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.
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.