The increasing resolution of earth observation sensors will require much higher data rates for the data downlink in future
than is feasible with conventional RF-technology. This applies for earth observation satellites as well as for aeronautic
observation platforms, such as aircraft or stratospheric high altitude platforms. The most promising solution for this data
downlink bottleneck is the application of optical free space transmission technologies. DLR has built diverse
atmospheric flight terminals and performed several trials of optical downlinks from space (together with partnering
organizations) as well as from atmospheric carriers in recent years. Here we present and compare results of such
communication system trials.
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.
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.
The tracking and pointing performance of the Laser Utilizing Communications Equipment (LUCE) equipped on the
Optical Inter-orbit Communications Engineering Test Satellite (OICETS) is presented. The operation characteristics of
LUCE observed in the ground-satellite communication demonstrations are focused on. Since the angular movement of LUCE's optical antenna required in the ground-satellite trials exceeds the specification demanded in the inter-satellite communications, the marginal performance for proper tracking and pointing can be observed. During the ground-satellite communication trials, the ground stations found periodical discontinuances in the optical link. By looking at the
LUCE's telemetries, the cause of the repeated breaks is revealed.
Optical LEO downlinks from the Japanese OICETS to the optical ground station built by the German Aerospace Center
(DLR) near Munich have been performed. This was the first optical LEO downlink on European grounds. The ground
station received a 50-Mbit/s OOK signal at 847 nm on its 40-cm Cassegrain telescope and sent two spatially displaced
beacon beams towards OICETS. Five out of eight trials could be performed successfully while the other three were
hindered by cloud blockage. A BER of 10-6 has been reached. The elevation angle above the horizon ranged between 2°
and 45°. The Fried parameter and the scintillation were measured with instruments inside the ground station. The beacon
power received by the LUCE Terminal onboard OICETS has also been recorded. This paper describes the setup of the
experiment and highlights the results of the measurement trials.
An optical link has been established between the Canary Islands La Palma and Tenerife. A 1064-nm transmitting laser was located on La Palma whereas a BPSK communication receiver and measurement instruments were installed in ESA's OGS on Tenerife. Beside the demonstration of a high-data-rate coherent signal transmission, the goal of the experiment was to measure the effects of the atmosphere on the beam propagation in order to estimate its impact on optical links. In particular, wavefront distortions have been investigated by means of a DIMM instrument and scintillation was observed by imaging the pupil of the OGS telescope on a CCD camera. Strong scintillation was observed during all the experiment with scintillation peaks at sunsets and sunrises, and saturation at about noon. Because of the narrowness of the beam (15-μrad divergence), beam wander has been a serious issue. Statistical results are compared with theory. Recommendations regarding the specifications of optical coherent systems in such detrimental conditions are given.
Optical communication at 1.25Gbps was successfully demonstrated in a downlink from a stratospheric balloon platform at 22km altitude to a Transportable Optical Ground Station. The experiments took place at ESRANGE, Kiruna, Sweden in August 2005. In addition to optical communications, several atmospheric measurement instruments (Differential Image Motion Monitor, Turbulence Profiler) were used to study the influence of atmospheric turbulence on the optical link. A description of the measurement instruments is given and results of the turbulence instruments (Fried parameter r0, Cn2 profile) are presented.
A high bitrate optical downlink was performed by the stratospheric optical payload experiment (STROPEX), a part of the EU CAPANINA project. The STROPEX objectives were to design and build the necessary hardware to demonstrate an optical backhaul downlink from a stratospheric platform to the ground and to carry out channel measurements on the link. A successful measurement campaign at ESRANGE near Kiruna, Sweden achieved all of these objectives. The transportable optical ground station received an almost error free 1.25 Gbit/s data signal from the payload over a distance of 64.3 km with a bit error rate of better than 10-9. This paper gives an overview of the stratospheric optical payload experiment, focusing on the airborne free-space experimental laser terminal (FELT). Additionally, the successful measurement campaign is described and the operation of the experiment is outlined.
This paper summarizes the results of a coherent transmission feasibility study which has been carried out for the coherent optical downlink from the German TerraSAR-X satellite. The receiver is located at an optical ground station. To evaluate the quality of the downlink experiment, effects of atmospheric refractive-index turbulence are investigated.
By means of numerical simulations, the influence of turbulence on the communication system is analyzed. The simulated distorted complex field at the receiver is focused by means of a Fourier transform and superimposed on the local oscillator. The impact on the performance of a shot-noise-limited receiver is studied. This investigation showed that a mean bit error rate of 10-9 is achieved easily for all cases and turbulence conditions.
The impact of phase-piston temporal fluctuations on the optical phase locked loop is investigated analytically. Beam motion with respect to the turbulence cells is taken into account. The calculated values for the additional residual phase noise due to the atmosphere are too small to cause significant deterioration of the receiver performance.
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