Improving Reliability of Laser Communication on CubeSats - Lessons Learned from PIXL-1 to QUBE

Kurzfassung

Free-space optical communication can improve operational space-based services in terms of the data throughput compared to classical radio channels. Use cases that generate high data volumes like Earth observation or remote sensing can benefit from the higher data rates. Replacing or supporting classical channels by laser communication requires a similar level of reliability, especially in operational scenarios. The German Aerospace Center (DLR) carried out and led the PIXL-1 mission to demonstrate the capabilities of OSIRIS4CubeSat in orbit. OSIRIS4CubeSat is one of the first examples of a laser communication terminal, that can be used in an operational use case on a CubeSat. During the PIXL-1 mission, more than 280 laser downlink experiments were performed of which less than 15% were at least partly successful. Furthermore, the maximum connection time did rarely overcome the duration of one minute. As nearly none of the outages was caused by the optical terminal itself, DLR analyzed the root causes of these limitations and investigated how to improve the reliability in preparation of future missions and potential operational use cases. The analysis of the PIXL-1 mission identified mainly three aspects with room for improvement. First, the link availability is highly dependent on the attitude control system. Reliable sensors are crucial for the exact orientation of the satellite, especially when the satellite is performing the coarse pointing for the laser terminal. But also, if the coarse pointing is done by the laser terminal itself, its orientation also relies on the satellite's attitude sensors, like the star trackers, to keep the required sub-degree pointing precision during a complete overpass. Secondly, the pointing must not only be precise, but also accurate to ensure that the laser hits the correct location. The mechanical structure of the satellite and the payload must be well aligned and robust to mitigate settling effects or deformation during launch. Deviations between the coordinate systems of attitude sensor, satellite and laser terminal lead to mispointing of the laser and require sophisticated search algorithms and in-orbit calibration. Thirdly, optical ground stations send a laser beacon to illuminate the satellite for the link acquisition. Hence, it must be guaranteed that the orbit determination accuracy on the ground station is accurate enough that the satellite is within the beacon's illumination cone. Otherwise search algorithms to find the satellite must be used by the ground station as well. The insights from the PIXL-1 mission were considered in the design of the QUBE mission led by the Center for Telematic (ZfT), in which a similar optical terminal, OSIRIS4QUBE was part of the demonstration. Therefore, the whole attitude determination and control system in QUBE underwent intensive testing on ground. This increased the reliability of the system significantly, especially on the sensors and the attitude determination. This did not only increase the number of successful links but also the duration of the optical connections in the QUBE mission. Placement and mounting of the sensor relative to the laser were a critical part of the satellite design process already early on in the project, while mechanical countermeasures improved the overall structural stability of the spacecraft. The precise alignment between the different coordinate systems in QUBE avoided search algorithms and in-orbit calibration. More accurate orbit files in QUBE enabled a faster link acquisition and improved the link duration and the downlink performance. Also here, sophisticated offset mitigation technics could be avoided. This paper details the root cause analysis of the PIXL-1 mission, describes the mitigation measures in preparation for the QUBE mission and shows the improvement of the availability of the optical connections by comparing the link success rate of the two missions.