Atmospheric Optical Turbulence Research Articles

Impact of exposure time on optical-phase measurements in turbulence

In this paper, we explore the impact of exposure time on optical-phase measurements collected on light that has propagated through atmospheric-optical turbulence. We model the exposure time by phase averaging over a convective distance, and we quantify the associated impact of imposing an exposure time using the piston- and tilt-removed phase variance. We accomplish this analysis through the development of an analytic solution and wave-optics simulations. In turn, we show that the analytic solution and simulation results are in good agreement when U c τ/D≲0.25, where U c is the convective velocity, τ is the exposure time, and D is the aperture diameter. When U c τ/D≳0.25, the analytic solution underestimates the piston- and tilt-removed phase variance relative to the simulation results, and we discuss these differences. This work, at large, informs wavefront sensing and adaptive-optics efforts, where either the wind speed is high, the system is on a high-speed platform, the beacon is on a high-speed platform, or the beacon signal is very faint thereby requiring long-exposure data collections.

Optical turbulence profiling at the Table Mountain Facility with the Laser Communication Relay Demonstration GEO downlink

We report the first measurement of the atmospheric optical turbulence profile using the transmitted beam from a satellite laser communication terminal. A ring image next generation scintillation sensor (RINGSS) instrument for turbulence profiling, as described in Tokovinin [MNRAS 502, 747 (2021)10.1093/mnras/staa4049], was deployed at the NASA/Jet Propulsion Laboratory’s Table Mountain Facility (TMF) in California. The optical turbulence profile was measured with the downlink optical beam from the Laser Communication Relay Demonstration (LCRD) geostationary satellite. LCRD conducts links with the Optical Communication Telescope Laboratory ground station and the RINGSS instrument was co-located at TMF to conduct measurements. Turbulence profiles were measured at day and night and atmospheric coherence lengths were compared with other turbulence monitors such as a solar scintillometer and Polaris motion monitor. RINGSS sensitivity to boundary layer turbulence, a feature not provided by many profilers, is also shown to agree with a boundary layer scintillometer at TMF (R = 0.85). Diurnal evolution of optical turbulence and measured profiles are presented. The correlation of RINGSS with other turbulence monitors (R = 0.75 − 0.86) demonstrates the concept of free-space optical communications turbulence profiling, which could be adopted as a way to support optical ground stations in a future Geostationary feeder link network. These results also provide further evidence that RINGSS, a relatively new instrument concept, correlates well with other instruments in daytime and nighttime turbulence.

Design and Construction of an Operational Prototype of Optical Atmospheric Turbulence Parameters Measurement Device Based on Moiré Deflectometry

Design and Construction of an Operational Prototype of Optical Atmospheric Turbulence Parameters Measurement Device Based on Moiré Deflectometry

Exploring atmospheric optical turbulence: observations across zenith angles

We present measurements of the atmospheric optical turbulence as a function of zenith angle using two identical instruments, Shack-Hartmann Image Motion Monitors (SHIMMs), to measure atmospheric parameters concurrently. One instrument was pointed near zenith, while the other collected data by tracking a single star until it set and thus sampling zenith angles continuously to the horizon. By comparing these measurements, we can attribute changes in the atmospheric parameters to the changing zenith angle rather than variations in local turbulence conditions. The primary purpose of this experiment is to make comparisons between the measurements of the scintillation index, σ I 2, and Fried parameter, r0, with current theories. In this demonstration, we find that there is a strong agreement between the models and the instrument up until zenith angles of 70∘, above which model and measurements begin to deviate. We discuss various ways in which limitations in models and our instrument may cause these deviations.

Atmospheric optical turbulence prediction method for satellite-ground laser communication (invited)

Atmospheric optical turbulence prediction method for satellite-ground laser communication (invited)

A Real-Time Measurement System for Atmospheric Turbulence Intensity and Distribution Based on the GLAO System

Measuring the intensity and distribution of atmospheric optical turbulence at large-aperture astronomical telescope sites is crucial to optimizing turbulence correction for different layers. A real-time measurement of turbulence distribution in large-aperture telescopes would be valuable for the parameter optimization of adaptive optics (AO) systems, especially for large field-of-view AO systems such as multi-conjugate adaptive optics (MCAO) and ground-layer adaptive optics (GLAO). Based on the GLAO system of NVST at FSO, a real-time measurement system was deployed to assess the site’s atmospheric turbulence intensity and distribution. This system is, to our knowledge, the first real-time turbulence parameter measurement system in the world with an AO system. We adopt pseudo-open loop methods to restore the turbulence information from the close-loop data of GLAO and measure the turbulence strength and distribution. Multiple subaperture pairs are used instead of a pair of subapertures for fitting calculation to increase the measurement accuracy. Two conventional measurement algorithms, SLODAR and S-DIMM+, are compared with the data from the open-source simulator SOAPY, to cross-verify the correctness of our calculation based on the data process of pseudo-open loop data and multiple subaperture pairs. The simulation results show that for two layers’ turbulence input, approximately 93% of the turbulence is correctly detected with the SLODAR method and the given parameters of wavefront sensors and correctors, while the S-DIMM+ is 87%. Real-time measurements of atmospheric turbulence at the NVST site were carried out on 28 May 2023. The observation results indicated that approximately 80% of the turbulence was located below an altitude of 2000 m; only a few appear in the upper height.

Continuous daytime and nighttime forecast of atmospheric optical turbulence from numerical weather prediction models.

Future satellite-to-ground optical communication systems will benefit from accurate forecasts of atmospheric optical turbulence; namely for site selection, for the routing and the operation of optical links, and for the design of optical communication terminals. This work presents a numerical approach based on the Weather Research and Forecasting software that enables continuous forecast of the refractive index structure parameter, C n2, vertical profiles. Two different C n2 models are presented and compared. One is based on monitoring the turbulent kinetic energy, while the other is a hybrid model using the Tatarskii equation to depict the free atmosphere region, and the Monin-Obukhov similarity theory for describing the boundary layer. The validity of both models is assessed by using thermosonde measurements from the Terrain-induced Rotor Experiment campaign, and from day and night measurements of the coherence length collected during a six-day campaign at Paranal observatory by a Shack-Hartmann Image Motion Monitor. The novelty of this work is the ability of the presented approach to continuously predict optical turbulence both during daytime and nighttime, and its validation with measurements in day and night conditions.

A global turbulence model shows excellent atmospheric optical turbulence conditions over the Ethiopian plateau during non-rainy season

ABSTRACT A worldwide search is currently being conducted to determine the most appropriate sites for the next generation of large optical and infrared telescopes. Here, we report a global map of atmospheric optical turbulence-associated parameter (atmospheric coherence length) based on the fifth generation of European Centre for Medium Range Weather Forecasts re-analysis data set and results for atmospheric coherence lengths at European Southern Observatory and Tibetan Plateau locations (all at mid-latitudes). These confirm the accuracy of the global atmospheric optical turbulence model, but also the simulation result reveals that an excellent atmospheric calm site exist at the Ethiopian Plateau (at a latitude of ∼10°N) outside of the rainy season. The Ethiopian Plateau is the highest plateau in Africa, known as the ‘Roof of Africa’, with an average elevation of over 2500 m, and thus potentially provides good opportunities for astronomy and astrophysics. The median atmospheric coherence length at the Ethiopian Plateau was 17.7 cm and approximately 75 per cent of the time above 15.6 cm for dry conditions, indicating a very calm atmosphere. Although favourable atmospheric turbulence conditions at the Ethiopian Plateau were found from the global turbulence model, due to the complete lack of infrastructure it has never been visited, meaning that additional validation experiments are required.

Hybrid optical turbulence models using machine-learning and local measurements.

Accurate prediction of atmospheric optical turbulence in localized environments is essential for estimating the performance of free-space optical systems. Macro-meteorological models developed to predict turbulent effects in one environment may fail when applied in new environments. However, existing macro-meteorological models are expected to offer some predictive power. Building a new model from locally measured macro-meteorology and scintillometer readings can require significant time and resources, as well as a large number of observations. These challenges motivate the development of a machine-learning informed hybrid model framework. By combining a baseline macro-meteorological model with local observations, hybrid models were trained to improve upon the predictive power of each baseline model. Comparisons between the performance of the hybrid models, selected baseline macro-meteorological models, and machine-learning models trained only on local observations, highlight potential use cases for the hybrid model framework when local data are expensive to collect. Both the hybrid and data-only models were trained using the gradient boosted decision tree architecture with a variable number of in situ meteorological observations. The hybrid and data-only models were found to outperform three baseline macro-meteorological models, even for low numbers of observations, in some cases as little as one day. For the first baseline macro-meteorological model investigated, the hybrid model achieves an estimated 29% reduction in the mean absolute error using only one day-equivalent of observation, growing to 41% after only two days, and 68% after 180days-equivalent training data. The data-only model generally showed similar, but slightly lower performance, as compared to the hybrid model. Notably, the hybrid model's performance advantage over the data-only model dropped below 2% near the 24 days-equivalent observation mark and trended towards 0% thereafter. The number of days-equivalent training data required by both the hybrid model and the data-only model is potentially indicative of the seasonal variation in the local microclimate and its propagation environment.

A Review of Daytime Atmospheric Optical Turbulence Profile Detection Technology

Atmospheric turbulence has been confirmed as the primary source affecting the quality of ground-based telescope image. To reduce the effect of atmosphere, a good site should be selected, and adaptive optics (AO) should be installed for the telescope. In general, the daytime atmospheric turbulence is more intense than that at night under the effect of solar radiation. Numerous solar telescopes have built AO systems worldwide. Conventional AO is only capable of improving the image quality in a small field of view, whereas it cannot satisfy the needs of a large field of view. The novel wide field adaptive optical system is capable of achieving a large field of view and high-resolution images, whereas the atmospheric turbulence profile should be accurately detected, which is the prerequisite and key parameter of the novel AO system. Moreover, the astronomical high-resolution technology in accordance with the turbulence imaging theory requires more detailed detection of turbulence. Accordingly, a brief review about the latest detection technology of the daytime optical turbulence profile is valuable for astronomical observations. Besides, the parameters of atmospheric turbulence are briefly introduced. Subsequently, SNODAR, SHABAR, MOSP, DIMM+, A-MASP, and other detection technologies of the stratified atmospheric turbulence for daytime are primarily presented, and the advantages and disadvantages of the different technologies are summarized.

SHIMM: a versatile seeing monitor for astronomy

ABSTRACTCharacterization of atmospheric optical turbulence is crucial for the design and operation of modern ground-based optical telescopes. In particular, the effective application of adaptive optics correction on large and extremely large telescopes relies on a detailed knowledge of the prevailing atmospheric conditions, including the vertical profile of the optical turbulence strength and the atmospheric coherence time-scale. The Differential Image Motion Monitor (DIMM) has been employed as a facility seeing monitor at many astronomical observing sites across the world for several decades, providing a reliable estimate of the seeing angle. Here, we present the Shack–Hartmann Image Motion Monitor (SHIMM), which is a development of the DIMM instrument, in that it exploits differential image motion measurements of bright target stars. However, the SHIMM employs a Shack–Hartmann wavefront sensor in place of the two-hole aperture mask utilized by the DIMM. This allows the SHIMM to provide an estimate of the seeing, unbiased by shot noise or scintillation effects. The SHIMM also produces a low-resolution (three-layer) measure of the vertical turbulence profile, as well as an estimate of the coherence time-scale. The SHIMM is designed as a low-cost, portable instrument. It is comprised of off-the-shelf components so that it is easy to duplicate and well suited for comparisons of atmospheric conditions within and between different observing sites. Here, the SHIMM design and methodology for estimating key atmospheric parameters will be presented, as well as initial field test results with comparisons to the Stereo-SCIntillation Detection And Ranging instrument.

Demonstrating 24-hour continuous vertical monitoring of atmospheric optical turbulence.

We report what is believed to be the first example of fully continuous, 24-hour vertical monitoring of atmospheric optical turbulence. This is achieved using a novel instrument, the 24-hour Shack-Hartmann Image Motion Monitor (24hSHIMM). Optical turbulence is a fundamental limitation for applications such as free-space optical communications, where it limits the achievable bandwidth, and ground-based optical astronomy, restricting the observational precision. Knowledge of the turbulence enables us to select the best sites, design optical instrumentation and optimise the operation of ground-based optical systems. The 24hSHIMM estimates the vertical optical turbulence coherence length, time, angle and Rytov variance from the measurement of a four-layer vertical turbulence profile and a wind speed profile retrieved from meteorological forecasts. To illustrate our advance we show the values of these parameters recorded during a 36-hour, continuous demonstration of the instrument. Due to its portability and ability to work in stronger turbulence, the 24hSHIMM can also operate in urban locations, providing the field with a truly continuous, versatile turbulence monitor for all but the most demanding of applications.

Study of the Cn2 Model through the New Dimensionless Temperature Structure Function near the Sea Surface in the South China Sea

The refractive index structure constant Cn2 near the ocean surface is an important parameter for studying atmospheric optical turbulence over the ocean. The measured refractive index structure constant and meteorological parameters, such as temperature and three-dimensional wind speed, near the sea surface on the South China Sea during the period from January to November 2020 were analyzed. On the basis of Monin–Obukhov similarity theory, the dimensionless temperature structure parameter function fT near the sea surface was established, and a new parameterized model of the near-sea surface was proposed. The new model improved the error of the widely used model proposed by Wyngaard in 1973 (W73) and better reproduced the daily variation in the measured Cn2. Further analysis of the seasonal applicability of the new model indicated that the correlation coefficients between the estimated and measured Cn2 in the spring, summer, autumn, and winter were 0.94, 0.94, 0.95, and 0.89, respectively, and the root mean square errors were 0.32, 0.41, 0.46, and 0.40 m−2/3, respectively. Compared with the Cn2 estimated by the W73 model, the correlation coefficient of Cn2 estimated by the new model and measured by the micro-thermometer increased by 0.05–0.27 and the root mean square error decreased by 0.04–0.56. The improved fT demonstrated higher accuracy than the existing models, which can lay a foundation for estimating turbulence parameters in different sea areas.

Multistep ahead atmospheric optical turbulence forecasting for free-space optical communication using empirical mode decomposition and LSTM-based sequence-to-sequence learning

Although free-space optical communication (FSOC) is a promising means of high data rate satellite-to-ground communication, beam distortion caused by atmospheric optical turbulence remains a major challenge for its engineering applications. Accurate prediction of atmospheric optical turbulence to optimize communication plans and equipment parameters, such as adaptive optics (AO), is an effective means to address this problem. In this research, a hybrid multi-step prediction model for atmospheric optical turbulence, EMD-Seq2Seq-LSTM, is proposed by combining empirical mode decomposition (EMD), sequence-to-sequence (Seq2Seq), and long short-term memory (LSTM) network. First, using empirical mode decomposition to decompose the non-linear and non-stationary atmospheric optical turbulence dataset into a set of stationary components for which internal feature information can be easily extracted significantly reduces the training difficulty and improves the forecast accuracy of the model. Second, sequence-to-sequence is combined with LSTM networks to build a prediction model that can eliminate time delay and make full use of long-term information and then use the model to predict each component separately. Finally, the prediction results of each component are combined to obtain the final atmospheric turbulence forecasting results. To validate the performance of the proposed method, three comparative models, including WRF, LSTM, and sequence-to-sequence-LSTM, are demonstrated in this study. The forecasting results reveal that the proposed model outperforms all other models both qualitatively and quantitatively and thus can be a powerful method for atmospheric optical turbulence forecasting.

Analysis of atmospheric optical turbulence model— methods and progress

Stratification is a significant characteristic of atmospheric turbulence, especially high-altitude turbulence. At a fixed height, the real optical turbulence value fluctuates by 1–2 orders of magnitude or even greater on the average value. The turbulence profile model based on the observed data is a statistical average result. It can neither represent the stratification characteristics of an actual atmospheric turbulence profile nor have the prediction function, and can not fully meet the demand of optical engineering. Owing to the limitation of the capacity and speed of the computer, it is impossible to solve the Navier Stokes equation through direct numerical simulation (DNS) and large eddy simulation (LES) to predict the optical turbulence. The solution is to predict the conventional gas parameters through the mesoscale weather numerical prediction model MM5/ WRF, and then calculate the turbulence parameters through the turbulence parameterization scheme. In this paper, the prediction methods and research results of <inline-formula><tex-math id="M6">\begin{document}$ C_n^2 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20221986_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20221986_M6.png"/></alternatives></inline-formula> in surface layer,boundary layer and free atmosphere layer are introduced. Tatarski formula is derived in detail from the turbulence kinetic energy prediction equation and the temperature fluctuation variance prediction equation, and the physical meaning and applicable conditions of the formula are summarized. The latest research progress of neural network prediction and Antarctic astronomical site selection is mainly introduced. The characteristics and differences among different models, such as the empirical model fitted with experimental data, the parameter model with conventional meteorological parameters based on Kolmogorov turbulence theory, the prediction model related to mesoscale meteorological model, and the neural network method based on data driving and so on, are analyzed. It is emphasized that Kolmogorov turbulence theory is the theoretical basis of the existing atmospheric optical turbulence parameter models.

Seasonal Changes in Atmospheric Optical Turbulence in a Near Maritime Environment Using Turbulence Flux Measurements

Light propagation through the atmosphere is affected by fluctuations in the refractive index along the path of propagation, called optical turbulence. In the atmospheric surface layer, these fluctuations are due mostly to turbulent mixing of variations in temperature. To this end, a characterization of the atmospheric surface layer above the water in Maryland, USA, is presented. Meteorological data were collected from a sensor array comprising two sonic anemometers and an infrared gas analyzer (IRGASON) over the 12-month period from January to December, 2021. The instruments were positioned vertically at heights of approximately 4 m, 6 m, and 8 m above the water. The temperature structure parameter CT2 was calculated and scaled using Monin–Obukhov assumptions. We find that the distribution of atmospheric stability presents as an approximately normal distribution during the winter and early spring, but is skewed negatively during the summer and late fall. We hypothesize that this could be due to a more stable stratification of the underlying water during summer months. Under unstable (−z/L) conditions, turbulent quantities including the temperature structure parameter scale in a manner consistent with the literature, but with generally better agreement during the summer months. Additionally, the CT2 estimations appear more consistent during the summer months across all values of (z/L). Under stable (+z/L) conditions, there are wide variations in CT2, which is also consistent with the literature.

Color mixing in targets seen through turbulence

We propose a novel imaging technique for assessing turbulence strength based on the color fluctuations of remote targets. Their chromatic changes, due to color mixing produced by the atmospheric optical turbulence, is characterized. We take advantage of a RGB Fourier transform-based color space to analyze the chromatic information on target images; then, through a model of primary color overlays, a chromatic shift measure is defined by the statistical dispersion of pixel colors. It is shown that such color changes can be related to the strength of refractive index fluctuations defined by the structure constant, .

Estimation and characterization of the refractive index structure constant within the marine atmospheric boundary layer.

We verified the feasibility of an alternative solution to generate temperature and pressure profiles with the U.S. standard atmosphere model (USSA-76). We simultaneously integrated this model with conventional meteorological parameters measured by a weather station in the course of estimating the refractive index structure constant (C n2). Moreover, a continuous-time-series estimation method of the refractive index structure constant was established within the marine atmospheric boundary layer (ABL) based on wind data obtained from a coherent Doppler wind lidar (CDWL). We also analyzed the optical turbulence characteristics induced by wind shear during the conducted experiment. Laminated and patchy stratified turbulences, which affect the performance of imaging and light transmission systems, were found within the marine ABL. Additionally, the relationship between the ABL and all atmospheric optical turbulence factors shows that the ratio of marine ABL in the entire layer differs from that reported in previous studies. Moreover, the influence of thermal turbulence factors within the marine ABL was less than that of the entire layer in our case. We report a real-time C n2 estimation method based on a CDWL. The characteristics of the marine ABL C n2 constitute a reference for optoelectronic applications.

Atmospheric optical turbulence mitigation using iterative image registration and least squares lucky look fusion.

This paper presents an atmospheric optical turbulence mitigation method that uses a sequence of short-exposure frames. An iterative block matching registration method is proposed for image dewarping. The dewarped frames are combined in a least squares lucky look (LL) fusion process. Here image patches are weighted so as to produce a fused image that is consistent with a theoretical LL optical transfer function (OTF) model. Finally, a Wiener filter is applied to provide deconvolution of the LL OTF. We also explore the LL selectivity tradespace. As the selectivity increases, the LL OTF becomes more favorable but the signal-to-noise ratio suffers, and vice versa. A restoration algorithm is applied to simulated data for quantitative analysis and two different real-world datasets for subjective evaluation. The proposed approach provides improved results compared with the benchmark methods.

Modeling and simulation of multispectral imaging through anisoplanatic atmospheric optical turbulence

We explore the modeling and simulation of multispectral imaging through anisoplanatic atmospheric optical turbulence. We analyze the impact of wavelength on a number of key atmospheric optical turbulence statistics. This includes the impact of wavelength on tilt and tilt variance. The modeling analysis also includes the impact of wavelength on the atmospheric optical transfer function. Here, we investigate the balance between diffraction and turbulence degradation as a function of wavelength. We also present a method for simulating atmospheric degradation for multispectral imagery using numerical wave propagation. Our approach uses a phase screen resampling method to allow for modeling the same atmospheric realization but with sampling parameters tailored to each wavelength. A number of multispectral simulation results, along with a validation study that compares the empirical statistics from the simulation to their theoretical counterparts, are presented. Real image data are also studied to validate theoretical multispectral tilt statistics.