Showing papers on "Scintillometer published in 2015"

Underwater optical communication performance for laser beam propagation through weak oceanic turbulence.

Abstract: In clean ocean water, the performance of a underwater optical communication system is limited mainly by oceanic turbulence, which is defined as the fluctuations in the index of refraction resulting from temperature and salinity fluctuations. In this paper, using the refractive index spectrum of oceanic turbulence under weak turbulence conditions, we carry out, for a horizontally propagating plane wave and spherical wave, analysis of the aperture-averaged scintillation index, the associated probability of fade, mean signal-to-noise ratio, and mean bit error rate. Our theoretical results show that for various values of the rate of dissipation of mean squared temperature and the temperature-salinity balance parameter, the large-aperture receiver leads to a remarkable decrease of scintillation and consequently a significant improvement on the system performance. Such an effect is more noticeable in the plane wave case than in the spherical wave case.

Intensity fluctuations of multimode laser beams in underwater medium.

Abstract: In an oceanic optical communications link, the received intensity fluctuations, quantified by the scintillation index, are formulated and evaluated when a multimode laser is used. The variations of the scintillation index versus the oceanic turbulence parameters are examined for different multimode laser structures. Oceanic turbulence parameters used are the rate of dissipation of kinetic energy per unit mass of fluid, the rate of dissipation of mean-squared temperature, the Kolmogorov inner scale, and the parameter w that defines the ratio of temperature to salinity contributions to the refractive index spectrum. The results in this paper can be used to improve performance in the design of oceanic optical communications links.

Characterizing the Footprint of Eddy Covariance System and Large Aperture Scintillometer Measurements to Validate Satellite-Based Surface Fluxes

Abstract: To validate satellite-based surface fluxes by ground measurements properly, several numerical simulations were carried out at a homogeneous alpine meadow site and mixed cropland site, considering various atmospheric conditions and different land cover distribution types. By comparing various pixel selection methods, the results showed that footprint was significant in insuring a consistent spatial scale between ground measurements and satellite-based surface fluxes, particularly for heterogeneous surface and high-resolution remote sensing data. Because large aperture scintillometer measurements cover larger areas than eddy covariance (EC) system measurements, the spatial heterogeneity at a subpixel scale in complicated surface should be further considered in validating coarse satellite data. Thus, more accurate validation data and scaling methods must be developed, such as measuring surface fluxes at the satellite pixel scale by a flux measurement matrix or airborne EC measurements.

Upscaling Sensible Heat Fluxes With Area-to-Area Regression Kriging

Abstract: Surface sensible heat flux (SHF) is a critical indicator for understanding heat exchange at the land-atmosphere interface. A common method for estimating regional SHF is to use ground observations with approaches such as eddy correlation (EC) or the use of a large aperture scintillometer (LAS). However, data observed by these different methods might have an issue with different spatial supports for cross-validation and comparison. This letter utilizes a geostatistical method called area-to-area regression kriging (ATARK) to solve this problem. The approach is illustrated by upscaling SHF from EC to LAS supports in the Heihe River basin, China. To construct a point support variogram, a likelihood function of four parameters (nugget, sill, range, and shape parameters) conditioned by EC observations is used. The results testify to the applicability of ATARK as a solution for upscaling SHF from EC support to LAS support.

Infrared and millimetre-wave scintillometry in the suburban environment – Part 2: Large-area sensible and latent heat fluxes

Abstract: . A millimetre-wave scintillometer was paired with an infrared scintillometer, enabling estimation of large-area evapotranspiration across northern Swindon, a suburban area in the UK. Both sensible and latent heat fluxes can be obtained using this "two-wavelength" technique, as it is able to provide both temperature and humidity structure parameters, offering a major advantage over conventional single-wavelength scintillometry. The first paper of this two-part series presented the measurement theory and structure parameters. In this second paper, heat fluxes are obtained and analysed. These fluxes, estimated using two-wavelength scintillometry over an urban area, are the first of their kind. Source area modelling suggests the scintillometric fluxes are representative of 5–10 km2. For comparison, local-scale (0.05–0.5 km2) fluxes were measured by an eddy covariance station. Similar responses to seasonal changes are evident at the different scales but the energy partitioning varies between source areas. The response to moisture availability is explored using data from 2 consecutive years with contrasting rainfall patterns (2011–2012). This extensive data set offers insight into urban surface-atmosphere interactions and demonstrates the potential for two-wavelength scintillometry to deliver fluxes over mixed land cover, typically representative of an area 1–2 orders of magnitude greater than for eddy covariance measurements. Fluxes at this scale are extremely valuable for hydro-meteorological model evaluation and assessment of satellite data products.

Infrared and millimetre-wave scintillometry in the suburban environment - Part 1: Structure parameters

Abstract: . Scintillometry, a form of ground-based remote sensing, provides the capability to estimate surface heat fluxes over scales of a few hundred metres to kilometres. Measurements are spatial averages, making this technique particularly valuable over areas with moderate heterogeneity such as mixed agricultural or urban environments. In this study, we present the structure parameters of temperature and humidity, which can be related to the sensible and latent heat fluxes through similarity theory, for a suburban area in the UK. The fluxes are provided in the second paper of this two-part series. A millimetre-wave scintillometer was combined with an infrared scintillometer along a 5.5 km path over northern Swindon. The pairing of these two wavelengths offers sensitivity to both temperature and humidity fluctuations, and the correlation between wavelengths is also used to retrieve the path-averaged temperature–humidity correlation. Comparison is made with structure parameters calculated from an eddy covariance station located close to the centre of the scintillometer path. The performance of the measurement techniques under different conditions is discussed. Similar behaviour is seen between the two data sets at sub-daily timescales. For the two summer-to-winter periods presented here, similar evolution is displayed across the seasons. A higher vegetation fraction within the scintillometer source area is consistent with the lower Bowen ratio observed (midday Bowen ratio

A dual-pass data assimilation scheme for estimating surface fluxes with FY3A-VIRR land surface temperature

Abstract: In this work, a dual-pass data assimilation scheme is developed to improve predictions of surface flux Pass 1 of the dual-pass data assimilation scheme optimizes the model vegetation parameters at the weekly temporal scale, and Pass 2 optimizes the soil moisture at the daily temporal scale Based on ensemble Kalman filter (EnKF), the land surface temperature (LST) data derived from the new generation of Chinese meteorology satellite (FY3A-VIRR) are assimilated into common land model (CoLM) for the first time Six sites, Daman, Guantao, Arou, BJ, Miyun and Jiyuan, are selected for the data assimilation experiments and include different climatological conditions The results are compared with those from a dataset generated by a multi-scale surface flux observation system that includes an automatic weather station (AWS), eddy covariance (EC) and large aperture scintillometer (LAS) The results indicate that the dual-pass data assimilation scheme is able to reduce model uncertainties and improve predictions of surface flux with the assimilation of FY3A-VIRR LST data

Estimation of Turbulent Sensible Heat and Momentum Fluxes over a Heterogeneous Urban Area Using a Large Aperture Scintillometer

Abstract: The accurate determination of surface-layer turbulent fluxes over urban areas is critical to understanding urban boundary layer (UBL) evolution. In this study, a remote-sensing technique using a large aperture scintillometer (LAS) was investigated to estimate surface-layer turbulent fluxes over a highly heterogeneous urban area. The LAS system, with an optical path length of 2.1 km, was deployed in an urban area characterized by a complicated land-use mix (residential houses, water body, bare ground, etc.). The turbulent sensible heat (Q H) and momentum fluxes (τ) were estimated from the scintillation measurements obtained from the LAS system during the cold season. Three-dimensional LAS footprint modeling was introduced to identify the source areas (“footprint”) of the estimated turbulent fluxes. The analysis results showed that the LAS-derived turbulent fluxes for the highly heterogeneous urban area revealed reasonable temporal variation during daytime on clear days, in comparison to the land-surface process-resolving numerical modeling. A series of sensitivity tests indicated that the overall uncertainty in the LAS-derived daytime Q H was within 20%–30% in terms of the influence of input parameters and the nondimensional similarity function for the temperature structure function parameter, while the estimation errors in τ were less sensitive to the factors of influence, except aerodynamic roughness length. The 3D LAS footprint modeling characterized the source areas of the LAS-derived turbulent fluxes in the heterogeneous urban area, revealing that the representative spatial scales of the LAS system deployed with the 2.1 km optical path distance ranged from 0.2 to 2 km2 (a “micro-a scale”), depending on local meteorological conditions.

Scaling Flux Tower Observations of Sensible Heat Flux Using Weighted Area-to-Area Regression Kriging

Abstract: Sensible heat flux (H) plays an important role in characterizations of land surface water and heat balance. There are various types of H measurement methods that depend on observation scale, from local-area-scale eddy covariance (EC) to regional-scale large aperture scintillometer (LAS) and remote sensing (RS) products. However, methods of converting one H scale to another to validate RS products are still open for question. A previous area-to-area regression kriging-based scaling method performed well in converting EC-scale H to LAS-scale H. However, the method does not consider the path-weighting function in the EC- to LAS-scale kriging with the regression residue, which inevitably brought about a bias estimation. In this study, a weighted area-to-area regression kriging (WATA RK) model is proposed to convert EC-scale H to LAS-scale H. It involves path-weighting functions of EC and LAS source areas in both regression and area kriging stages. Results show that WATA RK outperforms traditional methods in most cases, improving estimation accuracy. The method is considered to provide an efficient validation of RS H flux products.

Observing crosswind over urban terrain using scintillometer and Doppler lidar

Abstract: . In this study, the crosswind (wind component perpendicular to a path, Ub) is measured by a scintillometer and estimated with Doppler lidar above the urban environment of Helsinki, Finland, for 15 days. The scintillometer allows acquisition of a path-averaged value of Ub ( Ub ), while the lidar allows acquisition of path-resolved Ub (Ub (x), where x is the position along the path). The goal of this study is to evaluate the performance of scintillometer Ub estimates for conditions under which Ub (x) is variable. Two methods are applied to estimate Ub from the scintillometer signal: the cumulative-spectrum method (relies on scintillation spectra) and the look-up-table method (relies on time-lagged correlation functions). The values of Ub of both methods compare well with the lidar estimates, with root-mean-square deviations of 0.71 and 0.73 m s−1. This indicates that, given the data treatment applied in this study, both measurement technologies are able to obtain estimates of Ub in the complex urban environment. The detailed investigation of four cases indicates that the cumulative-spectrum method is less susceptible to a variable Ub (x) than the look-up-table method. However, the look-up-table method can be adjusted to improve its capabilities for estimating Ub under conditions under for which Ub (x) is variable.

Determination of Turbulent Sensible Heat Flux over a Coastal Maritime Area Using a Large Aperture Scintillometer

Abstract: Scintillometers have been widely used in estimating the surface-layer sensible heat flux $$(Q_\mathrm{H})$$ over natural and urban surfaces, but their application over water bodies is rare. Here, a large aperture scintillometer (LAS) was deployed over a coastal maritime area (‘a beach’) with an optical path distance of 1 km to investigate LAS capability in estimating the sensible heat fluxes. The measurements were conducted for clear days in the cold season, characterized by a warmer sea surface than the overlying air throughout the studied days. The LAS-derived $$Q_\mathrm{H} $$ showed a significant diurnal variability of 10-150 W m $$^{-2}$$ at the coastal site, and it was found that local thermal advection and tidal change at the site largely influenced the diurnal variability. A series of sensitivity tests indicated that the uncertainty in the LAS-derived $$Q_\mathrm{H} $$ was less than 11 %, except when De Bruin’s similarity function was used. The overall results demonstrate that the LAS system can detect the magnitude and variability of the turbulent heat exchange at the coastal site with high temporal resolution, suggesting its usefulness for estimating $$Q_\mathrm{H} $$ in the coastal maritime environment.

A new method for measuring the imaginary part of the atmospheric refractive index structure parameter in the urban surface layer

Abstract: . The atmospheric refractive index consists of both real and imaginary parts. The intensity of refractive index fluctuations is generally expressed as the refractive index structure parameter, with the real part reflecting the strength of atmospheric turbulence and the imaginary part reflecting absorption in the light path. A large aperture scintillometer (LAS) is often used to measure the structure parameter of the real part of the atmospheric refractive index, from which the sensible and latent heat fluxes can further be obtained, whereas the influence of the imaginary part is ignored or considered noise. In this theoretical analysis study, the relationship between logarithmic light intensity variance and the atmospheric refractive index structure parameter (ARISP), as well as that between the logarithmic light intensity structure function and the ARISP, is derived. Additionally, a simple expression for the imaginary part of the ARISP is obtained which can be conveniently used to determine the imaginary part of the ARISP from LAS measurements. Moreover, these relationships provide a new method for estimating the outer scale of turbulence. Light propagation experiments were performed in the urban surface layer, from which the imaginary part of the ARISP was calculated. The experimental results showed good agreement with the presented theory. The results also suggest that the imaginary part of the ARISP exhibits a different diurnal variation from that of the real part. For the wavelength of light used (0.62 μm), the variation of the imaginary part of the ARISP is related to both the turbulent transport process and the spatial distribution characteristics of aerosols.

Ultimate turbulence experiment: simultaneous measurements of Cn2 near the ground using six devices and eight methods

Abstract: We have performed a series of experiments in order to simultaneously validate several devices and methods for measurement of the path-averaged refractive index structure constant ( 𝐶𝑛 2). The experiments were carried out along a horizontal urban path near the ground. Measuring turbulence in this layer is particularly important because of the prospect of using adaptive optics for free-space optical communications in an urban environment. On one hand, several commercial sensors were used: SLS20, a laser scintillometer from Scintec AG, BLS900, a largeaperture scintillometer, also from Scintec, and a 3D sonic anemometer from Thies GmbH. On the other hand, we measured turbulence strength with new approaches and devices developed in-house. Firstly, an LED array combined with a high-speed camera allowed for measurement of 𝐶𝑛 2 from raw- and differential image motion, and secondly a two-part system comprising a laser source, a Shack-Hartmann sensor and a PSF camera recoded turbulent modulation transfer functions, Zernike variances and angle-of-arrival structure functions, yielding three independent estimates of 𝐶𝑛 2. We compare the measured values yielded simultaneously by commercial and in-house developed devices and show very good agreement between 𝐶𝑛 2 values for all the methods. Limitations of each experimental method are also discussed. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Using an eye-safe laser rangefinder to assist active and passive electro-optical sensor performance prediction in low visibility conditions

Abstract: Laser rangefinders are used in various electro-optical (EO) fire control systems. They often operate at eye-safe wavelengths around 1.55 μm, which extends their utility. The paper investigates the use of a modified eye-safe laser rangefinder at 1.55 μm to obtain information on atmospheric attenuation and couple that information to the performance of active and passive EO sensors with an emphasis of lower visibility conditions. Such information can be of great value both for estimating own sensor capabilities at a given moment as well as estimating the threat capability. One obvious example is ship defense where it is difficult to obtain visibility along variable and slant atmospheric paths, especially in darkness. The experimental equipment and the results from measurements of atmospheric backscatter along various atmospheric paths are presented. The backscatter curve is used to evaluate the extinction. These extinction values are compared with those deduced from a point visibility meter and from echo measurements against two similar nets positioned at two ranges from the sensor. TV and IR images of test targets along a 1.8 km path close to sea surface in the Baltic Sea were collected in parallel with the lidar. A weather station and a scintillometer collected weather and turbulence parameters. Results correlating the lidar attenuation with the imaging performance will be given.

Runway Wake Vortex, Crosswind, and Visibility Detection with a Scintillometer at Schiphol Airport

Abstract: We evaluate the performance and investigate the capability of a scintillometer to detect wake vortices, crosswind and visibility near an airport runway An experiment is carried out at Schiphol airport (Amsterdam, The Netherlands), where an optical scintillometer is positioned alongside a runway An algorithm is developed to detect wake vortices, and also the strength of the wake vortex, from the variance in the scintillation signal The algorithm shows promising results in detecting wake vortices and their strengths during the night During the day, the scintillometer signal is dominated by environmental turbulence and wake vortices are no longer detectable The crosswind measured by the scintillometer is compared with wind-speed and wind-direction data at the airport Our results show that, after applying an outlier filter, the scintillometer is able to measure the crosswind over the short time period of 3 s required for aviation applications The outlier filter does not compromise the capability of the scintillometer to obtain the maximum 3 s crosswind over a 10-min time frame correctly Finally, a transmission method is used to obtain the visibility from the scintillometer signal, which is then compared with that obtained from a visibility sensor The scintillometer is able to identify periods of low visibility correctly, although it shows a high amount of scatter around the exact visibility value

Experimental measurements of atmospheric Turbulence through optical triangulation method for FSO applications

Abstract: Optical Turbulence affects the performance of Free Space Optical (FSO) systems. This phenomena degrades the light wave front, changes the spatial optical power distribution and wander the beam in the receiver plane. The atmospheric turbulence is evaluated by the Refractive Index Structure Constant (Cn2). In this paper we present atmospheric turbulence measurements, using a simple and low cost device based on optical triangulation method, by tracking the beam hotspot in real time on the receiver plane.

High-resolution ground layer turbulence from inside the CFHT dome using a lunar scintillometer

Abstract: For ground layer adaptive optics systems, knowledge of the local height- and time- resolved ground layer (GL) turbulence is crucial to link local topography with optical turbulence. Such turbulence profiles have been obtained in the years 2009 and 2010 over 250 hours on Mauna Kea, Hawaii. Results from measurements inside the Canada-France-Hawaii Telescope (CFHT) dome indicate severe degradation of image quality due to a poorly vented dome and thus provide input for dome modifications and design aspects for a new ground layer adaptive optics system. The outside median GL seeing above 6 metres was determined to be 0.48±0.01".

Measurement of optical intensity fluctuation and study of transmission characteristics under different environment conditions

Abstract: There are some differences in optical intensity signals under different transmission environment conditions,which is affected by atmospheric turbulence.In order to study the transmission characteristics of optical intensity signals and the differences between them,this paper presents some experiment results of 4measurements of laser transmission over the road and the sea surface environment by using the large aperture scintillometer,and the optical path distances are 1.4km and 0.9km,respectively.Based on the measured data,firstly,the change trends of atmospheric refractive index structure constant and Rytov variance are analyzed and the differences of the degree of turbulence and optical intensity fluctuation are studied over the road and the sea surface environment.Secondly,the transmission characteristics are studied by analyzing the optical intensity signals of aperture averaging over these two kinds of environment.It is anticipated that this kind of signal data analysis is good for the study on atmospheric optics and free space optical communication system development.

Characterization of the horizontal optical turbulence (C n 2 ) data measured at Kongju and Cheonan

Abstract: When light from an object or an astronomical star propagates in the earth’s atmosphere, atmospheric turbulence can distort and move the image in various ways. A quantitative measure of the intensity of optical turbulence with a refractive index structure parameter, C 2 , is widely used in the statistical characterization of the random refractive index fluctuations generally referred to as optical turbulence. I this study, we investigated the horizontal optical turbulence in the near infrared region (850nm) at two sites in South Korea (Kongju and Cheonan) by using a scintillometer. The scintillometer measured the refractive index structure parameter C 2 over 2.1- and 0.4-km paths, respectively, in Kongju and Cheonan. The first path was over an urban area characterized by a complicated land-use mix (residential houses, a river, bare ground, etc.) whereas the second path was a building-to-building path at a 15-m height on a university campus. In addition to the scintillometer, an independent weather station recorded meteorological conditions such as wind speed, relative humidity, and temperature. Study results indicate the general patterns of the optical turbulence at both sites agree with previous-reported diurnal patterns; they have two dips in C2n, one at around sunrise and the other at sunset, but the night profiles varied strongly depending on the atmospheric conditions. The average values of C 2 for the measurement period were × 10−15 and 2.90 × 10−14 m−2/3 in Kongju and Cheonan, espectively, thus confirming that the optical field is clearer in the former. In addition, the average values of the Fried parameter, r0, were accordingly estimated to be 8.0 and 2.5 cm over a 2-km optical distance at Kongju and Cheonan, respectively.

Determine the optical turbulence parameter (Cn 2 ) in Babylon City-Iraq

Abstract: This research present to obtain optical turbulence structure and refractive index information a long a near horizontal (0.035 Km) free space path length. Measurement the optical turbulence structure (Cn2), refractive index (nr), Pressure and temperature for several case in winter, spring, summer and autumn. Optical measurements were investigated using three different wavelengths of laser sources (808, 632 and 1064 nm) .we found the change in refractive index is very small but the change in refractive index structure is very strong.

Measurement of optical intensity fluctuation over a real atmospheric turbulent path

Abstract: In order to investigate the effects of atmospheric turbulence on laser communication, an experiment is carried out to research intensity scintillation which caused by atmospheric turbulence. 15,000-frame gray images which grabbed by high-speed camera every ten minutes are processed and analyzed. Diurnal variation trend of scintillation index in different seasons are provided in the experiment, and the probability density distribution of intensity scintillation are analyzed in detail. The results indicate that diurnal variation of scintillation index are different in seasons in the weak turbulence conditions. It's also indicate that probability density is lognormal when scintillation index is large, and probability density is normal when scintillation index is small.

섬광계를 이용한 비균질 도시 지표에서의 현열속 산정

Abstract: A large aperture scintillometer (LAS) was deployed with an optical path length of 2.1 km to estimate turbulent sensible heat flux (Q H ) over a highly heterogeneous urban area. Scintillation measurements were conducted during cold season in November and December 2013, and the daytime data of 14 days were used in the analysis after quality control processes. The LAS-derived Q H show reasonable temporal variation ranging 20~160W m ?2 in unstable atmospheric conditions, and well compare with the measured net radiation. The LAS footprint analysis suggests that Q H can be relatively high when the newly built-up urban area has high source contribution of the turbulent flux in the study area (‘northwesterly winds’). Sensitivity tests show that the LAS-derived Q H are highly sensitive to non-dimensional similarity function for temperature structure function parameter, but relatively less sensitive to surface aerodynamic parameters and meteorological variables (temperature and wind speed). A lower Bowen ratio also has a significant influence on the flux estimation. Overall uncertainty of the estimated daytime Q H is expected within about 20% at an upper limit for the analysis data. It is also found that stable atmospheric conditions can be poorly determined when the scintillometry technique is applied over the highly heterogeneous urban area.

Effects of Corrugated Temperature Sheets on Optical Propagation along Quasi-Horizontal Paths in the Stably Stratified Atmosphere

Abstract: : Optical propagation through the clear atmosphere is affected by small-scale refractive-index fluctuations which are caused mainly by temperature fluctuations. In the stably stratified atmosphere, these temperature fluctuations are the result of a combination of (1) more or less homogeneous and isotropic turbulence and(2) non-turbulent, quasi-horizontal interfaces, or sheets''. Collocated in-situ and optical field measurements conducted in the atmospheric surface layer confirmed that angle-of-arrival fluctuations and irradiance fluctuations observed with large-aperture telescopes (36 cm aperture diameter) are consistent with theoretical predictions based on Taylor's frozen-turbulence hypothesis and the geometrical-optics approximation. Short-term (less than a few seconds) fluctuations are dominated by turbulence while longer-term fluctuations are dominated by horizontally extended sheets. Direct numerical simulations of isotropic turbulence showed very good agreement with the turbulence spectrum predicted by Hill's 1978model. A theoretical model of corrugated sheets was developed and analyzed.

Measuring crosswind using scintillometry

Abstract: This thesis demonstrates that a scintillometer is able to obtain a path-averaged value of the crosswind (i.e., wind component perpendicular to a path). A scintillometer consists of a transmitter and a receiver spaced a few hundred meters to a few kilometres apart. The transmitter emits light with a certain wavelength, which is refracted by scintillations in the atmosphere (eddies with a different temperature and moisture content than their surrounding). The atmosphere is turbulent, thus the receiver of the scintillometer measures the intensity of the fluctuations of the light. Two scintillometer setup next to each other in principle measure the same eddy field except for a time-shift. It is known that this time-shift is linked to the crosswind: the lower the time shift the stronger the crosswind. This thesis shows that experimental calibration in the field to measure the crosswind with a scintillometer is not necessary. Also we developed two new algorithms, which are able to obtain the crosswind from the scintillometer signal. First, the algorithms were validated with measurements made above a flat grassland field. Later, measurements took place over more complex terrains (i) above the city of Helsinki, Finland and (ii) next to a runway at Schiphol airport, the Netherlands. We highlight that even in these complex terrains the scintillometer was able to obtain the crosswind correctly. At Schiphol airport also other applications of scintillometry were investigated: visibility measurements, and wake vortex detection. To use scintillometers as a visibility sensor, more research is necessary. In contrast, the scintillometer proved to be able to detect wake vortices created by airplanes during the night, when the atmospheric turbulence is low.