Atmospheric Turbulence Strength Research Articles

Atmospheric optical communication with a Gaussian Schell beam.

We consider a wireless optical communication link in which the laser source is a Gaussian Schell beam. The effects of atmospheric turbulence strength and degree of source spatial coherence on aperture averaging and average bit error rate are examined. To accomplish this, we have derived analytic expressions for the spatial covariance of irradiance fluctuations and log-intensity variance for a Gaussian beam of any degree of coherence in the weak fluctuation regime. When spatial coherence of the transmitted source beam is reduced, intensity fluctuations (scintillations) decrease, leading to a significant reduction in the bit error rate of the optical communication link. We have also identified an enhanced aperture-averaging effect that occurs in tightly focused coherent Gaussian beams and in collimated and slightly divergent partially coherent beams. The expressions derived provide a useful design tool for selecting the optimal transmitter beam size, receiver aperture size, beam spatial coherence, transmitter focusing, etc., for the anticipated atmospheric channel conditions.

Generalized scintillation detection and ranging results obtained by use of a modified inversion technique.

The measurement of the strength of atmospheric optical turbulence by use of a modified generalized SCIDAR (scintillation detection and ranging) inversion technique is outlined and demonstrated. This new method for normalizing and inverting scintillation covariances incorporates the geometry specific to generalized SCIDAR. Examples of profiles from two astronomical observation sites are presented.

Profiling of atmospheric turbulence strength and velocity using a generalised SCIDAR technique

The measurement of the strength and veloc- ity of atmospheric optical turbulence using a generalised SCIDAR technique is outlined and demonstrated. This method allows the full turbulent prole to be characterised from the telescope pupil up to any desired altitude. A number of example proles from various astronomical ob- serving sites are presented.

Attempt to assess astronomical ?seeing? using a model

The degradation of astronomic images by optical inhomogeneities in the earth's atmosphere is generally called “seeing”. It represents the angular diameter of the stellar images as seen through a turbulent medium. Several techniques can be used to determine this parameter. The knowledge of the optical strength of atmospheric turbulence, namely, the integrated structure coefficient of the atmospheric refractive index Cn2 allows to predict the atmospheric optical quality in terms of seeing. We tried in this study to assess an astronomical seeing using a model forecast using meteorological data collected in three stations in Latin America from 1958 to 1991. The efficiency of the model is tested by comparison with simultaneous seeing measurements, at Chiliean astronomical sites.

On the relationship between the strength of atmospheric radar backscatter and the intensity of atmospheric turbulence

A commonly accepted expression relating the strength of radar backscatter and the strength of atmospheric turbulence is re-investigated. It is found that the previously accepted relation between the two quantities should have an extra dependence on the Richardson number included, and the implications for radar and in-situ studies of the atmosphere are discussed.

Application of similarity theory for calculating the acoustic refractive index structure parameter.

The acoustic refractive index structure parameter is important for determining the strength of atmospheric turbulence. The current methods for calculating the acoustic refractive index structure parameter is to calculate the temperature and wind structure parameter from tedious hot-wire anemometer measurements. Also most methods only provide a measurement of the turbulence strength at one height. An alternative method to calculate the refractive index structure parameter is to calculate the temperature and wind structure parameters using similarity theory. The similarity model requires easily obtained meteorological data at one height along with an estimate of the roughness length. The model uses similarity theory to calculate the temperature and wind structure parameters with height. This provides the capability to calculate the refractive index structure parameter with height. A comparison with turbulence data [Johnson etal., J. Acoust. Soc. Am. 81, 638–646 (1987)] will show how well the model works. This type of model predicts the turbulence strength with height through the surface layer for different stability states of the atmosphere.

Measurements of atmospheric turbulence strength by coherent optical scintillation

The strength of the atmospheric turbulence is measured by an optical scintillation method. The obtained quantity is the structure constant at optical frequencies. As all turbulence parameters, this quantity is strongly affected by meteorological and local conditions, therefore it is of interest to measure it in different conditions and locations. The present measurements were made to characterize the turbulence at Florence.

The effects of atmospheric turbulence on telescopic observations

Telescopic observations to terrestrial and extraterrestrial objects are affected by atmospheric turbulence. The structure of atmospheric turbulence is reviewed in tems of the refractive index structure parameter. The mean and fluctuating refraction effects of the turbulent atmosphere on the propagation of electromagnetic waves are addressed. The known formulae for the variance and the spectrum of the angle-of-arrival fluctuations are presented and experimental results are summarised. The angular resolution is compared with the pointing precision of a telescope for visual observations through a turbulent atmosphere. The ultimate precision of direction measurements is shown to be a function of instrumental design parameters, the strength of atmospheric turbulence, and the length of the averaging period.

Remote probing of atmosphere and wind velocity by millimeter waves

A technique is developed to probe the atmospheric turbulence strength C_{n}^{2} and the wind velocity along a path using millimeter waves as a tool. Data obtained in a line-of-sight millimeter-wave propagation experiment are processed and used as the source of information. The averaged C_{n}^{2} and wind velocity together with their gradients along the propagation path are calculated by inverting a set of integral equations. A numerical method is used to yield the least-square-error solutions. Comparison is made between the theoretically calculated wind velocity over a 33-hour period and that measured by a conventional anemometer.

Remote probing of the optical strength of atmospheric turbulence and of wind velocity

A procedure for determining the optical strength of turbulence of the atmosphere and the wind velocity at various altitudes by measuring the spatial and temporal covariance of scintillation is developed. Emphasis is placed on the development of the formal relationships that have to be inverted to obtain the desired results. For determination of optical strength of turbulence, it is a linear integral equation that is developed. However, for determination of remote wind velocity, a nonlinear integral equation is obtained. A computer approach for solving each of the equations is suggested. The configuration and performance requirements of the measurement apparatus are discussed.

Limiting Resolution Looking Down Through the Atmosphere

The turbulence of the atmosphere places an upper limit on the quality of an image of ground objects obtained by long-exposure photography from high altitudes in the atmosphere or in space. (By making the imaging optics good enough, the film resolution fine enough, and the platform stable enough, this limit could be approached but not exceeded.) A useful quantity for indicating the magnitude of this limit is the integral of the MTF associated with the turbulence. Treating the integral as a two-dimensional bandwidth, one-half the inverse of its square root can be associated with a resolution length, or angle, in the same manner that an electrical engineer associates a rise time with one-half the inverse of the bandwidth of an R–C filter. Based on published data for typical strength of atmospheric turbulence, the integral of the MTF was calculated as a function of altitude and the corresponding resolution computed. This resolution is shown to correspond to a length of about 4.6 cm on the ground. It is shown that as an observer goes deeper into space, this limiting ground resolution remains constant, but the diameter of the optics needed to approach the limit goes up. Graphs of achievable ground resolution at any altitude and of the diameter of the optics needed to approach this limit are presented.