Showing papers on "Scintillometer published in 2004"

Passive optical monitor for atmospheric turbulence and windspeed

Abstract: Measurement of the atmospheric index of refraction structure constant (Cn^2) is critical for predicting the performance of a free-space optical laser communication (FSO lasercomm) link. A Cn^2 monitor based on angle-of-arrival (AOA) fluctuations has been built for characterization of atmospheric conditions at the NRL FSO Lasercomm Test Facility across the Chesapeake Bay. The monitor used existing lights in various locations as point sources for determining AOA fluctuations. Real time analysis of the AOA fluctuations was performed to determine the power spectrum of the fluctuations every few seconds. This additional power spectrum information allows much greater understanding of atmospheric conditions including estimation of average wind speed based on frequency shifts in the power spectrum distribution. The performance of the monitor was tested over short paths by comparison to a commercial scintillometer. In addition, the monitor was used at other sites to determine atmospheric conditions at a variety of locations. Results of these experiments are presented.

Influence of atmospheric turbulence on FSO link performance

Abstract: Free-space optical transmission can provide high-speed links for a variety of applications. However, inhomogeneities in the temperature and pressure of the atmosphere lead to variations of the refractive index along the transmission path. This atmospheric turbulence can deteriorate the quality of the image formed at the receiver, and can cause fluctuations in both the intensity and the phase of the received signal. Laser beams experience three effects under turbulence: beam wander, beam spreading and scintillation. Of the three turbulence effects, free space optical systems might be most affected by scintillation. Random interference with the wave front can cause peaks and dips, resulting in receiver saturation or signal loss. Based on Kolmogorov’s theory, the Signal-to-Noise Ratio (SNR) and Bit Error Rate (BER) from scintillation were drawn in this paper. For a FSO system, after choosing a certain beam and communication range, link performance characterized by its SNR or BER is related to the refractive index structure coefficient C n 2 and turbulence depth L t . Considering all the three effects, we find for a weak turbulence model, a FSO system should be applied within a communication range of 2.5km and above the ground at least 5m.

Estimating optical turbulence effects on free-space laser communication: modeling and measurements at ARL's A_LOT facility

Abstract: We evaluate a similarity-based optical turbulence model that estimates diurnal values for Cn2 from easily obtained local terrain and environmental information by comparing it with scintillometer data taken at the Army Research Laboratory’s A_LOT facility in Adelphi, Maryland. The A_LOT facility is characteristic of many planned urban sites for free-space laser communication. One end of the test site is on top of a two-story building, and the other end is a water tower about 70 meters high. This comparison examines the effects of the asymmetric location, such as the non-uniform height above ground and surface roughness length. We found that by emphasizing the terrain type directly in front of the receiver and assuming the height above ground to be the height of the receiver, model results compare favorably with experimental data.

Scintillation index of a multiwavelength beam in turbulent atmosphere

Abstract: We characterize the scintillation index of a multiwavelength plane-wave optical beam that is subjected to a turbulent optical channel. It is assumed that the level of turbulence in the atmosphere ensures a weak-turbulence scenario and that the turbulence is due to the fluctuations in the index of refraction of the medium. It is assumed that the propagation path is nearly horizontal and that the heights of the transmitter and receiver justify a near-ground propagation assumption.

A scheme for pixel-scale aerodynamic surface temperature over hilly land

Abstract: Hilly-land satellite pixel-scale aerodynamic surface temperatures (AdST) are investigated using LAS (Large Aperture Scintillometer) and meteorological observations during 21–22 May 2001, indicating that the calculated temperatures are predominantly subject to estimated roughness lengths and, to a less extent, to estimated Bowen ratios, with errors to within 3.0 K between the AdST calculations and hilly radiometric surface temperatures retrieved from satellite data with the split window model. The errors depend heavily on the model used and the zenith angles and azimuth of the satellite and sun with respect to the observational site.

Analysis of the Factors Impacting Evapotranspiration Estimation Using Remote Sensing Data

Abstract: There are two major methods of estimating regional evapotranspiration using remote sensing data. One, based on the surface balance equation, is to estimate net radiance, soil heat flux and sensible flux using remote sensing, and then to get evapotranspiration using residual method; the other is to estimate directly evapotranspiration through combining Penman-Monteith equation with the surface energy balance equation. In this paper, limiting factors are analyzed, which play an important role in the accurate of estimating evapotranspiration using remote sensing data. Investigation on the factors which impact the regional evapotranspiration estimated by using remote sensing data would help the establishment of high accurate evapotranspiration model. Based on the analysis of the two major methods in this area, this paper points out that these factors include mainly: (1) the selection of remote sensing data; (2) the inversion accuracy of land surface parameters such as Albedo, emissivity and surface temperature; (3) the models of aerodynamic resistance and surface resistance; (4) the validation method about the estimated results; and (5) the extension of time scale. As illustrated in this paper, the remote sensing data, which are used in estimating regional evapotranspiration, must have not only the channels of visible lights, near infrared, thermal infrared but also the characteristic of multi-angle and multi-wave-band. The data of MODIS and ASTER are qualified data source for this application. The introduction of BRDF(Bidirectional Reflectance Distribution Function) and multi-angle and multi-wave-band into the inversion algorithm of Albedo helps to improve the accuracy and the usage of laser to measure territorial emissivity on airborne and spaceborne would also increase the inversion accuracy of surface temperature. However, with the difference between aerodynamic temperature and surface temperature, the aerodynamic roughtness and the narrow-wave-band of remote sensing, the evapotranspiration model using remote sensing is not yet perfect and it is not solved perfectly on the extension of time scale either due to the turbulence and the uncertainty of the clouds. It is also illustrated in this paper that the development of scintillometer measurement gives an opportunity to validate the estimated results.With these analysis and the illustrations, this paper concludes that the solving of the key techniques related to these factors would expedite the application and the development of the regional evapotranspiration estimation using remote sensing data.

An integrated msg-scintillometer network system to monitor sensible and latent heat fluxes.

Abstract: The rationale of the RAO-MSG project, entitled "Development of an operational algorithm to derive the surface energy balance components from MSG, HIRLAM and a network of ground-based scintillometers" is presented. An outline is given how to come to an integrated MSG-scintillometer system to determine surface fluxes. Recent results of scintillometry research are presented also, revealing that scintillometry can provide accurate flux observations on MSG pixel scale. Various field experiments set up for our RAO-MSG project are described. Examples of two approaches we intend to develop in future are presented as well.

Study on the optical scintillation for Gaussian-beam propagation in the slant-path

Abstract: In the paper, according to the ITU-R atmosphere model (ITU-R Document 3J/31-E, 2001) presented in 2001 by ITU-R, we study the Gaussian-beam wave scintillation index with altitude varying from weak fluctuation to strong fluctuation by applying a modification of the Rytov method. At the same time, we include the effect of a finite outer-scale in addition to the inner-scale. With a finite outer-scale, the scintillation index can be substantially lower in strong turbulence than that predicted by a model with an infinite outer-scale.

Description and measurement of the strength of an optical turbulence

Abstract: In the theory of light wave propagation in turbulence the refractive index structure constant is taken as a measure of the strength of the turbulence. A lot of efforts have been made to measure this parameter at different regions and seasons. However, this parameter doesn't indicate the fluctuation strength directly and for a non-Kolmogorov turbulence its unit is not fixed. Such a unit-varying parameter shouldn't be a suitable measure of a physical quantity. It is found that the strength of an optical turbulence is the variance of the refractive index. In the inertial subrange the structure function of refractive index generally depends on both the variance and the turbulence outer scale. The light propagation effects could be interpreted more clearly through these turbulence parameters. Instead of the structure constant the variance, the outer scale, the scaling power and the inner scale should be measured in the study of light propagation.

Analysis of the factors influencing surface sensible heat fluxes with large aperture scintillometers

Abstract: Large aperture scintillometers (LAS), Bowen ratio, eddy covariance measurements, soil moisture in various depths with measuring masses and routine weather observation such as visibility, cloud, wind speed and weather phenomenon with manual work were employed to study sensible heat flux over homogeneous bare soil surface from March 20th to April 20th, 2002, at XiaoTangshan area, Beijing. The diurnal variation of sensible heat flux from LAS is analyzed in this paper, and the relation between sensible heat flux and weather conditions is discussed. Moreover, the correlation coefficient between LAS based surface sensible heat fluxes and the influencing factors such as soil surface temperature and wind speed were analyzed. The analysis will help us to improve the accurate of LAS measurements. Further, the comparisons of the scintillometer flux measurements with the measurements of Bowen ratio and eddy correlation methods were done in order to make more research on scaling-up of surface sensible heat flux from point to area

Scintillometer fluxes of sensible and latent heat over a heterogeneous area - a contribution to LITFASS-2003

Abstract: Surface fluxes of sensible heat (H) and evaporation (LvE) are important in many atmospheric processes and can be measured with reasonable accuracy over homogeneous areas. However, surface fluxes that are representative for large natural landscapes (comparable to the grid box size of numerical models or satellite remote sensing pixels) are more difficult to determine because at these scales the earth’s surface is always heterogeneous. The scintillation technique is one of the few techniques that can provide fluxes at scales of several kilometers (10km). For example the combination of a large aperture optical scintillometer (LAS) and a microwave scintillometer (MWS), also known as the twowavelength method (Andreas, 1989), can provide the fluxes of both H and LvE at kilometer scales. So far there have been only a few experiments with (optical) scintillometers operated over heterogeneous terrain (see e.g. Beyrich et al., 2002; Meijninger et al., 2002a) and only one experiment that systematically investigated the two-wavelength method using mircowave scintillometers (Meijninger et al., 2002b). In this study we will present the results of scintillometer measurements carried out during the LITFASS-2003 experiment (Lindenberg, Germany) using three large aperture scintillometers (LAS) and one microwave scintillometer (MWS). The main goal of this study is to investigate the performance of the

Using Large Aperture Scintillometer to validate pixel heat flux based on remote sensing models

Abstract: To validate the accuracy of remote sensing flux model and to test the sensitivities of parameters, one Large Aperture Scintillometer (LAS), one Eddy Covariance (EC) system and other supporting observations were used to estimate sensible heat flux of a typical natural surface. The remote sensing model is Surface Temperature - Resistance (STR) model. The results obtained from different spatial scales were inter-compared. It shows that (1) there are better changing trend of sensible heat fluxes obtained by LAS and EC and STR methods; (2) the remote sensing assessed sensible heat flux was a good agreement with measurements by using LAS and EC, HSTR is rough 8% larger than HLAS; (3) LAS is reliable to validate the pixel sensible heat flux derived by remote sensing model

Study of scintillation for a Gaussian beam propagating in atmospheric turbulence on Earth-space paths

Abstract: On Earth-space paths, by applying a modification of the Rytov method that incorporates a spatial frequency filter function under strong fluctuation conditions, a model for the scintillation index of a Gaussian laser beam wave that is valid under moderate-to-strong irradiance fluctuations is developed. The scintillation indexes for a collimation Gaussian beam are predicted by using this analytic model, based on the ITU-R C/sub n//sup 2/ model. The results agree with theoretic expected scintillation. This scintillation model can be converted into a plane or a sphere wave scintillation index model on Earth-space paths, and also reduced to a Gaussian beam wave model on horizontal sight paths of invariable C/sub n//sup 2/.