Transportable optical ground station for high-speed free-space laser communication

Near real-time data downlinks from aircrafts, satellites and high altitude platforms via high-speed laser communication links is an important research topic at the Institute of Communications and Navigation of the German Aerospace Center (DLR). Ground stations for such scenarios are usually fixed at a certain location. With a motivation to provide a ground station that is quickly and easily deployed anywhere in the world, a transportable optical ground station (TOGS) has been developed. TOGS features a pneumatically deployable Cassegrain-type telescope with main mirror diameter of 60 cm, including optical tracking and receiving system. For calibration of position and attitude, multiple sensors like dual-antenna GPS and inclination sensors have been installed. In order to realize these systems, robust software that operates and controls them is essential. The software is platform independent and is aimed to be used on both mobile and ground terminals. It includes implementation of accurate pointing, acquisition and tracking algorithms, hardware drivers, and user interfaces. Important modules of the software are GPS tracking, optical tracking, starand satellite tracking, and calibration of the TOGS itself. Recently, a first successful data-downlink from an aircraft to TOGS using GPS tracking has been performed. To streamline the software development and testing process, some simulation environments like mount simulator, aircraft path simulator, tracking camera simulator and tracking error analysis tool have also been developed. This paper presents the overall hardware/software structure of the TOGS, and gives results of the tracking accuracy improvement techniques like GPS extrapolation and optical tracking.