Showing papers on "Scintillometer published in 1989"

Modeling of structure constant and inner scale of refractive index fluctuations - an experimental investigation

Abstract: Structure constants and inner scales of temperature fluctuations were derived from ultrasonic anemometer-thermometer measurements taken at heights of 48m and 80m above the ground. They were shown to follow local Monin-Oboukhov similarity from very unstable to very stable atmospheric stratification. A direct empirical expression for the stability dependence of the nondimensional inner scale is given.A bichromatic scintillometer based on the wavelengths 0.63/im and 10.6/im was operated near the ground. Comparison between optically measured path averaged inner scales to those derived from point measurement of vertical velocity fluctuations yielded excellent agreement. The observed dispersion of refractivity fluctuations was used to separate structure constants of temperature and humidity. Surface fluxes obtained from parameterizations requiring only simple meteorological input data were applied to scale optically measured structure constants of temperature and inner scale. The parameterized fluxes were shown to be sufficiently accurate to be used in Monin-Oboukhov similarity based models.1. INTRODUCTIONFor most optical applications in the atmosphere, turbulent refractive index fluctuations can sufficiently be described by the structure constant Cn2 and the inner scale t^. Cn2 is the spectral amplitude of the refractive index fluctuations in the inertial subrange of turbulence and is the most important parameter for optical propagation. in determines the upper cut-off frequency of that spectrum and is thus of considerable importance in applications being sensitive to fluctuations at high spatial wavenumbers, such as estimation of electrooptical systems performance degradation by scintillation interference, beam profile correction and correction of turbulence degraded images. Due to the rapid decrease of Cn2 with height the largest values of Cn occur near the ground. tn grows with decreasing dissipation rate of turbulent kinetic energy. So the smallest values of ^ are found near the surface where turbulent friction is pronounced. Both effects make the surface layer most disturbing to clear air optical propagation.Refractive index fluctuations are mainly caused by turbulent fluctuations of temperature and humidity with the corresponding structure constants CT2 and Cq2 and inner scales l^ and tq. Therefore modeling of Cn2 and ln has to be based on these quantities. Their adequate measurement is one of the basic tasks in experimental work in this field. Path averaging optical methods have a lot of advantages compared with conventional point measurements. A promising optical technique is the bichromatic scintillometer.1 Recent experiments have demonstrated the performance of this technique for tn measurement.2 If the wavelengths are properly chosen it should also be capable of separating Cn into its G/ and Cq2 components. A more detailed investigation on the use of this technique in experiments on optical turbulence modeling was one of the goals of the experimental study presented here.t£. Azoulay is on a sabbatical leave from Soreq Nuclear Research Center, Yavne 70600, Israel

Extended Communication Path Length Scintillation Measurements And Model: A Discussion Of Results

Abstract: Successful design of an atmospheric laser communications terminal requires an understanding of atmospheric turbulence induced scintillation of the optical signal. Laser beam scintillation is small scale interference within the beam cross section due to turbulence induced fluctuations of the refractive index of the atmosphere, causing variations in the spatial power density at the receiver. The variations in the spatial power density at the receiver manifest themselves as fades and surges of the detected optical signal. By understanding the statistics and power spectrum of the fades and surges, communication terminals can be designed to achieve needed levels of performance by employing optimized choices of increased link margin and error coding. As part of the HAVE LACE (Laser Airborne Communications Experiment) program, amplitude scintillation data was collected and analyzed for extended propagation path lengths. The analysis included the determination of the statistics and temporal power spectrum of the scintillation and the effect on communications performance. Since the HAVE LACE terminals used direct detection of pulsed laser energy, the random variations in the received signal strength was used to evaluate only the atmospheric turbulence induced amplitude scintillations. The collected data has been reduced and compared with a model for extended path length channels. The objective of this comparison was to verify the performance of the model against the collected data. The results from the comparison show a reasonable degree of correlation between the data and the model which warrants further investigation of this approach. This analysis is presented in a form which is consistent with an understanding of the implications of the effect of the communications channel on system performance.

Extending the range of validity of optical scintillometer measurements

Abstract: Measurements of the level of turbulence Cn2 have been successfully performed with the optical scintillometer. The success of this instrument is based on the observed fact that the variance of aperture-averaged scintillation is described by weak scattering theory even for conditions under which strong scintillation is observed for small apertures. However, for sufficiently long propagation paths the aperture-averaged variance is affected by strong scattering. The effects of strong scattering are calculated theoretically and compared to experiment. The physics of this regime are discussed and the important parameters investigated in order to extend the range of validity of optical scintillometer measurements.

The Intermittence Of Atmospheric Turbulence And Its Effects On Wave Propagation

Abstract: A kind of intermittent turbulent flows which was observed using a hot wire anemometer and a platinum wire thermometer on tower in Beijing at night has been introduced . This turbulence flow produced by wind shear in the stable stratified flow is consist of some individual or a crowdof cold eddies .The temperature spectral power law of this turbulence lies between -2 and -5/3 so that the C m value measured by various methods which correlate with scale size of eddies would be different.The mathematical representation of intermittent turbulent flows in the atmosphere was developed. The abrupt variance of temperature at the surface of cold eddies may cause a reflection of radio wave or sound wave , therefore the value of C m measured by various methods such as radar,scintillometer,thermometer, would be different.

Optical Scintillometer/Doppler Radar Instrument For Profiling Turbulence

Abstract: The scintillation of starlight contains information about the refractive turbulence strength in the atmosphere. The fluctuations to each two-dimensional spatial wavenumber in the scintillation pattern are caused by turbulent features that have the same two-dimensional wavenumber. Therefore, a receiver that spatially filters the scintillations in starlight can measure the amount of turbulence in the atmosphere in a narrow band of wavenumbers. If the entire atmosphere were moving with constant velocity, the dominant wavenumber would produce a constant frequency as the turbulence moved across the filter. However, wind velocity typically varies with altitude, and turbulence at different altitudes will produce different frequencies. If the wind velocity profile is known, the vertical profile of Cn2 can be inferred from the frequency distribution of scintillations at a particular wavenumber.