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 Proceedings of COAT-2019 Workshop

Communications and Observations through Atmospheric Turbulence :

characterization and mitigation


 02 to 03 DECEMBER 2019 / ONERA, Châtillon FRANCE


COAT-2019 has been organized in the context of "Journées Scientifiques ONERA (JSO)" by Jean-Marc Conan (ONERA) with the support
- of the following Scientific Committee :
Géraldine Artaud (CNES), James Osborn (Durham Univ.), Ramon Mata-Calvo (DLR), Szymon Gładysz (Fraunhofer IOSB), Aniceto Belmonte (Technical Univ. of Catalonia), Olivier Meyer (DGA), Anne Durécu (ONERA), Clélia Robert (ONERA), Laurent Mugnier (ONERA) ;
- of the following Organizing Committee at ONERA :
Marylène Baroin, Alain Broc, Frédéric Cassaing, Anthony Drouaire, Elodie Esperanssa, Caroline Lim, David Mariette, Aurélie Montmerle-Bonnefois, Nicolas Védrenne, Marie-Thérèse Velluet.

Sessions :

  1. Atmospheric turbulence : modeling, characterization & prediction 
  2. Impact of turbulence on beam propagation
  3. Adaptative optics for space observation
  4. Novel turbulence mitigation strategies
  5. Ground-space optical links : concepts and applications
  6. Free space communication links : experimental demonstrations


Scope :

In a large variety of fields, the development of ever more demanding optical systems now requires a thorough understanding of the impact of atmospheric turbulence together with the development of dedicated mitigation strategies. Originally driven by astronomy, these issues now concern near ground imaging, satellite observation, ground-space and near ground optical telecommunications and frequency dissemination, laser focusing... The workshop aims at giving an overview of current research activities in this field. It will address turbulence impact and mitigation, especially when facing strong turbulence, strong scintillation, high apparent wind speeds induced by target tracking, anisoplanatism effects due to the field of view or point-ahead angle... The following topics will, therefore, be addressed during the workshop: knowledge on turbulence conditions (modelling, characterization & prediction of both Cn² and wind profiles); study of system performance with and without mitigation; mitigation strategies based on innovative optical solutions such as single aperture adaptive optics, multi-aperture or aperture diversity solutions, integrated optics architectures, new wave-front sensing approaches, sensorless solutions, laser guide stars, high performance control; lastly, image/signal pre- and post-processing techniques (deconvolution, PSF prediction, digital communications techniques), possibly in a co-design approach.



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Session 1 : Atmospheric turbulence: modeling, characterization & prediction

[hal-03206072] Atmospheric Turbulence Statistics and Profile Modeling. Local to DLR Oberpfaffenhofen

Laser communication (lasercom) is influenced by atmospheric turbulence, a quality measured by the refractive index structure parameter Cn². This paper quantifies the degree of improvement to lasercom link budgets afforded by using ground-level measurements of turbulence in vertical turbulence models. Ground-level Cn² is measured with an off-the-shelf scintillometer for a path adjacent to DLR's optical ground station (OGS). Measurements are in agreement with literature on turbulence; nighttime Cn² is well represented by a log-normal distribution. Comparisons are drawn between profiles by comparing link budget parameter estimates generated by four turbulence profile models: HV-5/7, HV with Cn²(h0), and HV and HAP models with Cn²(h0) and fitting to downlink experiment data. Vertical turbulence profiles are converted to scintillation index sigma²_I by way of theory described in the literature on weak and strong turbulence. Normalised root-mean-squared-error is used to establish goodness-of-fit of modeled sigma²_I to downlink beam parameter measurements . Use of Cn²(h0) in a profile model improves upon the fit beyond HV-5/7 by ~8.3%. Improvements in the mean expectation from specific fits to satellite downlink experiments improve the NRMSE 30%. However, the variability in margin estimation due to changes in Cn²(h0) indicates fitting might not be a consistent improvement over the HV-5/7 model. This paper describes the setup of the scintillometer, six months of measurements, the use of Cn²(h0) measurements in vertical profile models to find the path integrated intensity scintillation index (sigma²_I), and a comparison of modeled-path integrated scintillation index to the scintillation index of downlink ground measurements at DLR's OGS.

Session 2 : Impact of turbulence on beam propagation

[hal-03146214] Wave-front Aberrations of Laser Beams Propagating through the Atmosphere

Small temperature variations carried by the turbulent velocity field in the Earth's atmosphere produce small phase perturbations in laser beam propagating through it. Perturbations in the phase cause wave-front errors and may significantly decrease the Strehl ratio of the laser beam. For compensation of atmospheric distortions, adaptive optics systems are used. Before the design of any adaptive optics system, the simulation of the atmospherically distorted laser beam is important. In this work, we describe the purpose, theory, implementation, and results of a wave optics propagation simulation code LAtmoSim and we investigate the effect of laser system parameters on the degree of the wave-front error caused by turbulence.

Session 3 : Adaptative optics for space observation

[hal-02895885] Adaptive Optics Facility. When cutting-edge technology meets operational robustness and performance

The Adaptive Optics Facility (AOF) is an ESO project started in 2005, which transformed Yepun, one of the four 8m telescopes in Paranal, into an adaptive telescope. This has been done by replacing the conventional secondary mirror of Yepun by a 1172 actuator Deformable Secondary Mirror (DSM) and attaching four Laser Guide Star (LGS) Units to its centerpiece, each of them featuring a high stability narrow-band 20 W laser. Additionally, two Adaptive Optics (AO) modules (GALACSI serving MUSE a 3D spectrograph, and GRAAL, serving Hawk I a wide field infrared imager) have been assembled onto the telescope Nasmyth adapters, each of them incorporating four LGS WaveFront Sensors (WFS) and one tip-tilt sensor used to control the DSM at 1 kHz frame rate. These WFSs are based on 0 Read-out Noise (RoN) detectors. The complete AOF has been operated on-sky for more than 2 years and is routinely delivering science. This paper presents the most important and amazing features of the AOF, focusing on cutting-edge technology in use, operational concept and overall performance. In the first part of the paper, the AOF design is recalled, with a focus on the DSM, the lasers, the WFS cameras and the Real Time Computer technology. Then, the acquisition sequence and overall on-sky operation efficiency will be detailed; finally, on-sky performance of AOF will be presented.

[hal-03146244] Data-based modelling of low-order modes for AO control: what do on-sky experiments tell us?


[hal-03206079] LEO satellite imaging with adaptive optics


Session 4 : Novel turbulence mitigation strategies

[hal-02887698] Modal Wavefront Sensorless Adaptive Optics with Karhunen-Loève Functions

In this work we propose an optimal modal decomposition technique for gradient-based wavefront sensorless adap-tive optics. We implement statistically independent Karhunen-Loève functions for iterative blind correction, and analyze their optimal correction performance in various turbulence conditions in temporally evolving turbulence simulations.

[hal-03143101] Laboratory Validation of Sequential Optimization of Adaptive Receivers in Downlink Laser Communications


[hal-03143521] Optimal modal basis approaches fundamental limits to free-space optical system capacity imposed by atmospheric turbulence


[hal-03146222] Coupling of atmospheric perturbed optical beams with guided modes of propagation


[hal-03146226] Using event cameras for imaging through atmospheric turbulence


Session 5 : Ground-space optical links : concepts and applications

[hal-03143529] Optical feeder links for high throughput satellites and the H2020 VERTIGO project

The demand from satellite operators for ever increasing capacity during these years has led to the emergence of VHTS (Very High Throughput Satellites) system solutions. Such satellites exhibit as a minimum several times the capacity of conventional satellites and possibly up to Terabit per second and beyond. One of the main challenges associated to the implementation of these very high capacity systems is to feed the satellite in an efficient way to limit the cost of the system, in particular that of the ground segment. While RF bands are facing saturation and are submitted to strict frequency regulation, leading to a large number of ground stations, optical feeder links are considered as a promising technology to meet the future VHTS system requirements while strongly reducing the ground segment. Nevertheless, optical feeder links are still facing some implementation uncertainties, beyond the obvious issue of nebulosity which can be alleviated through site diversity approach. While several feeder link architectures are envisaged leading to significant implementation differences, atmospheric propagation impairments and their mitigation techniques together with high power generation and management as well as efficient modulation use are of primary importance in the design and sizing of the optical feeder link. During past years several experimentations on ground or in-flight have demonstrated part of these concepts and subsystems necessary to implement such high capacity systems. Simultaneous combination of all these concepts in comprehensive demonstrations has however not been implemented yet. This paper recalls the main drivers of satellite systems design based on optical feeder links and introduces the H2020 VERTIGO project that is specifically addressing the topic of the GEO-ground optical link and the associated technological challenges (high optical power generation, high efficiency waveforms, atmospheric impairments mitigation techniques).

[hal-03143532] Protection schemes for optical communication between optical ground station and a satellite


[hal-03206080] Adaptive Optics for GEO-Feeder Links: from Performance Analysis via Reciprocity Based Models to Experimental Demonstration


Session 6 : Free space communication links : experimental demonstrations

[hal-03146231] Dynamic and environmental study of a mid-infrared wavelength channel for a horizontal telecom link

This study characterizes a horizontal atmospheric telecom channel in terms of evolution time of the intensity and the phase of the electromagnetic field received. The channel temporal characterization comes from a turbulence data base recorded during a Cn² profiler experiment (SCINDAR). The coherence times of the intensity and phase for an overcast and unclear sky are computed, these values bring information on the bit rate achievable and the speed of an adaptive optics system.