Florian Moll

Experimental verification of optical backhaul links for high-altitude platform networks: Atmospheric turbulence and downlink availability

Optical backhaul downlinks from high-altitude platforms (HAPs) are investigated. An experiment demonstrated the advantages of optical links: a small and lightweight terminal with low power consumption was launched to the stratosphere and data transmitted down to a ground station at a rate of 1.25 Gbit/s: Owing to the chosen system parameters and the high budget margin, disturbing turbulence effects did not decrease the link performance. The scientific aspect of the experiment was to study turbulence effects in order to design future systems with higher transmission performance. On the day of the experiment, measured scintillation and wavefront distortions were minimal in the morning. The best atmospheric conditions were observed about 3 h after sunrise with a peak of the atmospheric coherence length r0 at 16 cm. An r0 of 4 cm was measured as the worst case before sunrise and later during the day. This trend could also be observed for power- and intensity scintillation index. The latter changed from 0.28 (best case) to 1.12. For small scintillation index a lognormal intensity probability density function was measured. Apart from the robust intensity modulation scheme with direct detection which was used for the trial, future improved systems could benefit from a coherent transmission scheme. According to the r0 measurements and further simulations on heterodyne efficiency it turned out that the aperture size can be decreased from 40 to 10 cm without any significant change in the link margin. Future stratospheric optical links between HAPs or links from platforms to satellites will not suffer from cloud blockage but it remains an issue for up/downlinks to a ground station. This can be mitigated by ground-station diversity. Four optical ground stations in the southern part of Europe can lead to an availability of over 98%. The separation distance of the ground stations is about 900 km with a negligible correlation of cloud cover. A change of wavelength from the employed 1.55 to a wavelength around 11 microns with minimum cloud attenuation would increase the link availability for thin clouds.

Wavelength selection criteria and link availability due to cloud coverage statistics and attenuation affecting satellite, aerial, and downlink scenarios

The choice of wavelength is essential for the variety of different communication scenarios in the field of free space optics (FSO). Possibilities are Satellite and HAP (High Altitude Platform) Downlinks, HAP-HAP links, HAP-Satellite links and all kinds of links involving aeronautical vehicles. This paper addresses the influence of the wavelength dependent attenuation of clouds, the atmospheric transmission in the NIR and MIR and a statistical analysis of cloud coverage data for an estimation of link availability. Regarding the calculation of atmospheric transmission the free available simulation tools libRadtran and GENLN2 have been used. To identify advantageous wavelengths to increase link availability, cloud attenuation is determined by Mie scattering calculations of particle size distributions of various cloud types. Here the MIR wavelength interval between 10 μm and 12 μm has been found to give the lowest attenuation in clouds. However in most cases clouds will block the optical link. For that matter a statistical analysis of satellite based data from the European Cloud Climatology (ECC) is done to reveal favorable places with high availability in Europe. The improvement of link availability when a concept of ground station diversity is applied has also been investigated. An availability of almost 99 % is reached with four hypothetical stations in southern Europe. Further the difference between availability values of single years decreases with multiple stations.

Transmitter diversity verification on ARTEMIS geostationary satellite

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.

Optical high-capacity satellite downlinks via high-altitude platform relays

Earth-observation (EO) satellite missions produce a large amount of data using high-resolution optical or radar sensors. During the last decades the amount of data has steadily increased due to improved sensor technologies with increased temporal resolution, sensor resolution, and pixel count. As a consequence EO satellite missions have become limited by the downlink data rates of microwave communication systems, which are inhibited by spectrum restrictions, manageable antenna sizes, and available transmit power. Optical downlinks from EO satellites with data rates of several Gbps mitigate the limiting effects of microwave communication systems; however optical links do not provide the necessary link availability through the atmosphere due to cloud blockage above the ground station. Apart from diversity concepts with several ground stations or satellite networks, a stratospheric High Altitude Platform (HAP) could act as a relay station to forward the optical communication beam over the last 20km through the atmosphere to the ground station, where short-range, high data-rate microwave systems are feasible. This paper will discuss the capabilities of HAP and GEO relay stations to increase the downlink capacities of LEO satellites. Environmental aspects for the deployment of HAP relays and regulatory/technology issues for a microwave downlink on the last 20km to the ground will be discussed.

Terabit-throughput GEO satellite optical feeder link testbed

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.

Demonstration of High-Rate Laser Communications From a Fast Airborne Platform

In this paper, we report on the demonstration of a high-rate free-space optical communication downlink from a fast airborne platform to a ground station. The flight platform used was a Panavia Tornado with a laser communication terminal installed in an attached avionic demonstrator pod. A transportable optical ground station equipped with a free-space receiver front end was used as the receiver station. Downlink wavelength for communication and uplink wavelength for beacon laser were chosen to be compatible with the C-band DWDM grid. New optomechanical tracking systems were developed and applied on both sides for link acquisition and stabilization. The flight tests were carried out at the end of November 2013 near the Airbus Defence & Space location in Manching, Germany. The campaign successfully demonstrated the maturity and readiness of laser communication for aircraft downlinks at a data rate of 1.25 Gbit/s. We outline the experiment design based on link budget assessments, the developed optomechanical terminal technology, and the results of the flight campaign. The experiment itself focused on the tracking performance of the airborne terminal and the ground station. Performance could be measured at aircraft speeds up to Mach 0.7, and video data from an onboard camera was transmitted. Tracking accuracy of up to 20 μrad rms for the airborne terminal and the ground station were achieved at instantaneous tracking errors below 60 and 40 μrad, respectively. The tracking link worked up to a horizontal distance of 79 km, and data transmission was possible up to 50 km.

High-speed, high-volume optical communication for aircraft

Transportable ground stations that receive high data volumes from aircraft offer a solution for monitoring unforeseen events.

Channel characterization for air-to-ground free-space optical communication links

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.

Air to Ground Quantum Key Distribution

To enable global scale quantum key distribution1-3 (QKD), satellite based systems 4,5 are the most promising approach. So far, free-space QKD has already been demonstrated on communication channels with attenuation comparable to satellite downlinks,6 and classical laser communications with satellites and aircrafts is heavily explored.7-10 Here, combining both these challenges, we demonstrate an aircraft to ground QKD transmission obtaining a sifted key rate of 145 bit/s and a QBER, larglely dominated by background events and stray light, of 4:8 %.

Ground stations for aeronautical and space laser communications at German Aerospace Center

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.