FSO systems present many advantages like high data rate, license-free bandwidth and tap-proof communication allowing the download of vast amounts of data from LEO satellites. However, the atmospheric channel is quite challenging, because of spurious effects such as absorption, scattering and scintillation that in turn vary the link losses in correspondence to the elevation. In order to maximize the downlink throughput, it should start around 5° elevation. This leads to a design with suboptimal performance for higher elevations when constant data rates are used. Therefore, DLR is developing a system to adjust the data rate according to elevation and atmospheric channel conditions. This data rate variation is achieved by determining the maximum rate for higher elevation and then for lowering the data rates a bit-level repetition is performed. The presented system enables a fast transition between the different data rates. Additionally, this system allows the satellite to transmit data at rates even lower than those nominally supported by the physical transceiver. At the receiver side, the system complexity increases as it should be able to acquire, detect, and filter the signal for different data rates. DLR proposes a system that mirrors the operation of its transmitter counterpart by sampling the acquired signal at the maximum data rate. Then an FPGA processes the signal by majority decision algorithm followed by voting system that filters and detects the intended data rate in real-time. This enables replication and parallelization of the filtering and detection processes enabling the automatic detection of the received data rate. In order to provide noise stability, the transition between data rates is governed by a hysteresis process. This scheme allows the detection and selection of the proper data rate in the range of few microseconds for a system operating between 10 Gbps and 1.25 Gbps in steps of factor of 2, ignoring the propagation delays.
Quantum key distribution (QKD) is one of the most mature quantum technologies and can provide quantum-safe security in future communication networks. Since QKD in fiber is limited to a range of few hundred kilometers, one approach to bridge continental scale distances may be the use of high altitude pseudo satellites (HAPS) as mobile trusted nodes in the stratosphere. In parallel, free-space laser communication for high rate data transmission has been a subject of research and development for several decades and its commercialization is progressing rapidly. Important synergies exist between classical free-space communication and QKD systems since the quantum states are often implemented using the same degrees of freedom such as polarization or field amplitude and phase. These synergies can be used to benefit from the progress in classical free-space laser communication in QKD applications. In this paper, the use case of QKD in a stratospheric environment is described wherein HAPS may serve as relay station of secret keys and encrypted data. The mission scenario and HAPS capabilities are analyzed to derive special requirements on the stratospheric laser terminal, the link geometry and the ground segment with respect to a feasibility demonstration. To obtain a flexible and compatible system, discrete variable and continuous variable QKD protocols are considered to be implemented side by side in the HAPS payload. Depending on the system parameters, it can be beneficial to use the one or the other kind of protocol. Thus, a direct comparison of both in one and the same system is of scientific interest. Each of the protocols has particular requirements on coupling efficiency and implementation. Link budget calculations are performed to analyze possible distances, key rates and data transmission rates for the different schemes. In case of the QKD system, the mean coupling efficiency is of main interest, i.e. signal fluctuations arising from atmospheric turbulence must be taken into account in the security proof, but the buffered key generation relaxes real-time requirements. This is different to classical communications, where the corresponding fading loss must be assessed. A system architecture is presented that comprises the optical aircraft terminal, the optical ground terminal and the most important subsystems that enable implementation of the considered QKD protocols. The aircraft terminal is interfaced with the dedicated quantum transmitter module (Alice) and the ground station with the dedicated quantum receiver module (Bob). The optical interfaces are SMF couplings which put high requirements on the receiving optics, in particular the need for wave-front correction with adaptive optics. The findings of the system study are reviewed and necessary next steps pointed out.
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.
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.
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.
Free-space optical (FSO) communication is a very attractive technology offering very high throughput without spectral regulation constraints, yet allowing small antennas (telescopes) and tap-proof communication. However, the transmitted signal has to travel through the atmosphere where it gets influenced by atmospheric turbulence, causing scintillation of the received signal. In addition, climatic effects like fogs, clouds and rain also affect the signal significantly. Moreover, FSO being a line of sight communication requires precise pointing and tracking of the telescopes, which otherwise also causes fading. To achieve error-free transmission, various mitigation techniques like aperture averaging, adaptive optics, transmitter diversity, sophisticated coding and modulation schemes are being investigated and implemented. Evaluating the performance of such systems under controlled conditions is very difficult in field trials since the atmospheric situation constantly changes, and the target scenario (e.g. on aircraft or satellites) is not easily accessible for test purposes. Therefore, with the motivation to be able to test and verify a system under laboratory conditions, DLR has developed a fading testbed that can emulate most realistic channel conditions. The main principle of the fading testbed is to control the input current of a variable optical attenuator such that it attenuates the incoming signal according to the loaded power vector. The sampling frequency and mean power of the vector can be optionally changed according to requirements. This paper provides a brief introduction to software and hardware development of the fading testbed and measurement results showing its accuracy and application scenarios.
Free-space laser communications are subject of current research and development in many research and industrial
bodies. Long distance air-ground and space-ground can be implemented in future communication networks as feeder,
backbone and backhaul links for various air- and space-based scenarios. The Institute of Communications and
Navigation of the German Aerospace Center (DLR) operates two ground stations to investigate the communication
channel and system: the Optical Ground Station Oberpfaffenhofen and the Transportable Optical Ground Station. The
first one is a fixed installation and operated as experimental station with focus on channel measurements and tests of new
developments. Various measurement devices, communication receivers and optical setups may easily be installed for
different objectives. The facility is described with its dome installation, telescope setup and infrastructure. Past and
current deployment in several projects is described and selected measurement achievements presented. The second
ground station is developed for semi-operational use and demonstration purposes. Based on experience with the
experimental ground station, this one is developed with higher level of integration and system robustness. The focus
application is the space-ground and air-ground downlink of payload data from Earth observation missions. Therefore, it
is also designed to be easily transportable for worldwide deployment. The system is explained and main components are
discussed. The characteristics of both ground stations are presented and discussed. Further advancements in the
equipment and capability are also presented.
Some current and future airborne payloads like high resolution cameras and radar systems need high channel capacity to
transmit their data from air to ground in near real-time. Especially in reconnaissance and surveillance missions, it is
important to downlink huge amount of data in very short contact times to a ground station during a flyby. Aeronautical
laser communications can supply the necessary high data-rates for this purpose. Within the project DODfast
(Demonstration of Optical Data link fast) a laser link from a fast flying platform was demonstrated. The flight platform
was a Panavia Tornado with the laser communication terminal installed in an attached avionic demonstrator pod. The air
interface was a small glass dome protecting the beam steering assembly. All other elements were integrated in a small
box inside the Pod’s fuselage. The receiver station was DLR’s Transportable Optical Ground Station equipped with a
free-space receiver front-end. Downlink wavelength for communication and uplink wavelength for beacon laser were
chosen from the optical C-band DWDM grid. The test flights were carried out at the end of November 2013 near the
Airbus Defence and Space location in Manching, Germany. The campaign successfully demonstrated the maturity and
readiness of laser communication with a data-rate of 1.25 Gbit/s for aircraft downlinks. Pointing, acquisition and
tracking performance of the airborne terminal and the ground station could be measured at aircraft speed up to 0.7 Mach
and video data from an onboard camera has been transmitted. Link distances with stable tracking were up to 79 km and
distance with data transmission over 50 km. In this paper, we describe the system architecture, the flight campaign and
Near real-time data downlinks from aircrafts, satellites and high altitude platforms via high-speed laser commu-
nication 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 mo-
tivation 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 implementa-
tion of accurate pointing, acquisition and tracking algorithms, hardware drivers, and user interfaces. Important
modules of the software are GPS tracking, optical tracking, star- and 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.