly reducing their size and mass [12]. Furthermore, miniaturized back-end optics were designed utilizing COTS-based optics and FSM, enabling rapid development. The integrated front-end and back-end optics design was optimized to increase the received signal power and enhance the SNR (signal-to-noise ratio) margin of the optical link channel. The remainder of this paper is organized as follows.