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The high data rates enabled by free-space optical communication links and additional factors, such as the unregulated spectrum, make them an attractive solution in future geostationary feeder link applications with capacities beyond one Terabit per second. However, optical links through the atmosphere are heavily affected by atmospheric turbulence, leading to significant scintillation. Transmitter diversity is an effective way to improve the signal quality of optical links. Transmitter diversity with Phase-Division in Bit-Time is a novel scheme for mitgating interference and thus enabling the use of equal wavelengths for multiple transmitters. This paper will evalute the Phase-Division in Bit-Time transmitter diversity scheme and present measurement results from a lab testbed.

The adoption of optical inter-satellite links in future evolutions of Global Navigation Satellite Systems could enable key capabilities such as intra-system communication, time-transfer and precise ranging. This work reports on the development of a laboratory demonstrator designed to verify the performance of two-way optical links in a controlled environment. First test results on the performance of bi-directional frequency transfer and single directional ranging over a 30 m free-space link are reported.

Introducing optical inter-satellite links in a global navigation satellite system enhances orbit determination and offer frequency synchronization across the constellation. To show proof-of-concept and system capabilities a laboratory demonstrator has been developed and first tests in laboratory environment have been taken out. The demonstrator is buildup of two reciprocal units interconnected via bi-directional free-space optical links. Each unit is composed of opto-mechanical terminal, optical communication system and digital signal processing implemented on a field programmable gate array. Ranging and time transfer is realized utilizing a 25.55 Gigachip-per-second binary phase shift keying modulation scheme. Correlating the received ranging sequence with a local generated reference, pseudo range determination within the order of 100 µm accuracy is achieved. In addition, a 50 Megabit-persecond data signal is multiplexed onto the ranging sequence for exchanging satellite and time information as well as user data. Frequency transfer is realized using a cavity stabilized laser as carrier transferring its stability onto the phase locked local oscillator at the receiver side. Global synchronization at system level is achieved by means of synchronizing the optical carrier and spread sequence to a highly stable clock reference as well precise ranging and information distributed through the data channel. Using onsatellite ultra-stable optical oscillator and a frequency comb for radio frequency reference generation short-term frequency stability in the order of 10-15 s/s (Allan deviation at 1 s gate time) is achieved. The aim of this paper is to show the current status of the laboratory demonstrator development and present first measurements of the entirely setup system. A general overview of the digital signal processing is given and data transmission as well as time transfer are discussed in particular. The laboratory demonstrator setup and mechanical construction is presented. Clock stability transfer between high-accuracy radio frequency reference and the digital signal processing system is shown. Further, optical frequency transfer experiments are performed demonstrating successful synchronization between laboratories.

Global coverage of internet access is essential for digitalization in society, becoming a disruptive technology in industry, education or political participation for example. Satellite communications is a complementary approach to the terrestrial fiber network, which can provide world-wide coverage with few satellites in geostationary orbit or with low-earth-orbit constellations. Optical wavelengths offer multiple THz of available spectrum that can be used to connect satellites to the ground network with high-throughput links, solving the radiofrequency bandwidth bottleneck, without regulations. Cloud covereage and atmospheric turbulence are the main challenge in guaranteeing the same availability as in terrestrial fiber-based systems. While the former can be addressed by site diversity, for the latter, other mitigation strategies are required. Adaptive optics is a common approach to correct for atmospheric phase distortions and ensure stable fiber coupling. However, this approach requires a relatively complex active setup and therefore a collaboration between DLR Institute of Communications and Navigation and Cailabs has been formed to investigate alternative passive solutions for low-complexity ground stations. Coupling into multimode fibers does not require adaptive optics due to the large fiber core, however the coupled signal is distributed into multiple fiber-modes and is therefore incompatible with standard telecommunications components. Cailabs Multi-Plane Light Conversion (MPLC) technology overcomes this issue, selectively demultiplexing the fibermodes into single-mode fibers. Here, DLR’s adaptive optics system and the MPLC technology in a turbulence-relevant environment for GEO communications are compared, investigating the advantages of the MPLC approach for compensating strong turbulence. This paper presents an overview of the measurement setup and analyzes the single-mode fibers outputs of the spatial demultiplexer and the measured phase-distortions from a wavefront sensor.

Future spacecraft missions aim to communicate with the Earth using near-infrared lasers. The possible bit rate of free-space optical communication (FSOC) is orders of magnitude greater when compared to current radio frequency transmissions. The challenge of ground–space FSOC is that atmospheric turbulence perturbs optical wavefront propagation. These wavefront aberrations can be measured using a Shack–Hartmann wavefront sensor (SHWFS). A ground-based adaptive optics (AO) system can mitigate these aberrations along the optical path by translating wavefront measurements into deformable mirror commands. However, errors result from atmospheric turbulence continuously evolving, and there are unavoidable delays during AO wavefront correction. The length of an acceptable delay is referred to as the coherence time—a parameter dependent on the strength of turbulence profile layers and their corresponding wind-driven velocity. This study introduces a novel technique, to the best of our knowledge, for using SHWFS single-source observations, e.g., the downlink signal from a geostationary satellite, to measure the strength and velocity of turbulence profile layers. This work builds upon previous research and demonstrates that single-source observations can disentangle turbulence profile layers through studying the cross-covariance of temporally offset SHWFS centroid measurements. Simulated data are used to verify that the technique can recover the coherence time. The expected and measured results have a correlation coefficient of 0.95.

Future spacecraft will require a paradigm shift in the way information is transmitted, in light of the continuous increase in the amount of data requiring space links. Current radio-frequency-based communication systems impose a bottleneck on the volume of data that can be transmitted back to Earth due to technological as well as regulatory reasons. Free-space optical communication has finally emerged as a key technology for solving the increasing bandwidth limitations for space communication while reducing the size, weight and power of satellite communication systems, and taking advantage of a license-free spectrum.

Optical inter-satellite links are proposed in the Kepler constellation to connect satellites in a Global Navigation Satellite System (GNNS) constellation for optical ranging, time transfer and data transmission [1]. A laboratory demonstrator is being developed to verify all three aspects. The demonstrator is constituted by two terminals, performing a bidirectional free-space optical link in the laboratory, with single-mode fiber coupling in the receivers at both sites. The optomechanics is based on commercial off-the-shelf (COTS) components. The optical terminal includes a point-ahead assembly, which compensates for the point-ahead angle (PAA) between the two linked satellites. The absence of the PAA under laboratory conditions allows this mirror to be used for pointing jitter emulation instead, i.e. to emulate the expected satellite platform angular vibrations. The ranging is performed by using a 25.55 Gc/s binary phase shift keying (BPSK) phase modulation of the optical carrier. This high modulationrate allows ranging accuracy in the order of 100 µm. The data-communication channel is multiplexed to the ranging signal at a rate of 50 Mb/s and allows exchanging satellite and timing information. Both the optical carrier and the spreading sequence are synchronized to the on-board reference and a reference clock input for the sequence generator Field Programmable Gate Array (FPGA) is generated. Thus, the information exchanged though the data communication channel and precise ranging are used to support highly accurate two-way time transfer between the linked satellites. The objective of this paper is to present the hardware developments of optical terminals, which will demonstrate range measurements and communications. The first characterizations of the main terminal components are shown, namely the real-time Digital Signal Processing (DSP) system based on FPGA for ranging and data transfer and the optical phase-locked-loop (OPLL) for locking and tracking the incoming optical carrier. Further, static homodyne ranging estimation is evaluated and the impact on the frequency stability of the optical assembly and other main components is shown.

For the next generation of very high throughput communication satellites, TNO and DLR envision optical free-space communication between ground stations and geostationary telecommunication satellites to replace the traditional RF links. To mitigate atmospheric turbulence, an Adaptive Optics (AO) system will be used to apply uplink pre-correction. OFELIA, an ground terminal breadboard was developed to demonstrate the pre-correction principle over an realistic link. Currently, integration tests have been performed to verify the AO performance. Also a laser link experiment over 10 km distance has already been established, in a scenario relevant to ground-to-satellite links. The paper shows that AO is clearly beneficial for the downlink performance. In addition the first preliminary experimental results of the pre-correction show it is also beneficial for the uplink.

After more than 3 years of operational experiences with the Transportable Adaptive Optical Ground Station (T-AOGS) it is not any more the question whether optical communication through atmosphere is possible for Geo to ground applications. It is important to understand the performance of optical communication under different atmospheric-conditions and which the key parameters are to improve simplicity, robustness and availability of optical bi-directional satellite to ground links (SGL). We report within this paper on the characterization of the atmospheric channel for ground to GEO optical communication without adaptive optics correction in the uplink. Besides the telemetry data of the space segment and the T-AOGS, also a special measurement campaign was carried out using the 1m telescope of the ESA-OGS in parallel. An outlook for further analysis and activities is given.

Current satellite communication systems based on RF technology are often limited by the available spectrum. Free-Space Optical Communication Links are a promising solution to overcome this bottleneck. The available spectrum enables Terabit-per-Second data rates, and the point-to-point nature of FSO links makes spectrum regulation unnecessary. Therefore, the use of optical links is investigated in a number of scenarios, as e.g. for GEO feeder links serving multimedia applications. Transmitter Diversity might be a solution to overcome atmospheric impacts on the ground-to-satellite transmission in an optical GEO feeder link application. Transmitter Diversity can e.g. be implemented by using various spectral channels. Despite the virtually unlimited bandwidth available in atmospheric transmission windows, practical implementations of an optical GEO feeder link need to use available hardware, namely fiber amplifiers in the C- and L-band – Therefore, a more efficient use of the available spectrum will enable in a higher system throughput. In this paper, we present a bandwidth efficient transmitter diversity scheme, called "phase-division in bit-time". It is based on Intensity Modulation with Direction Detection, and makes use of phase modulation for the transmitters to mitigate the impact of atmospheric phase distortions on the quality of the received signal.

Broadband internet access has become a vertex for the future development of society and industry in the digital era. Geostationary orbit (GEO) satellite can provide global broadband coverage, becoming a complementary solution to optical fiber network. Low-earth-orbit (LEO) constellations have been proposed in the last years and they may become a reality soon, but still based on radiofrequency for the ground-to-satellite links. Optical technologies offer multiple THz of available spectrum, which can be used in the feeder link. The DLR’s Institute of Communications and Navigation has demonstrated Terabit-per-second throughput in relevant environment for GEO communications, in terms of the turbulent channel. In 2016 DLR set the world-record in freespace communications to 1.72 Tbit/s, and in 2017 to 13.16 Tbit/s. Two terminals, emulating the satellite and the ground station have been developed. Bi-directional communications link with single-mode-fiber coupling at both ends was demonstrated. Adaptive optics for the downlink and uplink (pre-distortion) improved the fiber-coupling in downlink and decreased signal fluctuations in uplink. A 80 Gbit/s QPSK system based on digital homodyne reception was also developed, demonstrating the use of coherent communications under strong turbulence conditions. These activities were performed in the frame of two internal DLR projects, THRUST and Global Connectivity Synergy project. Several measurement campaigns took place in the last years in a valley-to-mountain-top test-link. Turbulence has been monitored at both ends and the point-ahead-angle has been emulated by separating the downlink beacon from the receiving aperture. An overview of the system and the main results will be presented.

We present the results from a Monte Carlo computer simulation of adaptive optics (AO) pre-compensated laser uplink propagation through the Earth’s atmospheric turbulence from the ground to orbiting satellites. The simulation includes the so-called point-ahead angle and tests several potential AO mitigation modes such as tip/tilt or full AO from the downlink beam, and a laser guide star at the point ahead angle. The performance of these modes, as measured by metrics relevant for free-space optical communication, are compared with no correction and perfect correction. The aim of the study is to investigate fundamental limitations of free-space optical communications with AO pre-compensation and a point-ahead angle, therefore the results represent an upper bound of AO corrected performance, demonstrating the potential of pre-compensation technology. Performance is assessed with varying launch aperture size, wavelength, launch geometry, ground layer turbulence strength (i.e. day/night), elevation angle and satellite orbit (Low-Earth and Geostationary). By exploring this large parameter space we are able examine trends on performance with the aim of informing the design of future optical ground stations and demonstrating and quantifying the potential upper bounds of adaptive optics performance in free-space optical communications.

For the next generation of very high throughput communication satellites, free-space optical (FSO) communication between ground stations and geostationary telecommunication satellites is likely to replace conventional RF links. To mitigate atmospheric turbulence, TNO and DLR propose Adaptive Optics (AO) to apply uplink pre-correction. In order to demonstrate the feasibility of AO pre-correction an FSO link has been tested over a 10 km range. This paper shows that AO pre-correction is most advantageous for low point ahead angles (PAAs), as expected. In addition, an optimum AO precorrection performance is found at 16 AO modes for the experimental conditions. For the specific test site, tip-tilt precorrection accounted for 4.5 dB improvement in the link budget. Higher order AO modes accounted for another 1.5 dB improvement in the link budget. From these results it is concluded that AO pre-correction can effectively improve high-throughput optical feeder links.

© 2020 Optical Society of America]. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited. In a downlink scenario, the performance of laser satellite communications is limited due to atmospheric turbu- lence, which causes fluctuations in the intensity and the phase of the received signal, leading to an increase in bit error probability. In principle, a single-aperture phase-compensated receiver, based on adaptive optics, can over- come atmospheric limitations by adaptive tracking and correction of atmospherically induced aberrations. However, under strong turbulence situations, the effectiveness of traditional adaptive optics systems is severely compromised. We have developed an alternative intensity-based technique that corrects the wavefront by iter- atively updating the phases of individual focal-plane speckles, which maximizes the power coupled into a single- mode fiber. Here, we present the proof of concept for this method. We show how this technique offers around 4 dB power gain with fewer than 60 power measurements under strong turbulence conditions. It delivers a good performance in different turbulent regimes, and it shows robustness against severe deterioration of the signal-to-noise ratio.

Wavefront distortions of optical waves propagating through the turbulent atmosphere are responsible for phase and amplitude fluctuations, causing random fading in the signal coupled into single-mode optical fibers. Wavefront aberrations can be confronted, in principle, with adaptive optics technology that compensates the incoming optical signal by the phase conjugation principle and mitigates the likeliness of fading. However, real-time adaptive optics requires phase wavefront measurements, which are generally difficult under typical propagation conditions for communication scenarios. As an alternative to the conventional adaptive optics approach, here, we discuss a novel phase-retrieval technique that indirectly determines the unknown phase wavefront from focal-plane intensity measurements. The adaptation approach is based on sequential optimization of the speckle pattern in the focal plane and works by iteratively updating the phases of individual speckles to maximize the received power. We found in our analysis that this technique can compensate the distorted phasefront and increase the signal coupled with a significant reduction in the required number of iterations, resulting in a loop bandwidth utilization well within the capacity of commercially available deformable mirrors.

An optical communications system employing intradyne reception and offline digital signal processing is tested over a 10.45 km link through the atmosphere. 40 GBaud transmission using binary phase-shift keying in the C-band is demonstrated and compared with laboratory measurements. Simultaneous photodetector measurements show that the turbulence in the atmospheric channel is representative for relevant and worst-case conditions in the geostationary satellite uplink scenario.

TNO and DLR envision optical free-space communication between ground stations and geostationary telecommunication satellites to replace the traditional RF links for the next generation of Very High Throughput Satellites. To mitigate atmospheric turbulence, an Adaptive Optics (AO) system will be used. TNO and DLR are developing breadboards to validate Terabit/s communication links using an AO system. In this paper the breadboard activities and first results of the sub-systems will be presented. Performance of these subsystems will be evaluated for viability of terabit/s optical feeder links.

The SpaceDataHighway, the first operational service of high-speed data relay system based on optical inter-satellite links, has set a new milestone in space optical communications. Data relay systems are becoming crucial in applications such as Earth observation, where huge amounts of data need to be sent to Earth reliably and with low latency. Optical communications plays a major role in such high-speed applications, since no-regulations are needed, because the lack of interference among users, and the huge amount of available bandwidth. Since the end of the 1990’s, several experiments have shown the feasibility of such technology with several demonstrations from LEO, GEO and the Moon. The current state-of-the-art relay system architecture involves LEO and GEO satellites with optical inter-satellite links, and direct Ka-Band RF links from GEO to the Earth. Next generation systems may involve also UAVs, and may rely only on optical communications to exploit the full potential of these frequencies. The main challenges of using optical links are the turbulence effects, when the link traverses the Earth’s atmosphere, and the degrading impact of platform micro-vibrations because of the inherently small divergence of the transmitted beam. Such aspects have to be taken into account when designing future systems. Together with the modulation, the forward error correction (FEC) defines the communications performance of the system. Following CCSDS coding recommen-dations, the performance of several coding schemes is analyzed; concretely Reed-Solomon codes, convolutional codes, turbo codes and low-density parity check codes are taken into account. One of the main characteristics of the atmospheric channel is the correlation of fading events, which requires further data protection to compensate for erasure events. Interleaving and packet level coding in combination with FEC are compared through simulations. Finally, different approaches for data correction are considered. The complexity on board the GEO satellite can specially limit the use of the most advanced decoding schemes and data-protection for the upcoming generations of relay systems. The trade-off between performance and complexity is crucial in order to allow further system enhancements in terms of capacity, without endangering the whole system availability.

To overcome data rate limitations of RF communication links with satellites, TNO and DLR envision optical free-space communication feeder links for next generation high throughput satellites. This paper provides a feasibility assessment of such links and the technology needed. The main results of the link budget and the turbulence modeling of terabit/s optical links are presented. Based on these parameters, requirements and status of the link-subsystems are discussed, and a roadmap is presented, aimed at achieving terabit per second optical feeder links.

Free-space optical communications (FSOC) are rapidly becoming a key technology for terrestrial, aerial, and space communication, mainly because of its very high throughput capacity. To achieve multi-gigabit laser downstream, an efficient single-mode fiber coupling is required. However, atmospheric turbulence remains one of FSOC’s main limitations. The turbulence affects the communications performance by inducing wavefront distortions that develop into coupled power fluctuations. In regimes of very strong turbulence, the use of traditional adaptive optics systems is limited due to strong scintillation and higher number of phase singularities. These limitations could be solved by relying on systems based on the stochastic iterative maximization of the coupled power. The drawback of such systems is that a high number of iterations are required for signal optimization. We address this problem and propose a different iterative method that compensates the distorted pupil phasefront by operating directly on the focal plane. The technique works by iteratively updating the phases of individual speckles to maximize the received power coupled into a single-mode fiber. We show numerically and experimentally that the method can improve the quality of the received signal with reduced bandwidth utilization.

Reliable modelling of the power variations in atmospherically propagated optical waves is important for appropriate design of the necessary fading mitigation techniques like Forward Error Correction (FEC) or Automatic Repeat reQuest (ARQ) algorithms. A simulation tool modeling the combined impact of atmospheric turbulence and pointing jitter is presented and its performance is compared with measured power vectors.

The German Aerospace Center (DLR) and the European Southern Observatory (ESO) performed a measurement campaign together in April and July 2016 at Teide-Observatory (Tenerife), with the support of the European Space Agency (ESA), to investigate the use of laser guide stars (LGS) in ground to space optical communications. Atmospheric turbulence causes strong signal fluctuations in the uplink, due to scintillation and beam wander. In space communications, the use of the downlink channel as reference for pointing and for pre-distortion adaptive optics is limited by the size of the isokinetic and isoplanatic angle in relation to the required point-ahead angle. Pointing and phase errors due to the decorrelation between downward and upward beam due to the point-ahead angle may have a severe impact on the required transmit power and the stability of the communications link. LGSs provide a self-tailored reference to any optical ground-to-space link, independently of turbulence conditions and required point-ahead angle. In photon-starved links, typically in deep-space scenarios, LGSs allow dedicating all downlink received signal to communications purposes, increasing the available link margin. The scope of the joint DLR-ESO measurement campaign was, first, to measure the absolute value of the beam wander (uplink-tilt) using a LGS, taking a natural star as a reference, and, second, to characterize the decrease of correlation between uplink-tilt and downlink-tilt with respect to the angular separation between both sources. This paper describes the experiments performed during the measurement campaigns, providing an overview of the measured data and the first outcomes of the data post-processing.

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. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

The sensitivity characteristics of optical receiver frontends for high-speed data communications depend on modulation format, detector type, and specific operational constraints. A general mathematical model of the receiver sensitivity that fits to analytical as well as measured data is required to compare different receiver implementations and assess the reliability of data links under varying received power as common in free-space optical communication links. In this paper, a new approach based on Q-factor modeling is presented, compared with analytical receiver models, and applied to a multitude of exemplary receiver implementations. A methodology is introduced to generally apply the model to ideal or practical binary optical receiver frontends.

Future Very High Throughput Satellite Systems (VHTS) will perform at several Tbit/s throughput and thus face the challenge of limited feeder-link spectrum. Whereas with conventional RF feeder links several tens of ground gateway stations would be required, the total capacity can alternatively be linked through a single optical ground station using Dense Wavelength Division Multiplexing (DWDM) techniques as known from terrestrial fiber communications. While intermittent link blockage by clouds can be compensated by ground station diversity, the optical uplink signal is directly affected by scintillation and beam wander induced by the atmospheric index-of-refraction turbulence. The transmission system must be capable to mitigate these distortions by according high-speed tracking and fading compensation techniques. We report on the design of a near-ground long-range (10km) atmospheric transmission test-bed which is, with its relatively low elevation of 1.8 degrees, exemplary for a worst case GEO uplink scenario. The transmitting side of the test-bed consists of a single telescope with a a fine pointing assembly in order to track the atmospheric angle-of-arrival and precisely aim towards the beacon of the receiver. On the other side of the test-bed, the receiver telescope is also capable of fine pointing by tracking the transmitted signal. The GEO uplink scenario is modelled by a precise scaling of the beam divergence and the receiver’s field of view as well as by the beacon offset to model the point-ahead angle. In order to make the experimental test-bed correspond to an actual feeder link scenario, the link budget as well as the turbulence profile of the experimental scenario are modelled and compared to the GEO uplink. Several DWDM channels are multiplexed to reach the total link capacity of above one Tbit/s. We report on the preparatory lab tests, and the free-space test-bed design and verification, of the Terabit Throughput Satellite Technology Project THRUST.

The paper presents an experimental testbed, which will be used for the demonstration of more than 1 Tbps free-space optical (FSO) transmission in a scenario similar to that of GEO uplink. The chosen terrestrial scenario corresponds to worst-case conditions of a GEO uplink scenario according to the atmospheric channel characteristics and the geometry of the scenario. The testbed consists of a DWDM-based FSO link over 26km distance with less than 2° elevation. The communications system consists of several DWDM transmitters, which are multiplexed, amplified and transmitted in a single FSO channel. The transmitter as well as the receiver will be pointed and tracked by a high-precision fine-pointing mechanism. The state of the atmospheric transmission media will be monitored in realtime, allowing us to deepen our understanding of the optical satellite uplink transmission.

Binary phase shift keying (BPSK) optical transmission in the C-band with coherent intradyne reception is demonstrated over a long range (10.45 km) link through the atmosphere. The link emulates representative channel conditions for geostationary (GEO) optical feeder up-links in satellite communications. The digital signal processing used in recovering the transmitted data and the performed measurements are described. Finally, the bit-error-rate (BER) results for 10 Gbit/s, 20 Gbit/s and 30 Gbit/s of the outdoor experiments are presented and compared with back-to-back measurements and theory.

In this paper, a digital transmission scheme protected by a packet-level forward error correction (FEC) coding technique is proposed for optical feeder links in a satellite communication system. The architectures of the gateway and the satellite are defined, including the building blocks of the interface between the radio frequency (RF) front-end and the optical front-end, as well as the digital signal processor. The system is designed to cater for Terabit/s high-throughput satellite (HTS) applications. In addition, the turbulent atmospheric optical channel is modeled for different elevation setups and optical ground station (OGS) altitudes in untracked and tracked beam scenarios. The performance of the digital transmission scheme is evaluated in the forward and return link channels. It is shown that fade mitigation techniques such as packet-level FEC coding in the forward link, as well as beam tracking, and large-aperture OGS telescope in the return link are essential to close the link budget of a Terabit/s satellite transmission link.

NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft has embarked the Massachusetts Institute of Technology (MIT) Lincoln Laboratory’s (LL) Lunar Lasercom Space Terminal (LLST) as a secondary payload and part of the Lunar Laser Communications Demonstration (LLCD) experiment. The LLST was operated on four out of every seven days for one month during the commissioning phase of the LADEE spacecraft in lunar orbit, and then again for a shorter period of time, after the end of the primary scientific mission. ESA's Optical Ground Station (OGS) on the Canary island of Tenerife was one of two secondary participating ground stations – together with JPLs Table Mountain facility – involved in the experiment. We present, from ESA's perspective, the Lunar Optical Communications Link (LOCL) project including first results and lessons learned of the short-duration experiment using the OGS. The success of LOCL is of strategic importance for ESA for the development of future optical communications over "deep-space" distances.

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.

Optical feeder links will become the extension of the terrestrial fiber communications towards space, increasing data throughput in satellite communications by overcoming the spectrum limitations of classical RF-links. The geostationary telecommunication satellite Alphasat and the satellites forming the EDRS-system will become the next generation for high-speed data-relay services. The ESA satellite ARTEMIS, precursor for geostationary orbit (GEO) optical terminals, is still a privileged experiment platform to characterize the turbulent channel and investigate the challenges of free-space optical communication to GEO. In this framework, two measurement campaigns were conducted with the scope of verifying the benefits of transmitter diversity in the uplink. To evaluate this mitigation technique, intensity measurements were carried out at both ends of the link. The scintillation parameter is calculated and compared to theory and, additionally, the Fried Parameter is estimated by using a focus camera to monitor the turbulence strength.

In the context of the increasing demand in high-speed data link for scientific, planetary exploration and earth observation missions, the Italian Space Agency (ASI), involving Thales Alenia Space as prime, the Polytechnic of Turin and other Italian partners, is developing a program for feasibility demonstration of optical communication system with the goal of a prototype flight mission in the next future. We have designed and analyzed a ground level bidirectional Free Space Optical Communication (FSOC) Breadboard at 2.5Gbit/s working at 1550nm as an emulator of slant path link. The breadboard is full-working and we tested it back-toback, at 500m and 2.3km during one month. The distances were chosen in order to get an equivalent slant path cumulative turbulence in a ground level link. The measurements campaign was done during the day and the night time and under several weather conditions, from sunny, rainy or windy. So we could work under very different turbulence conditions from weak to strong turbulence. We measured the scintillation both, on-axis and off-axis by introducing known misalignments at the terminals, transmission losses at both path lengths and BER at both receivers. We present simulations results considering slant and ground level links, where we took into account the atmospheric effects; scintillation, beam spread, beam wander and fade probability, and comparing them with the ground level experimental results, we find a good agreement between them. Finally we discuss the results obtained in the experimentation and in the flight mission simulations in order to apply our experimental results in the next project phases.

Repeater systems in a geostationary orbit utilizing free-space optical-communication offer great potential to backup, process and archive large amounts of data collected or generated at remote locations. In contrast to existing or upcoming global satellite communication systems, such optical GEO relays are able to provide a huge return-channel data throughput with channel rates in the gigabit-per-second range. One of the most critical aspects of such data uplinks are atmospheric disturbances above the optical ground terminals used to connect to the space segment. In this study, we analyse the design drivers of optical ground stations for land-based applications. In particular, the effects of atmospheric attenuation and atmospheric turbulence are investigated. Moreover, we present implementation ideas of the necessary ground infrastructure and exemplify our results in a case study on the applicability of free-space optical satellite communication to the radio astronomy community. Our survey underpins pre-existing ventures to foster optical relay services like the Space-Data-Highway operating via the European Data Relay System. With well-designed, self-sufficient and small-sized ground terminals new user groups could be attracted, by offering alternatives to the emerging LEO mega-constellations and GEO-satellite communication systems, which operate at low return channel data rates across-the-board.