Benjamin Rödiger, German Aerospace Center (DLR)
René Rüddenklau, German Aerospace Center (DLR)
Roland Haber, Center for Telematics
Lisa Elsner, Center for Telematics
Benjamin Rödiger, Project Manager, German Aerospace Center (DLR)
OSIRIS4QUBE is an optical laser terminal for CubeSats to transmit experimental quantum states and is developed by the German Aerospace Center (DLR). It is based on the classical laser communication terminal OSIRIS4CubeSat which was successfully demonstrated during the PIXL-1 mission on the 3U CubeSat CubeL. The modular design of OSIRIS4CubeSat allows to extend the functionalities of the terminal with only minimal changes. OSIRIS4QUBE transmits a classical optical signal and additionally two quantum signals are provided by the partner payloads from Ludwig-Maximilians-University in Munich (LMU) and Max Planck Institute for the Science of Light in Erlangen (MPL). The three signals are coupled into the same optical path and are transmitted to and received by an optical ground station. The two quantum payloads and the laser communication terminal are integrated into the 3U CubeSat QUBE built by Center for Telematics (ZfT) The CubeSat uses the UNISEC Europe standard to provide a modular and efficient platform for the various payloads. ZFT also developed a precise attitude determination and control system, which is essential for the laser pointing maneuvers.
A precise and accurate pointing is crucial for a successful optical link establishment. The required optical power density on ground to collect enough photons can only be achieved with a low divergence of the laser. The divergence of the laser beam is below 200 microradians (full angle, 1/e²), which overcomes the capabilities of common pointing mechanism of a 3U CubeSat. Thus, OSIRIS4QUBE uses the pointing acquisition and tracking system where a laser bacon is sent from the optical ground station and illuminates the satellite. The fine pointing assembly of the terminal can orientate itself on this beacon and corrects the residual pointing error of the satellite. This method was used in several previous projects and ensures that the transmission laser hits the target where the beacon comes from, the optical ground station. The satellite itself does the coarse pointing and has to point within the field of regard of OSIRIS4QUBE. Therefore, the absolute pointing error must not exceed 1 degree in any direction. The attitude determination and control system of QUBE has a coarse pointing mode using sun sensors, magnetometer and gyroscopes to steer roughly into the direction of the optical ground station. Shortly before the link, the fine pointing mode, based on a star tracker takes over and controls the satellite with a pointing error of less than 1 degree to the target. It is necessary to keep the absolute pointing error below 1 degree during the whole overpass from 5 degree ascending to 5 degree descending.
This paper shows the first results of the launch and early orbit phase of the QUBE mission. It explains the pointing, acquisition and tracking system and with that the required subsystems of the space and the ground segment. The precision of the satellite QUBE is verified by the telemetry inside the satellite. Accuracy verification needs feedback from the ground such as camera pictures or an optical feedback. In this case, the transmission laser of OSIRIS4QUBE is used as immediate feedback as it can be seen on the infrared image of the camera inside the optical ground station. On the other side – the space segment – the beacon is acquired by the tracking sensor inside the laser terminal. The telemetry of OSIRIS4QUBE can therefore be correlated with the attitude telemetry of the satellite to evaluate the accuracy. The pointing of QUBE can be further improved based on these data. The next step is to include the data of the fine pointing assembly inside OSIRIS4QUBE directly into the Kalman filter as a sensor input. The results of the evaluation and corrections of precision and accuracy are discussed in this paper.