Satellite-to-ground communications and laser applications often suffer from turbulence, causing loss of signal quality. Key parameters include the coherence length of turbulence (𝑟0), the isoplanatic angle, and the Greenwood frequency. Specifically, this study focuses on the effects of different assumptions regarding mixing length scales on the vertical profile of optical turbulence. We report a comparison of various parametrizations, based on the ratio of mechanical and thermal heat exchange, alongside existing parametrizations found in literature. The range of mixing scale variations, spanning from centimeters to thousands of meters, poses a significant challenge and raises questions regarding the utilization of standard radiosonde data for turbulence estimation.
Accurate prediction of the vertical distribution of optical turbulence strength (see manuscript PDF for symbol) is essential for several applications, such as ground-to-satellite optical communications and astronomical seeing prediction. We have developed an algorithm that couples the calculation of (see manuscript PDF for symbol) profiles with the results of the Weather Research and Forecast Model (WRF), using the Tatarskii formula for (see manuscript PDF for symbol). The mixing length scale was identified as a critical parameter along with the gradients of potential temperature and wind speed. To determine the effects of the number of vertical levels used in the simulations and the effects of diurnal variations, we conducted a study using 100 to 500 levels in both daytime and nighttime simulations. We analyzed the results to identify differences and to suggest improvements in the calculations, taking into account the trade-off between longer simulation time and accuracy. We also compared our results to vertical profiles of (see manuscript PDF for symbol) derived from experimental data obtained from radiosonde ascents, for the Bergen radiosonde station in Germany. Our results contribute to refining the modelling of (see manuscript PDF for symbol) and improving the accuracy of the prediction, which will benefit various practical applications.
In the framework of two research groups of the Science and Technology Organization (STO) Fraunhofer IOSB and partners conducted a field trial in a humid coastal environment in Florida (USA). The research groups investigate the sensitivity of EO/IR-TDAs (Electro-Optical/Infrared Tactical Decision Aids) to environmental factors and physics-based EO/IR scene simulation tools for decision support systems. Main objective of the trial was the collection of a dataset, that can be used to study/improve the quality of the current EO/IR scene simulation toolsets and to evaluate the impact of different parameters on the reliability of EO/IR-TDA predictions. An overview of the trial and results of a first analysis are given.
An over-water propagation link of 3.6 km was set up between the Dutch city of Den Helder and the island of Texel. For 6 days in November 2021, a laser beam was propagated along this path. An analysis of the beam wander resulted in an estimate of the strength of optical turbulence C2n. Estimates compare quite well with independent measurements of C2n by two boundary layer scintillometers. Regional maps of C2n were produced by a numerical tool consisting of the WRF model coupled to a micrometeorological module. Regional differences in C2n could be explained in terms of surface conditions, and acceptable agreement was found between the numerical values of C2n for the trial site and the values provided by the scintillometers.
The investigation of transmission, refraction, and turbulence over the False Bay in South Africa and their influence on wave propagation was the main topic of First European South African Transmission ExpeRiment (FESTER). It yielded a 9-month continuous dataset of turbulence (Cn2) data, acquired by three Boundary Layer Scintillometers (BLS) and one ultrasonic anemometer. The data is analysed in terms of atmospheric stability, and relations are sought between the atmospheric state and the power spectrum of turbulence on the one hand, and the vertical gradients of turbulence strength on the other hand. This allows us to test various parameterizations of the z/L function, not only against experimental data, but also against numerical weather prediction (NWP) data. This work extends our previous analysis of specific case studies of the FESTER dataset.
We report the analysis of radio soundings launched at our permanent measurement site in North-western Germany. The data of potential temperature and wind speed are used to derive vertical profiles of the gradient Richardson number 𝑅𝑖𝑔 in the free troposphere, and subsequently, the strength of optical turbulence 𝐶𝑛 2. These values are compared to a numerical framework, which derives 𝐶𝑛 2 from mesoscale weather prediction data by two schemes based on the gradient Richardson number and Monin-Obukhov similarity theory, respectively.
Insight in the performance of laser systems as function of weather is relevant for both civilian and military applications. This paper focuses on a long-term experiment to characterize laser propagation conditions over a near-surface, across-water optical link over the outer Bay of Eckernförde in the German part of the Baltic Sea. Turbulence was characterized with a boundary layer scintillometer (BLS), and it is shown that saturation effects occurred over this long optical path. The turbulence is primary driven by thermal forces and correlates best with the air-sea temperature difference. Simulations exploiting numerical mesoscale weather prediction tools agree favorably with the observations. The effect of the environment on beam divergence and power-in-the-bucket is discussed.
Our aim is to characterize the optical turbulence over the urban areas. Since it is difficult to measure 𝐶2/𝑛 (see PDF) continuously over an urban area, we explore the possibility of using a mesoscale weather prediction model to predict 𝐶2/𝑛 (see PDF) over the urban area. To this end, the output of the Weather Research and Forecast model (WRF) was coupled with a micrometeorological parametrization, which allowed calculation of 𝐶2/𝑛 (see PDF) at each numerical grid point in the surface layer. Numerical results are compared to data of path-averaged measurements of optical turbulence performed with a large aperture scintillometer (BLS900) over the city of Ettlingen (southwestern Germany) during two time periods in Spring and Summer 2013. Effects of the heat island effect are revealed by high turbulence values, observed at night-time.
Military operations in arid regions of the world are becoming more and more regular. The atmospheric conditions in these regions impose severe restrictions on the performance of optical systems. In contrast to regions, where many airports are located and therefore the monitoring network of ground stations is very dense, only few ground measurements are available for arid regions. To a certain extent, measurements can be collected and generalized with large-scale measurement campaigns, but they are very cost-intensive and partly not achievable due to the political situation. Another possibility to close this gap of data is provided by satellite measurements. For various measurement parameters such as humidity, wind, solar radiation and aerosols, this works quite well with some limitations.
For this reason, models are a good complement to fill the lack of data in these regions. The study is concerned with identifying the turbulence in Western Sahara. The models used WRF (Weather Research and Forecasting Model) and ICON (Icosahedral Nonhydrostatic Model) have been sufficiently tested in different regions of the world. As there are no turbulence measurements in the Sahara, this is the first test to estimate the magnitude of the turbulence in order to discuss the need for an extensive measurement campaign. The models can be validated with previous trials of IOSB such as White Sands Missile Range (WSMR) in the USA, (New Mexico).
In the framework of a NATO research group Fraunhofer IOSB and partners conducted a field trial in an arid shrub land environment in southern New Mexico (USA). The group investigates environmental limitations of fielded EO-TDAs (Electro-Optical Tactical Decision Aids). Main objective of the trial was to study the impact of the atmosphere on imaging sensor performance with a focus on the effects of atmospheric extinction and near surface turbulence. An overview of the trial will be given, as well as an overview on EO-TDA development. Results of efforts to forecast the refractive index structure parameter using numerical weather prediction (NWP) models will be described, as well as the results of a perception study on the influence of turbulence on target acquisition ranges using MWIR imagery.
Electro-optical and laser systems are operated world-wide. Their performance in the outside atmosphere is mainly governed by the strength of optical turbulence Cn2 . The predictability of Cn2 using weather-forecast models is investigated by performing simulations with the Weather Research and Forecast Model (WRF). The WRF output data were combined with a micrometeorological parametrization to derive Cn2 . Simulation runs were performed for locations and times included in our worldwide data set of Cn2 obtained in several field trials over land and over the sea. Experimental data of point and integrated path measurements in the surface layer were compared to model calculations of Cn2 . The regions include different climatic conditions from South Africa, the US, as well as Central and Northern Europe. The applicability of WRF to predict Cn2 at the different locations will be discussed. It will be shown that WRF in a 1.1-km resolution is adequate to provide a first estimate of Cn2.
The atmospheric influence on wave propagation was investigated during the First European South African Transmission ExpeRiment (FESTER) from June 2015 to February 2016. The focus in this article was set on optical turbulence, the main atmospheric factor affecting the position and strength of Laser beams, the performance of electro-optical systems and imaging. Measurements were performed continuously during the campaign on three sites over the northwestern part of False Bay. The optical turbulence measurements include in situ measurements using an ultrasonic anemometer at the Roman Rock Island. Integrated optical turbulence measurements were performed at two sites, over a path of 1.8 km and a long distance path of 8.6 km. The sites may be affected by local effects of the coastal environment. For comparison, the optical turbulence was modeled using micrometeorological parameterization. Additionally, the optical turbulence was determined by simulations using the weather research and forecast model WRF. Simulation results were compared to measurements considering seasonal and meteorological variations. The representativeness of the measurements locations for offshore measurements will be discussed.
Atmospheric turbulence impacts on the propagation of electro-optical radiation. Typical manifestations of optical turbulence are scintillation (intensity fluctuations), beam wander and (for laser systems) reduction of beam quality. For longer propagation channels, it is important to characterize the vertical and horizontal distribution (inhomogeneity) of the optical turbulence. In the framework of the First European South African Transmission ExpeRiment (FESTER) optical turbulence was measured between June 2015 and February 2016 on a 2 km over-water link over False Bay. The link ran from the Institute of Maritime Technology (IMT) in Simons Town to the lighthouse at Roman Rock Island. Three Boundary layer scintillometers (BLS900) allowed assessing the vertical distribution of optical turbulence at three different heights between 5 and 12 m above the water surface. The expected decrease of Cn2 with height is not always found. These results are analyzed in terms of the meteorological scenarios, and a comparison is made with a fourth optical link providing optical turbulence data over a 8.7 km path from IMT to Kalk Bay, roughly 36° to the north of the three 2 km paths. The results are related to the inhomogeneous meteorological conditions over the Bay as assessed with the numerical weather prediction tool, the Weather Forecast and Research model WRF.
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