Prototype test results of a High-Altitude Pseudo-Satellite laser communication terminal

High-Altitude Pseudo-Satellites (HAPS) platforms offer a cost-effective solution for remote connectivity with uninterrupted 24/7 availability, presenting a compelling alternative to conventional orbiting satellites. Integrating laser communication terminals onto HAPS platforms enhances their capabilities, enabling high bandwidth and long-distance remote connectivity. In this context, we present Skyline S2, a laser communication terminal for the Zephyr High-Altitude Pseudo-Satellite, a solar-powered platform developed by Airbus, operating in the stratosphere. The Skyline S2 Laser Communication Terminal (LCT), a collaborative development effort between Airbus Netherlands and partners, will enable a 1 Gbit/s inter-Zephyr bi-directional link, spanning distances of up to 100 km. We have designed a novel compact 40 mm aperture LCT that meets the stringent requirements concerning mass, size, and power consumption, while being capable of withstanding the harsh environmental conditions of ambient temperature and air pressure. This paper focuses on the early stages of development, during which prototype lab tests were conducted to mitigate potential risks. The fine steering mirror control performance, the resulting fibre coupling efficiency, and 1 Gbit/s free space communications link test results are discussed. Additionally, this paper summarizes the Skyline S2 design and outlines our roadmap for future developments and Zephyr flight experiments.