Optical Satellite Downlinks have gathered increasing attention in the last years. A number of experimental payloads have become available, and downlink experiments are conducted around the globe. One of these experimental systems is SOTA, the Small Optical Transponder, built by the National Institute of Information and Communications Technology (NICT).
This paper describes the downlink experiments carried out from SOTA to the German Aerospace Center’s Optical Ground Stations located in Oberpfaffenhofen, Germany. Both the Transportable Optical Ground Station (TOGS) as well as the fixed Optical Ground Station Oberpfaffenhofen (OGS-OP) are used for the experiments. This paper will explain the preparatory work, the execution of the campaign, as well as show the first results of the measurements.
Robotic operations in space with telepresence systems require high data rates for sensor and video feedback in combination with very low delays for precise and transparent control. The ESA funded project HiCLASS-ROS (Highly Compact Laser Communication Systems for Robotic Operations Support) demonstrated the use of optical communication links for symmetrical and bi-directional high data rate links in combination with lowlatency channel coding for very low round trip times comparable to a LEO scenario.
Optical Direct-to-Ground data links for earth-observation satellites will offer channel rates of several Gbps, together with low transmit powers and small terminal mass and also rather small ground receiver antennas. The avoidance of any signal spectrum limitation issues might be the most important advantage versus classical RF-technology. The effects of optical atmospheric signal attenuation, and the fast signal fluctuations induced by atmospheric index-of-refraction turbulence and sporadic miss-pointing-fading, require the use of adaptive signal formats together with fading mitigation techniques. We describe the typical downlink scenario, introduce the four different modes of data rate variation, and evaluate different methods of rate-adaptive modulation formats and repetition coding techniques.
The German Aerospace Center’s Institute of Communications and Navigation developed the Free Space Experimental Laser Terminal II and has been using it for optical downlink experiments since 2008. It has been developed for DLR’s Dornier 228 aircraft and is capable of performing optical downlink as well as inter-platform experiments. After more than 5 years of successful operation, it has been refurbished with up-to-date hardware and is now available for further aircraft-experiments. The system is a valuable resource for carrying out measurements of the atmospheric channel, for testing new developments, and of course to transmit data from the aircraft to a ground station with a very high data rate. This paper will give an overview about the system and describe the capabilities of the flexible platform. The current status of the system will be described and measurement results of a recent flight campaign will be presented. Finally, an outlook to future use of the system will be given.
The optical satellite-ground channel is turbulent and causes scintillation of the power received by a ground based telescope. Measurements are important to quantify the effect and evaluate common theory. A telescope with 40 cm primary mirror is used to measure the signals from the OPALS terminal on the International Space Station and the SOTA terminal on the SOCRATES satellite. The measurement instrument is a pupil camera from which images are recorded and intensity scintillation index, power scintillation index, probability density function of intensity and intensity correlation width are derived. A preliminary analysis of measurements from three satellite passed is performed, presented and discussed. The intensity scintillation index ranges from ~0.25 to ~0.03 within elevations of 26 to 66 deg. Power scintillation index varies from ~0.08 to ~0.006 and correlation width of intensity between ~11 and ~3 cm. The measurements can be used to estimate the fluctuation dynamics to be expected for a future operational ground receiver. The measurements are compared to model calculations based on the HV5/7-profile. Good agreement is observed to some part in the intensity scintillation index. Agreement is less for the power scintillation index and intensity correlation width. The reason seems to be a reduction of aperture averaging in some sections of the measurements due to increased speckle size. Finally, topics for future work are identified to improve the measurement analysis and deeper investigate the origin of the observed behavior.
A worldwide growing interest in fast and secure data communications pushes technology development along two lines. While fast communications can be realized using laser communications in fiber and free-space, inherently secure communications can be achieved using quantum key distribution (QKD). By combining both technologies in a single device, many synergies can be exploited, therefore reducing size, weight and power of future systems. In recent experiments we demonstrated quantum communications over large distances as well as between an aircraft and a ground station which proved the feasibility of QKD between moving partners. Satellites thus may be used as trusted nodes in combination with QKD receiver stations on ground, thereby enabling fast and secure communications on a global scale. We discuss the previous experiment with emphasis on necessary developments to be done and corresponding ongoing research work of German Aerospace Center (DLR) and Ludwig Maximilians University Munich (LMU). DLR is performing research on satellite and ground terminals for the high-rate laser communication component, which are enabling technologies for the QKD link. We describe the concept and hardware of three generations of OSIRIS (Optical High Speed Infrared Link System) laser communication terminals for low Earth orbiting satellites. The first type applies laser beam pointing solely based on classical satellite control, the second uses an optical feedback to the satellite bus and the third, currently being in design phase, comprises of a special coarse pointing assembly to control beam direction independent of satellite orientation. Ongoing work also targets optical terminals for CubeSats. A further increase of beam pointing accuracy can be achieved with a fine pointing assembly. Two ground stations will be available for future testing, an advanced stationary ground station and a transportable ground station. In parallel the LMU QKD source size will be reduced by more than an order of magnitude thereby simplifying its integration into future free-space optical communication links with CubeSats.
The next five to ten years will see more and more free-space optical communication systems being put into practical use as technologies and techniques continue to mature, particularly in the area of mobile and satellite-to-ground communications. To meet the increasing demand of these types of systems, it is necessary to gain a deeper understanding of the various atmospheric effects at play in a free-space optical link in an effort to mitigate their impact on operational systems. In that context, the German Aerospace Center (DLR) has conducted a number of field trials between a Dornier 228 aircraft and its ground station in Oberpfaffenhofen, just south of Munich, Germany. These field trials have involved the concurrent measurement of atmospheric turbulence using three different techniques: pupil plane imaging, focus spot imaging and Shack-Hartmann wave-front sensing. To ensure the accurate synchronization of measurements between the three techniques, a concerted effort was made in the selection of computer hardware and the development of image acquisition software. Furthermore, power measurements in up- and downlink have been taken to be further correlated with the 3 primary instruments. It is envisioned that the resulting analysis of these measurements shall contribute to the implementation of new adaptive optics techniques to facilitate various air and space communication links. This paper shall describe the overall experiment design as well as some of the design decisions that led to the final experiment configuration.