Free-Space Optical (FSO) communication is on the verge of becoming an important backbone of the global network infrastructure, and the de facto standard for high-rate data links on satellites, in the near future. In an attempt to overcome link misalignment issues associated with low resolution Four Quadrant Detectors (4QDs) and to widen the scope of feasible optical link scenarios, a detection approach for optical position sensing based on Short-Wave Infrared (SWIR) image sensors is proposed. Thanks to the capability of resolving fine details over a large Field-of-View (FOV), image recognition algorithms can be employed to distinguish different beam objects within the focal plane, making it possible to separate the useful signal from unwanted stray light sources. This classification approach potentially enables Direct-to-Earth (DTE) links during daylight, as these rely on the suppression of the Earth’s albedo superimposing the user signal. Using image acquisition, multivariate tracking algorithms such as Kalman or Particle Filter can be set up to improve the stability of beam tracking needed for more challenging link topologies, such as fast fly-by maneuvers or Intersatellite Links (ISLs) between different orbits. Furthermore, an image-based beam detection system is a useful diagnostic tool for in-orbit calibration or validation of different atmospheric conditions.
Free-Space Optical Communications have gained increasing attention throughout the last years. It is intended to use optical links in a large number of application scenarios, such as communications or navigation. The Optical Satellite Links department at the German Aerospace Center's Institute of Communications and Navigation looks back to a heritage in free-space optical communication systems research of more than two decades. The researched topics include applications such as optical communications from the satellites to Earth and between satellites, development of optical ground stations, design of optimized adaptive optics systems, quantum key distribution systems, technologies for optical time-transfer and ranging, among others. This paper will give an overview of recent research activities carried out in the Optical Satellite Links department, and will give an outlook to future developments which are planned.
Free-space optical communications is an emerging field for a variety of use cases in the domain of satellite communications. It is deployed in Earth observation missions to downlink payload data, in telecom missions (GEO or LEO based) to up- and downlink data streams and in quantum communication missions to implement the needed quantum and classical channels. Part of the propagation path goes through the atmosphere where the propagating optical wave experiences wave-front distortions and consequently distortions of the irradiance field. These distortions lead to scintillation of the optical power in the focal plane of the optical ground stations receiver front-end which can cause signal outages. Characterization of power scintillation is very important to assess the needed fading margin in the communication link design. Thus, these power scintillations are matter of investigation in this paper. Measurement campaigns were conducted to experimentally characterize the power scintillation. Received power was measured with a 40 cm telescope located at the DLR site Oberpfaffenhofen, i.e. in a suburban area. Satellite source is the OSIRISv1 laser terminal on the LEO satellite Flying Laptop. The campaigns were conducted in the years 2018, 2019 and 2020 where data of 15 passes could successfully be recorded. The power scintillation statistics are analyzed over elevation. The power scintillation index shows different behavior from pass to pass which is due to the different environmental conditions during the individual passes. Median values and quartiles of power scintillation index over elevation are given. Furthermore, statistics of fading margin for link budget calculation are derived. Both can be used to define best, nominal and worst cases in the design of the LEO-ground communication link.
Optical communication systems are picking up pace in the various industries for its many scenarios in which it can be used. This includes satellite, aircraft and ground systems for which the DLR Institute of Communications and Navigation develops laser terminals. Free Space Optics (FSO) based optical systems have different applications ranging from terrestrial to airborne to space, and thus also the associated requirements to these application domains vary even though the primary functional goals remain the same. Furthermore, quantum communication became of interest for its inherently secure key generation process that allows two nodes to securely communicate with each other and share secrets for long term security. Out of the different use cases of free-space optical laser terminal came the concept of having a modular FSO optical system design that could quickly adapt to the changing needs from mission to mission. The idea of a modularity appeals to many despite its challenges in the beginning, as in the long run the pros outweigh the cons and the gained advantages make the initial effort worth the while. The concept was realised by carefully studying different requirements from various on-going and past projects and thereafter segmenting different aspects of the optical system based on three main categories, namely; the received path, the transmitted path and the shared path. To elaborate modularity in system here; a fast steering mirror (FSM) can be replaced with a fold mirror for a simpler design should the requirements not require it, or an exchangeable CPA with integrated telescope mounted at the aperture of the optical bench to allow for different magnifications and ex-aperture beam diameter, all these without major impact on other subsystems. This greatly reduces the effort involved in re-evaluating system performance in the long run. In this study the design and development of such an optical system is presented that lays its ground in the Institute of Communications and Navigation’s OSIRIS project, but still continues to serve as the primary optical system design of choice for future projects.
KEYWORDS: Satellites, Optical communications, Space operations, Satellite communications, Commercial off the shelf technology, Interfaces, Telecommunications
Space industry has undergone a significant change over the last years. The development moved from large and costly spacecrafts to cost-efficient designs and shorter development times. While the satellites became smaller, the resolution of high compact sensors increased which led to a high data-volume to be transmitted and increasing demands for higher data rates on small satellites. This motivated for a highly compact version of DLR’s optical communication payload OSIRIS for small LEO satellites. DLR’s Institute of Communications and Navigation has developed the OSIRIS (Optical Space Infrared Downlink System) program starting with payloads on the satellites Flying Laptop of Univ. of Stuttgart and BiROS of DLR. Combining miniaturization to the flight-proven developments with novel concepts, OSIRIS4CubeSat allows integration in a standard CubeSat bus. The development of OSIRIS4CubeSat (industrialized under the product name “CubeLCT”) is conducted in close collaboration with Tesat Spacecom, DLR’s commercialization partner. The first implementation will be demonstrated within the PIXL-1-Mission on a 3Unit CubeSat. Furthermore, OSIRIS4CubeSat (O4C) has been chosen to support scientific missions together with university partners in the field of Quantum Key Distribution (QUBE). In the future, the modular design will enable extensions for optical inter-satellite links. This paper will give an overview about the development of the O4C payload and the current status of the PIXL-1- Mission. Furthermore, it will show the adaptation of the payload for the scientific mission QUBE. Besides these projects, the paper will give an outlook for future extensions of the O4C payload and the necessity of high data-rates in other scenarios such as inter-satellite links.
The generated amount of data on high flying platforms like aircrafts, satellites and Unmanned Aerial Vehicles (UAV) increases continuously, due to the technical improvement of modern sensor systems. The resulting demands for higher data rates on airborne and space platforms motivates the development of Laser Communication terminals for aircrafts and satellites in the last years. DLR’s Institute of Communications and Navigation has a successful track record in developing Free Space Optical (FSO) terminals for flying platforms like stratospheric balloons, aircrafts and small satellites to transfer data from moving platforms down to earth in real-time. Beside the advantages of FSO such as high data rates and a secure transfer channel against Radio Frequency (RF) interferences, a direct line of sight is mandatory for a successful link. Traditional RF-Communication is more robust and less effected by atmospheric disturbances or weather conditions. Thus, new system concepts have been developed to benefit from the provided high data rates of the FSO and the reliability of RF-Communication technologies. As part of this trend, DLR has developed and demonstrated a Hybrid FSO/RF-communication system capable of overcoming the spurious effects of the atmosphere. This paper gives an overview about DLR’s current studies and developments with the goal to combine the advantages of FSO and RF-Communication. It discusses possible implementation concepts on different platforms and presents experimental results of the implemented FSO/RF hybrid communication system operating for airborne, optical downlinks at 1Gbps.
Optical satellite downlinks have gained increasing attention throughout the last years. Especially for the application of optical satellite downlinks, DLR's Institute of Communications and Navigation is developing a number of payloads for various satellites. Within the OSIRIS program, DLR develops experimental optical terminals and systems which are optimized for small satellites.
This presentation will give an update on DLR's existing payloads (i.e. on the satellites Flying Laptop and BIROS, which have been launched in 2017 and 2016, respectively). First results will be presented. Furthermore, an outlook to planned missions will be given. This includes OSIRIS4Cubesat, which is planned for launch in Q4/2018, and OSIRISv3, which will be installed at the Airbus DS Bartolomeo platform aboard the ISS in 2019.
Optical satellite links have gained increasing attention throughout the last years. Especially for the application of optical satellite downlinks. Within the OSIRIS program, DLR's Institute of Communications and Navigation develops optical terminals and systems which are optimized for small satellites. After the successful qualification and launch of two precursor terminals, DLR currently develops OSIRISv3, a 3rd generation OSIRIS terminal with up to 10 Gbps downlink rate, and OSIRIS4Cubesat, a miniaturized version optimized for Cubesat Applications. The University of Stuttgart's Institute of Space Systems develops small satellites, which are used to demonstrate novel technologies in the Space domain. Together, DLR and University of Stuttgart integrated the first OSIRIS generation onboard the Flying Laptop satellite, which was launched in July 2017 and has been successfully operated since. This paper will give an overview about DLR's OSIRIS program. Furthermore, it will show first results of OSIRISv1 on Flying Laptop. Therefore, the Flying Laptop satellite and OSIRISv1 will be explained. Preliminary results from the validation campaign, where optical downlinks have been demonstrated, will be given.
With the increasing need for higher data rates on small LEO spacecraft, highly compact laser communication systems are required to overcome the limitations in the downlink channel. The Optical Communication Systems (OCS) group at the German Aerospace Center (DLR) has been working on the program OSIRIS (Optical Space Infrared Downlink System) since 10 years. The OSIRIS program started with the development of optical communication payloads for the Flying Laptop and BiROS satellite, which are already in orbit. The program covers a very broad range from scientific experiments and developments to demonstrations of relevant technologies. The range of developments and demonstrations covers an OSIRIS application on CubeSats with a demonstration in 2018, as well as very high data rate applications in the scope of the 3rd OSIRIS generation with up to 100 Gbps. These developments are the basis for scientific measurements on the channel characteristics and new applications and technologies in space. In the process towards an industrial application, OSIRIS is also involved in standardization activities in the framework of CCSDS.
This paper will give an overview of scientific missions and developments in the OSIRIS program and give an outlook on the development path ahead.
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 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.
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.
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