Showing papers on "Scintillometer published in 2005"

Structure parameters for temperature and humidity from simultaneous eddy-covariance and scintillometer measurements

Abstract: Line-averaged values of the structure parameters of temperature and humidity, CT2 and Cq2, were estimated from simultaneous measurements with an optical and a microwave scintillometer over a path of 4.7 km length at the Meteorological Observatory Lindenberg during the LITFASS-2003 experiment. By cross-correlating the detected signals of the two scintillometers, the temperature-humidity structure parameter, CTq, and the temperature-humidity correlation, rTq, were also derived directly from the measurements. Comparison with corresponding values obtained from local measurements with an eddy-covariance system on a meteorological tower show a consistent behaviour in time (with some exceptions especially for Cq2 and rTq during nighttime). The deviations are of a magnitude between 20 % and 35 % of the typical daytime values of the structure parameters.

Turbulence statistics applied to calculate expected turbulence-induced scintillation effects on electro-optical systems in different climatic regions

Abstract: The refractive-index structure parameter C n 2 is the parameter most commonly used to describe the optically active turbulence. In the past, FGAN-FOM carried out long-term experiments in moderate climate (Central Europe, Germany), arid (summer), and semiarid (winter) climate (Middle East, Israel). Since C n 2 usually changes as a function of time of day and of season its influence on electro-optical systems should be expressed in a statistical way. We composed a statistical data base of C n 2 values. The cumulative frequency of occurrence was calculated for a time interval of two hours around noon (time of strongest turbulence), at night, and around sunrise (time of weakest turbulence) for an arbitrarily selected period of one month in summer and in winter. In October 2004 we extended our long-term turbulence experiments to subarctic climate (North Europe, Norway). First results of our turbulence measurement over snow-covered terrain indicate C n 2 values which are similar or even higher than measured values in Central European winter. The statistical data base was used to calculate the expected turbulence-induced aperture-averaged scintillation index for free-space optical systems (FSO system) in different climates. The calculations were performed for commercially available FSO systems with wavelength of 785 nm and 1.55 µm respectively and with aperture diameters of the receiver of 60 mm and 150 mm for horizontal path at two heights, 2.3 m and 10 m above ground.

Scintillation: theory vs. experiment

Abstract: In May 2004 a joint atmospheric propagation experiment was conducted between the Australian Defence Science and Technology Organisation, the Office of Naval Research and the University of Central Florida. A 45 mm divergent Gaussian beam was propagated along a horizontal 1500 meter path approximately 2 meters above the ground. At the receiver were 3 apertures of diameter 1mm, 5mm, and 13mm. The scintillation was measured at each aperture and compared to scintillation theory, recently developed for all regimes of optical turbulence. Three atmospheric parameters, Cn2 , lo and Lo , were inferred from these optical measurements. Simultaneously, a commercial scintillometer, which recorded values for Cn2 , was set up parallel to the optical path. In this paper, a numerical scheme is used to infer the three atmospheric parameters and comparisons are made with the Cn2 readings from the scintillometer.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The surface energy balance over drying semi-arid terrain in West Africa

Abstract: One of the fundamental aspects of current research in earth system science is the proper understanding of land-atmosphere interactions. The role of the land surface is crucial in the climate system, since a large fraction of incoming solar radiation passes through the atmosphere and is converted at the surface into turbulent fluxes. For numerous regions, including the semi-arid regions, only little knowledge is available about the diurnal and seasonal cycle of land surface interactions. The semi-arid areas pose a big challenge due to the large contrasts of dry and wet situations within a seasonal cycle. This is especially valid for the semi-arid region in West Africa , since it is one of the most climatically sensitive and ecologically unstable regions in the world . The variability of weather and climate in the region is strongly influenced by complicated interactions and feedbacks between the land and the atmosphere. To analyze and to predict these interactions and feedbacks it is inevitable to measure and model the involved components. Since standard methods for this purpose are not always applicable to the heterogeneous surface in West Africa , new measurement and modeling techniques have to be applied. The overall objective of this thesis is to analyze and to model the land surface interactions in the Volta basin, West Africa , by using meteorological data obtained in the framework of the GLOWA-Volta project. A focus is put on diurnal and seasonal time scales. For measuring turbulent fluxes the key instrument is the large aperture scintillometer. This robust method yields area-averaged fluxes over complex terrain, which are required when analyzing meteorological data from heterogeneous surfaces. It is found that it is a suitable technique for the kind of environment also in comparison to different measuring techniques. Based on the analysis of the measurements, two different land surface schemes are evaluated. Both schemes are not able to reproduce the measured seasonal cycle in surface fluxes. Several changes are proposed to obtain enhanced model performance. Based on the earlier findings a model is constructed, combining the best parts of each of the two land surface schemes. It is shown that the performance of the new formulation is more realistic. Using a factorial design as the sensitivity analysis method it is assessed, which parameters are the most important. Furthermore it is found that those important parameters and their interactions change significantly during one season. As a final step the gained knowledge is utilized to construct a simple satellite based algorithm to obtain surface water flux as the important flux on a regional basis. For evaluating this first order approach the large aperture scintillometer is utilized to evaluate fluxes on satellite pixel scale.

Measurements of aperture averaging on bit-error-rate

Abstract: We report on measurements made at the Shuttle Landing Facility (SLF) runway at Kennedy Space Center of receiver aperture averaging effects on a propagating optical Gaussian beam wave over a propagation path of 1,000 m. A commercially available instrument with both transmit and receive apertures was used to transmit a modulated laser beam operating at 1550 nm through a transmit aperture of 2.54 cm. An identical model of the same instrument was used as a receiver with a single aperture that was varied in size up to 20 cm to measure the effect of receiver aperture averaging on Bit Error Rate. Simultaneous measurements were also made with a scintillometer instrument and local weather station instruments to characterize atmospheric conditions along the propagation path during the experiments.

Low altitude horizontal scintillation measurements of atmospheric turbulence over the sea : Experimental results

Abstract: We present the current status and developments of a horizontal beam path laser propagation experiment over the sea performed off the coast of Puerto Rico. Atmospheric turbulence effects have been measured by a Shack-Hartmann wavefront sensor with a Dalsa CCD camera and by a scintillometer from Optical Scientific, Inc* (OSI). We present preliminary scintillation measurements for an approximate period of two days from the two optical systems during the month of July 2005, also suggestions for improvement in the software, data acquisition protocol and hardware are presented.

Advances in the study of areal surface fluxes with large aperture scintillometer

Abstract: It is very important to determine land surface fluxes, including sensible heat fluxes and latent heat fluxes, for the issues involving many aspects of climate, agriculture, hydrology, and water resources management. However, it is still very difficult to observe and study areal fluxes in the scale of several to tens kilometers, especially on heterogeneous land surface. The Large Aperture Scintillometer (LAS), which is becoming popular in recent years, offers a new perspective to solve this problem. Based on light scintillation in the turbulent atmosphere,it can measure sensible heat fluxes by emitting a beam of light over a horizontal path. Then, latent heat fluxes can be derived according to surface energy balance equation. The LAS is adapted to different kinds of land surface, homogeneous or heterogeneous. Test comparisons of LAS flux measurement with the measurements by eddy correlation system show good agreement. There are some key factors, which influence the precision of areal evapotranspiration determined by LAS, such as beam height, wind speed, roughness length, zero plane displacement, Bowen ratio, and net radiation, etc. Basic principle and some algorithms are introduced. A prosperous utilization of LAS in recent years is in the validation of areal surface fluxes retrieved from remote sensing models, such as the SEBAL and SEBS. Finally, this paper discusses the development trend of applying the large aperture scintillometer in the future.

Laser propagation through turbulence over land and sea (Invited Paper)

Abstract: This paper will describe single and double path laser link measurements over land and sea. The laser system consisted of a CO2 laser, a pointing and tracking head, a quadrant laser receiver, a 3-5 μm IR-camera, a TV camera and a laser range finder. For the naval scenario the laser system was placed in a building 18 m above water and corner cube targets and a single path receiver were placed on islands at 2.5, 5.5 and 16.5 km. For the land scenario the laser system was placed in a building about 13 meters above ground looking at targets and a single path receiver at 2 km range. Together with the laser registrations, separate instruments such as a scintillometer and a weather station were recording the meteorological parameters. The analysis contains evaluation of temporal and amplitude signal distributions, probability and mean time of fade and tracking performance. Results from single ended and double ended paths will be compared. Different ways of using this database for turbulence simulation and laser system performance predictions also in other wavelength regions will be discussed as well as the impact on applications including laser imaging, free-space laser communications and directed infrared countermeasures.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Characterization of the Shuttle Landing Facility as a laser range for testing and evaluation of EO systems

Abstract: The Shuttle Landing Facility runway at the Kennedy Space Center in Cape Canaveral, Florida is almost 5 km long and 100 m wide. Its homogeneous environment makes it a unique and ideal place for testing and evaluating EO systems. An experiment, with the goal of characterizing atmospheric parameters on the runway, was conducted in June 2005. Weather data was collected and the refractive index structure parameter was measured with a commercial scintillometer. The inner scale of turbulence was inferred from wind speed measurements and surface roughness. Values of the crosswind speed obtained from the scintillometer were compared with wind measurements taken by a weather station.

Intermittency of laser scintillation over sea

Abstract: The intermittency in optical scintillation was theoretically studied and experimentally observed by laser propagation through the turbulent atmosphere over sea surface. It is found that the intermittent characteristics of the atmospheric turbulence repeat itself in optical irradiance fluctuations, then a method of the singularity measurement analysis was introduced and the intermittency index was presented to quantitatively evaluate the intermittency strength. Experiment results show that the intermittency index in laser scintillation is around 0.1 and varies a little compared with the significant variation of the turbulence strength, which had been approximately estimated at two orders of magnitude in a whole day. The dependence of the intermittency on the wavelength was further studied and the relationship was obtained using a multi-wavelength scintillometer. Generally, the intermittency indices display some differences at the different optical wavelengths, especially in strong turbulence. However, the wavelength dependence is not so notable that the intermittency indices are assumed to be equal within less than 10%, even the maximum error is no more than 20%. Moreover, some statistical results of the intermittency strength are also obtained from a long-term plan of laser propagating in coastal surroundings.

Cloud-cover mitigation of the influence of atmospheric turbulence on propagation of laser beams

Abstract: Among the atmospheric properties that adversely affect laser propagation is air turbulence. One common optical parameter of air turbulence is the refractive index structure constant that quantifies the fluctuations in the refractive index caused by temperature fluctuations and hence air density fluctuations. There is a reason to believe, from theory and from sparse data that, when propagation occurs under widespread cloudy conditions, the refractive index structure constant is significantly reduced. Therefore the intensity of a propagating laser beam will not be degraded nearly as much as would be expected under clear or lightly scattered cloud conditions. New experimental data will be presented that support this hypothesis. The refractive index structure constant was measured for various cloud-cover conditions during daytime with additional factors present, such as changing crosswinds and precipitation. It was possible to observe the apparent pattern of the decrease of the refractive index structure constant by two orders of magnitude during the periods of increasing cloud-cover evaluated by the measurement of solar irradiance. The statistical correlation coefficient between the log of solar irradiance and the log of the refractive index structure constant was found to be around 0.9 (the closer it is to the maximum of 1.0, the stronger the correlation). The measurements were conducted with a commercial scintillometer/anemometer (1 m above ground, 500-m optical path length) in Northern Alabama in late spring and summer. The effect is believed to be due to the reduction of solar radiation, which caused the temperature gradient that initiated convection in the air. The results of this work can find their application in designing free space laser communication systems and military laser systems. Distribution A. Approved for public release; distribution unlimited. 1 Sep 04

Profile and character of atmospheric structure constants of refractive index

Abstract: Random fluctuation of turbulence brings random fluctuation of refractive index and makes atmosphere become a random fluctuation medium, which destroys the coherence of light-wave. Research of atmospheric turbulence is actually investigation of atmospheric refractive index.The atmospheric structure constant of refractive index is an important parameter of denoting atmospheric turbulence. In this paper, the atmospheric structure constant of refractive index is measured both noonday and at night everyday, at all seasons, respectively,using HTP-2 micro-thermal meter. The vertical profile of refractire index in Hefei and the North(0～30 km)is investigated by analysis of experimental data.

Optical method for simultaneously measuring refractive-index structure parameter and inner scale of atmospheric turbulence

Abstract: The principle is as follows: Supposing the inner scale has a specific value,the refractive-index structure parameters areequal for different wavelengths,which is based on the optical-propagation theory using the modified Hill spectrum with the inner scaleeffect.According to the proposed inversion principle,a three-wavelength scintillometer was designed for the reliability verification in thelaser propagation trials.The results showthat the inner scale near the land surface is generally between 2 and 10 mm,and it will vary ac-cordingly as the refractive-index structure parameter changes.Moreover,some experiments also demonstrate that the method is reliable bythe inverted values of any two wavelengths,of which the minimum correlation coefficient is 0.81.

Application of large aperture scintillometer on drought monitoring

Abstract: Drought is one of the major meteorological disasters to agriculture in north China so that the development of methods for effectively monitoring droughts is of great significance to dry land crops. This paper makes analysis of products of energy and water balances retrieved from LAS (Large Aperture Scintillometer) measurements, indicating that the structural parameter of LAS refractive index shows regular difference in daily variation between different weather backgrounds and remarkable difference in sensible heat flux on a seasonal basis, with higher negative correlation between such flux and soil humidity at 0 ~ 50 cm depth.

Photonic Propagation Noise On Long Atmospheric Paths

Abstract: Refractive index fluctuations along the path of an optical beam propagating in the atmosphere give rise to irradiance fluctuations (“scintillations”) on millisecond time scales. We present new analyses of concurrent weak (photonic) and bright (photocurrent) scintillation data obtained over atmospheric paths from 4–40 km in length.

Measurements of Aperture Averaging on Bit-Error-Rate

Abstract: We report on measurements made at the Shuttle Landing Facility (SLF) runway at Kennedy Space Center of receiver aperture averaging effects on a propagating optical Gaussian beam wave over a propagation path of 1,000 in. A commercially available instrument with both transmit and receive apertures was used to transmit a modulated laser beam operating at 1550 nm through a transmit aperture of 2.54 cm. An identical model of the same instrument was used as a receiver with a single aperture that was varied in size up to 20 cm to measure the effect of receiver aperture averaging on Bit Error Rate. Simultaneous measurements were also made with a scintillometer instrument and local weather station instruments to characterize atmospheric conditions along the propagation path during the experiments.