We present a method to develop a turbulence emulation bench for low-Earth-orbit satellite-to-ground optical communication links under strong turbulence. We provide guidelines to characterize the spatio–temporal dynamics of phase disturbances and scintillation produced by the emulator on a laser beam. We implemented such an emulator for a link at 10 deg elevation and discuss here its design method and characterization. The characterization results are compared to numerical simulations, and this characterization results in the validation of a digital twin of the emulator. The emulator will serve as a testing platform for adaptive optics systems and other free-space optical communication components under strong turbulence conditions.
These last years have seen a raising interest for ground to GEO satellites optical very high throughput links, i.e. GEO-feeder links, or GEO-FL. However, despite their potential, these applications have to overcome atmospheric turbulence, which requires the development of mitigation techniques, such as adaptive optics (AO). In the case of GEO-FL, AO performance is limited by the Point-Ahead Angle (PAA) induced anisoplanatism. We describe here how our feedback on our field experiments helped us to design ONERA’s AO-compensated ground station, FEELINGS, and the status of said ground station in the fall of 2022.
Space data highway networks are currently being deployed to provide high data rate connectivity to non-fibre-connected areas around the world and to prepare the incoming growth of Internet Of Things market. Among other solutions, GEO optical feeder links are seen as the backbone of the future globalized very high data rate satellite-based telecommunication networks. The FEEDELIO experiment developed for ESA in April 2019 demonstrated the relevance of angular decorrelation models to assess the statistical characteristics of the pre-compensated uplink irradiance that is deeply affected by anisoplanatism. Based on the conclusion drawn from this experiment, we present the results of a preliminary design study of an Adaptive Optics (AO) pre-compensated optical feeder links ground station. Exploiting end-to-end simulations under relevant turbulence conditions, time-correlated fading statistics are investigated to provide typical fading durations for AO pre-compensated optical channel, taking into account the impact of point-ahead mispointing errors.
The volume available on-board small satellites limit the optical aperture to a few centimetres, which limits the Ground- Sampling Distance (GSD) in the visible to approximately 3 m at 500 km. We present a performance analysis of the concept of a deployable CubeSat telescope. This payload will allow a tripling of the ground resolution achievable from a CubeSat imager, hence allowing very high resolution imaging from Low Earth Orbit (LEO). The project combines precision opto-mechanical deployment and cophasing of the mirrors segments using active optics. The payload has the potential of becoming a new off-the-shelf standardised system to be proposed for all high angular resolution imaging missions using CubeSats or similar nanosats. Ultimately, this technology will develop new instrumentation and technology for small satellite platforms with a primary mirror size equal or larger than 30 cm. In this paper, we present the breakdown of the different error sources that may affect the final optical quality and propose cophasing strategies. We show that the piston, tip and tilt aberrations may need to be as small as 15 nm RMS to allow for diffraction-limited imaging. By taking a co-conception approach, i.e. by taking into account the post-processing capability such as deconvolution, we believe these constraints may be somewhat released. Finally, we show numerical simulation of different solutions allowing the aberrations of the primary mirror segments.
Optical communication between satellites and optical ground stations is a promising technology for high-speed data transfer between Space and Earth. However, the atmospheric turbulence present in the last few tens of kilometres near the ground has a significant impact on the quality of the link and reduces the transfer rate due to signal fading. To explore the robustness of adaptive optics (AO) systems to mitigate the turbulence effects at low elevation angles, we have developed a bench representative of the propagation channel between the ground and the satellite. The aim of the paper is to present the system analysis, the definition choices and the simulated performance that have conducted to the actual bench. We also describe the set-up and the characterisation of the main components.
Due to the exponential increase in global data transfer, high-speed free space optical links between the ground and satellites are increasingly being envisaged. However, the availability of the optical link is affected by atmospheric attenuation and turbulence. To reliably estimate the overall optical system performance, accurate prediction of the link availability is required, which implies reliable characterization of the local atmospheric parameters. Such characterization will help for optimal selection of the optical ground station network and the prediction of their turbulence conditions. In this contribution, we propose several composite parametric models (profiles of Cn² and wind) based on analytical and bulk models fed by weather data. This flexible method can be applied to any location on the globe for an automatic characterization of the turbulent channel. In this paper we present the characterization methodology and the comparison of parametric and numerical models with in situ experimental measurements. Results show outstanding adequacy of the ground Cn² values obtained with a Bulk-model fed by the local weather station and those of micro-thermometer measurements and the Astro-Meso-NH model.
In the framework of NATO task group SET 226 on turbulence mitigation techniques for OA systems, a trial was conducted in the premises of RDDC-Valcartier, using indoor and outdoor facilities in September 2016. Images data sets were collected under various turbulence conditions, both controllable (indoor) and natural (outdoor). The imagery of this trial was used in the Grand Challenge, where different experts were asked to process identical input data with state-of-the-art algorithms. The trial also provided a data-base to validate theoretical and numerical models. The paper will give an overview of the experiment set-up (target, sensors, turbulence screens generators…) and present some preliminary results obtained with the collected data in terms of effectiveness of image processing techniques, new methods for turbulence characterisation, modelling of laser beam propagation.
KEYWORDS: Telescopes, Sensors, Space telescopes, Optical transfer functions, Optical design, Signal to noise ratio, Point spread functions, Interferometers, Optical telescopes, Optical instrument design
ONERA is currently conducting a study on the feasibility of an imaging interferometer for Earth observation from a GEO orbit. During this study, some key elements for the definition of such an instrument have been identified and studied. The results obtained so far confirm the applicability of wide-field optical interferometry with a Michelson-type instrument for Earth observation from a GEO orbit.
The goal of a cophasing sensor (CS) is to measure the phase disturbances between the sub-apertures or inside each aperture of a telescope. Three CSs are currently studied at ONERA. A first CS for Earth imaging is based on phase diversity on extended sources (cf companion paper by L. Mugnier). A second CS for intercalibration uses phase retrieval on a point source. The third CS for nulling interferometry (“DWARF”, for the ESA/DARWIN mission) is based on similar algorithms. To test performance of these CSs, ONERA has defined and integrated a multipurpose bench, “BRISE”. Its main features are the simultaneous imaging of a point source and of an extended source, the minimisation of absolute and differential disturbances, the use of any aperture configuration and the generation of pure calibrated piston/tip/tilt aberrations on three sub-apertures by a dedicated PZT-based device. Preliminary experimental results are consistent with numerical simulations and confirm nanometric performance.
This paper, “Wavefront sensors for the active control of earth observation optical instruments," was presented as part of International Conference on Space Optics—ICSO 1997, held in Toulouse, France.
In the framework of a European collaborative research project called ALWS (Airborne platform effects on lasers and Warning Sensors), the effects of platform-related turbulence on MAWS (missile approach warning systems) and DIRCM (directed infrared countermeasures) performance are investigated.
Field trials have been conducted to study the turbulence effects around a hovering helicopter and behind a turboprop aircraft on the ground, with engines running. The time dependence of the power in the bucket and the amplitude of the angle of arrival have been characterized during the trial. Temporal spectra of these two parameters present an asymptotic behavior typical of optical beams propagating through developed turbulence (Kolmogorov). Based on the formalism developed in the case of propagation through atmospheric turbulence, we have first estimated turbulence strength and wind velocity inside plume for different flight conditions. We have then proposed an approach to simulate times series of these two quantities in the same conditions. These simulated time series have been compared with the recorded data to assess their validity domain. This model will be integrated in a simulator to estimate the impact of the turbulence induced by the platform and calculate the system performance. In this model dedicated to plume and downwash effects, aero-optical effects are not taken into account.
Airborne platform effects on lasers and warning sensors (ALWS) has been a European collaborative research project to investigate the effects of platform-related turbulence on optical countermeasure systems, especially missile approach warning systems (MAWS) and directed infrared countermeasures (DIRCM). Field trials have been carried out to study the turbulence effects around a hovering helicopter and behind a turboprop aircraft with engines running on the ground. In addition different methods for modelling the effects have been investigated.
In the helicopter trials significant beam wander, scintillations and beam broadening were experienced by narrow divergence laser beams when passing through the down-wash of the hot engine exhaust gases. The measured effects considerably exceed the effects of atmospheric turbulence. Extraction of turbulence parameters for modelling of DIRCM-relevant scenarios show that in most cases the reduction of jamming power and distortion of jamming waveform can be expected to be small. The reduction of effects of turbulence is mainly related to the larger beam divergence and shorter Rayleigh length of DIRCM lasers compared to the experimental probe beams. Measurements using the turboprop platform confirm that tolerable effects on laser beam properties are found when the laser beam passes through the exhaust 15 m behind the outlet where the exhaust gases are starting to cool down.
Modelling efforts have shown that time-resolved computational fluid dynamics (CFD) calculations can be used to study properties of beam propagation in engine exhaust-related turbulence. Because of computational cost and the problem of validating the CFD results the use for system performance simulations is however difficult.
The hot exhaust gases emitted from aircraft engines create extreme optical turbulence in a local region. The effects on countermeasure system performance depend both on the system parameters and on the threat characteristics. With present-day DIRCM systems, the effects of even severe turbulence are often tolerable.
An optical link based on a multiplex of wavelengths at 1:55 μm is foreseen to be a valuable alternative to the conventional radio-frequencies for the feeder link of the next-generation of high throughput geostationary satellite. Considering the limited power of lasers envisioned for feeder links, the beam divergence has to be dramatically reduced. Consequently, the beam pointing becomes a key issue. During its propagation between the ground station and a geostationary satellite, the optical beam is deflected (beam wandering), and possibly distorted (beam spreading), by atmospheric turbulence. It induces strong fluctuations of the detected telecom signal, thus increasing the bit error rate (BER). A steering mirror using a measurement from a beam coming from the satellite is used to pre-compensate the deflection. Because of the point-ahead angle between the downlink and the uplink, the turbulence effects experienced by both beams are slightly different, inducing an error in the correction. This error is characterized as a function of the turbulence characteristics as well as of the terminal characteristics, such as the servo-loop bandwidth or the beam diameter, and is included in the link budget. From this result, it is possible to predict intensity fluctuations detected by the satellite statistically (mean intensity, scintillation index, probability of fade, etc.)). The final objective is to optimize the different parameters of an optical ground station capable of mitigating the impact of atmospheric turbulence on the uplink in order to be compliant with the targeted capacity (1Terabit/s by 2025).
In order to address the high throughput requested for both downlink and uplink satellite to ground laser links, adaptive optics (AO) has become a key technology. While maturing, application of this technology for satellite to ground telecommunication, however, faces difficulties, such as higher bandwidth and optimal operation for a wide variety of atmospheric conditions (daytime and nighttime) with potentially low elevations that might severely affect wavefront sensing because of scintillation. To address these specificities, an accurate understanding of the origin of the perturbations is required, as well as operational validation of AO on real laser links. We report here on a low Earth orbiting (LEO) microsatellite to ground downlink with AO correction. We discuss propagation channel characterization based on Shack–Hartmann wavefront sensor (WFS) measurements. Fine modeling of the propagation channel is proposed based on multi-Gaussian model of turbulence profile. This model is then used to estimate the AO performance and validate the experimental results. While AO performance is limited by the experimental set-up, it proves to comply with expected performance and further interesting information on propagation channel is extracted. These results shall help dimensioning and operating AO systems for LEO to ground downlinks.
In collaboration between CNES, NICT, Geoazur, the first successful lasercom link between the micro-satellite SOCRATES and an OGS in Europe has been established. This paper presents some results of telecom and scintillation first data analysis for 4 successful links in June & July 2015 between SOTA terminal and MEO optical ground station (OGS) at Caussols France. The telecom and scintillation data have been continuously recorded during the passes by using a detector developed at the laboratory. An irradiance of 190 nW/m2 and 430 nW/m2 has been detected for 1549 nm and 976 nm downlinks at 35° elevation. Spectrums of power fluctuation measured at OGS are analyzed at different elevation angles and at different diameters of telescope aperture to determine fluctuations caused by pointing error (due to satellite & OGS telescope vibrations) and caused by atmospheric turbulence. Downlink & Uplink budgets are analyzed, the theoretical estimation matches well to measured power levels. Telecom signal forms and bit error rates (BER) of 1549 nm and 976 nm downlink are also shown at different diameters of telescope aperture. BER is 'Error Free' with full-aperture 1.5m telescope, and almost in ‘good channel’ with 0.4 m sub-aperture of telescope. We also show the comparison between the expected and measured BER distributions.
The performance of electro-optical platform protection systems can be degraded significantly by the propagation
environment around the platform. This includes aero-optical effects and zones of severe turbulence generated by engine
exhausts. For helicopters rotor tip vortices and engine exhaust gases that are pressed down by the rotor airflow form the
so called downwash phenomena. The downwash is a source for perturbations. A wide range of spatial and temporal
fluctuations in the refractive index of air can occur. The perturbations from the turbulent flow cause detrimental effects
on energy delivery, angle of arrival fluctuations, jam-code transmission, tracking accuracy and imaging performance in
general. Therefore the effects may especially have a severe impact on the performance of laser-based protection systems
like directed infrared countermeasures (DIRCM). The chain from passive missile detection and warning to obtaining an
optical break-lock by the use of an active laser system will be influenced.
To anticipate the installed performance of an electro-optical defensive aids suite (DAS) for helicopter platforms it is
necessary to develop models for the prediction of the perturbations. Modelled results have to be validated against
experimental findings. However, the data available in open literature on the effects of rotor downwash from helicopters
on optical propagation is very limited. To collect necessary data and to obtain a first impression about the magnitude of
occurring effects the European defence agency group (EDA) on “airborne platform effects on lasers and warning sensors
(ALWS)” decided to design and perform a field trial on the premises of the Italian Air Force Flight Test Center in Pratica
di Mare, Italy. ALWS is a technical arrangement under the Europa MoU among France, Germany, Italy, Sweden and the
United Kingdom.
Active imaging can be used for surveillance or target identification at long range and low visibility conditions. Its principle is based on the illumination of a scene with a pulsed laser which is then backscattered to the sensor. The signal to noise ratio and contrast of the object over the background are increased in comparison with passive imaging. Even though, range and field of view (FOV) are limited for a given laser power. A new active imaging system presented here aims at overcoming this limitation. It acquires the entire scene with a high-speed scanning laser illumination focused on a limited region, whereas at each scan the full frame active image is acquired. The whole image is then reconstructed by mosaicking all these successive images. A first evaluation of the performance of this system is conducted by using a direct physical model. This end-to-end model, realistic in terms of turbulence effects (scintillation, beam wandering ...), gives us a sequence of images a synthetic scenes. After presenting this model, a reconstruction method of the total scene is described. And the performances of this new concept are compared to those of a conventional flash active camera by using usual metrics ( SNR, MTF ...). For various mean laser powers, we quantify the gains expected in terms of range and field of view of this new concept.
The performance of optical systems is degraded by atmospheric turbulence. Over propagation distances that exceed
several kilometers, it is difficult to evaluate its impact because of terrain variability - a factor that should be taken into
account. However, to optimize performance, the turbulence characteristics and its effect on optical wave propagation
along the propagation path should be known. The understanding of turbulence impact is one of the main objectives of the
NATO group SET 165: "Adaptive Optics (AO) for laser beam delivery, passive and active imaging and turbulence
mitigation". In this paper we describe experiments performed by the NATO SET 165 research group, namely, a set of
atmospheric experiments over a 7 km distance, and discuss some preliminary results of data processing. The experiments
were conducted at the University of Dayton Intelligent Optics Laboratory (UD/IOL) in October 2011. It benefited
significantly from the available optical setups and the infrastructure on the UD/IOL site.
Free-space optical (FSO) communication systems have currently a restricted range, because of atmospheric effects
which reduce their application range. The goal of the SCALPEL project is to study the feasibility of long range FSO
systems (goal: 20 km), i.e. to estimate how dedicated devices could enhance the range of FSO communication systems,
for example by changing the link's wavelength for a better atmospheric transmission and weaker turbulence effects,
and/or by using an innovative adaptive optics device to compensate, at least partially, turbulence perturbations.
In this paper, we study how the atmosphere constrains the system design in terms of transmission and turbulence.
We show that the system cannot work unless it has a full-wave adaptive optics correction, and that a wavelength around
4 μm presents several advantages toward the usual wavelength, i.e. 1.55 μm. A first design of the system is then
presented, including the source and the sensor.
Onera, The French Aerospace Lab, has developed an active burst illumination imaging system with a short time gating.
This imaging device is used to obtain a passive or an active image of a small volume of the illuminated scene. To better
understand and evaluate the relevant physical phenomena (scintillation, speckle...) impacting on the performance on
burst illumination imaging system, Onera has implemented a code (PIAF). The aim of this paper is to describe the model
and to present some results. Efforts have done on 3D target geometries and surface properties. We analyze each
contribution like the incoherent solar field or the incident laser field. We adapt classical and physical models for light
reflection. Speckle contributions are also treated using data bases generated by an Onera tool.
Data-rates of long-range free-space optical communication links are deteriorated by atmospheric turbulence which
causes power in the bucket fluctuations. In order to compensate for those effects the use of adaptive optics is
envisioned. Different solutions have been proposed for the correction. We study here the performances of several
compensation methods, encompassing both amplitude and phase and phase-only precompensation. In the case
of phase-only precompensation we studied two system designs, one which is dedicated to symmetrical communication
systems and the other to dissymmetric systems. In the dissymmetric case we studied two ways of driving
the deformable mirror: the use of a Shack-Hartmann wavefront sensor and a model-free phase modulation. For
each compensation architecture simulation results covered weak, moderate and strong turbulence conditions.
Different statistical properties of laser illumination through atmospheric turbulence have already been derived either in the small or strong perturbation regimes. However, modeling the instantaneous illumination remains useful for the estimation of optical system performances. using an end-to-end propagation model based on a split step algorithm leads up to heavy calculations. We propose a simplified model to provide instantaneous laser illuminations according to the C2/n profile. This simplified method has been implemented. Hypothesis taken into account and cross check with a numerical end-to-end model developed at Onera are presented.
Shack-Hartmann wavefront sensor is widely used for the measurement of phase perturbations induced by turbulence.
Such a wavefront sensor relies on the measurement of the image displacements in the lenslet array focal
plane. Different algorithms can be used to estimate this displacement. This paper is dedicated to the analysis
and comparison of their performances. Special attention will be paid to correlation techniques which are well
suited to extended sources.
The speckles produced in the image of a laser-illuminated object introduce errors in the wavefrontmeasurement by a Shack
Hartmann (SH) Wave-Front Sensor (WFS). The speckles are similar to a spatial noise onto the image and contribute to the error in the computation of the image position for each lenslet of the sensor. This speckle error impacts directly the performance of an adaptive optics system. We express this error term for different cases without and with turbulence and show that it is an additive white noise. Based on end to end numerical simulations, we quantify the speckle error and compare it to the turbulent wavefront variance in different turbulence regimes.
From the local wavefront slope measurement given by a Shack Hartmann wavefront sensor (SH), it is possible to evaluate the decentering aberrations introduced by optical system misalignments. These slopes can be expressed as a function of the centered system aberrations and of the optical axis displacements in the image and pupil planes. After calibration of the wavefront sensor, the system alignment can be, for small aberrations, automatically controlled. The validity of the concept was proved on a representative observation system.
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