Design and Implementation of an Integrated Optical Signal Sensor Device for Telescopes for Optical Satellite Data Reception and Measurements of Signal Variations

Abstract

Satellites are largely dependent on radio frequency (RF) for communications. Rapidly expanding capabilities of smaller satellites in the low earth orbits is leading to a larger data generation by these satellites. Due to limited bandwidth for data downlinks, the RF communications pose a bottleneck in the link. Optical space-to-ground communication has the potential to achieve much higher data rates. However, space-to-ground links are challenged by the atmospheric disturbances and local weather conditions. The Optical Communications Group at Deutsches Zentrum für Luft-und Raumfahrt (DLR), Oberpfaffenhofen (OP) investigates new technologies to increase the data throughput between satellites and ground stations, by employing free-space optical (FSO) links. This requires sophisticated setups in space-and ground-segment. Prototypes of such optical ground stations exist in the institute which are used in the investigation of the quality of the communication channel. Atmospheric turbulence causes random intensity fluctuations on the optical signals whereas clouds coverage which are weather dependent leads to loss of link which reduces the Optical Ground Station (OGS) availability per satellite pass. This necessitates the need for an OGS site diversity. Building multiple sophisticated OGS setups is expensive to deploy in large numbers. However, with relaxed requirements regarding data rate and reliability, a simplified ground station assembly based on modified COTS-devices is feasible. It can be used for testing of new space assets and also perform transmission channel measurements independently and also in parallel to the existing OGS. In this study, a new simple optical ground station focal assembly (SOFA) has been designed and implemented. The SOFA then has been attached to a commercial off the shelf (COTS) telescope and tested. The SOFA is designed in such a way that a ground station beacon assembly can later be included in the focal assembly setup as many future satellites will rely on a beacon signal from the ground for accurate pointing during the tracking and data acquisition phase. The SOFA design is based on the parameters from the optical terminal onboard the satellite Flying Laptop currently in orbit, as one of its payload carrying DLR built optical downlink terminal named OSIRIS V1. The design drivers for the optics as well as the sensors have been derived from the analysis of the downlink budget which has been discussed in details in this work. COTS search for the optomechanics to hold the optics has also been carried out. Finally, with mainly COTS components the SOFA setup was assembled in the optics laboratory in the Institute of Communications and Navigation (IKN), DLR. This device is then also tested with a passive stationary target from the roof of the IKN building.