Troy Leclerc

Deep turbulence propagation of a Gaussian-beam wave in anisotropic non-Kolmogorov turbulence

In the conventional Kolmogorov model of turbulence the turbulent fluctuations of the index of refraction are assumed to be statistically homogeneous and isotropic, and there is a specific mathematical form for the power spectral density of the index of refraction fluctuations. Development of the turbulent theory of passive scalar transfer has shown that although the conventional Kolmogorov spectrum model with a 11/3 power-law index is generally correct near the ground (within the inertial subrange), it constitutes only one part of the more general behavior of passive scalar transfer in a turbulent flow. Hence, deviations from the conventional Kolmogorov model are possible. In this study we develop theoretical models for beam spot size, spatial coherence, and scintillation index that are valid in weak irradiance fluctuation regimes as well as in deep turbulence, or strong irradiance fluctuation regimes. These theoretical models are based on power-law index variations 3

Near-ground vertical profile of refractive-index fluctuations

The most commonly used altitude-dependent model for refractive index fluctuations, over long high-altitude slant paths or ground/space links, is the Hufnagel-Valley model. For the near-ground turbulence portion of the path, this model uses an exponential decay term suggested by Valley to connect ground level turbulence with the original Hufnagel model which was constructed for turbulence above 3 km. However, it has long been observed that refractive-index fluctuations in the first few hundred meters near the ground decrease with altitude raised to the -4/3 power rather than exponentially. Recent and some earlier measurements of refractive-index fluctuations are presented in this paper along with a theoretical modification of the Hufnagel model to account for low-altitude turbulence exhibiting this power-law behavior.

Observation and analysis of aero-optic effects on the ORCA laser communication system

In this paper we show evidence of aero-optic effects on the measured beacon beam as the gimbal angle of a nosemounted turret changes from 0 to 90 degrees and greater with respect to the line of flight. Data from the beacon beam was collected with a new technology 3-aperture scintillometer over a 82km to 104km air-to-ground downlink during field testing of the ORCA system in Nevada in May 2009. In this paper we present data analysis on the impact of an aero-optic boundary layer on a laser link between an aircraft and a ground-based stationary node. Particularly we look at the impact of an aero-optic boundary layer on the mean, variance, scintillation, probability density function (PDF), power spectral density (PSD), and fading of the received irradiance. We find that the most compelling argument for the presence of strong aero-optic effects comes from calculating the PSD of the received beacon intensity. We also find the cumulative effect of the aero-optic boundary layer differs depending on the transmitted beam parameters, i.e. collimated or divergent.

Prediction of the ground-level refractive index structure parameter from the measurement of atmospheric conditions

Evaluation of the methods developed by Bendersky, Kopeika, and Blaunstein1 to predict the refractive index structure parameter from the direct measurement of macroscopic atmospheric conditions were investigated. Measurements of ground-level temperature, relative humidity, wind speed, solar flux, and aerosol loading taken by the University of Central Florida weather station were compared against concurrent measurements of the refractive index structure parameter made by Scintec SLS-20 scintillometers positioned near the weather station. Wind measurements were obtained by three, three-axis sonic anemometers (capable of resolving a three-dimensional wind vector) positioned at heights of 1, 1.5, and 2.5 meters above the ground. Temperature measurements were taken at ground level, and at heights of 1 and 1.5 meters. Data were collected for two days atop Antelope Peak, NV. Collection times covered both daytime and nighttime measurements.

Creating a C n 2 profile as a function of altitude using scintillation measurements along a slant path

Using a three-aperture scintillometer system (TASS) to measure irradiance fluctuations along a slant path, it is possible to create a Cn2 profile model as a function of altitude up to (and possibly beyond) the maximum altitude of a laser beam along the propagation slant path. This technique was demonstrated recently in June 2011 on a beacon beam transmitted between Hollister Airport in California and Fremont Peak at a slant range of 17 km. Although the primary experiment was to test a hybrid optical RF communication system (FOENEX), the beacon signal at the transmitter was intercepted by the TASS from which weighted path-average values of Cn2, inner scale l0, and outer scale L0 were determined. Path-average values were then entered into an algorithm that determines the parameters of the HAP Cn2 profile model (a variation of the HV profile model). In this paper we report on these recent measurements and how this method of constructing the HAP model can be used over other propagation paths.

Analysis of fading in the propagation channel for the ORCA laser communication system

Irradiance data were collected over an air-to-ground path using several different sized receiving apertures. The data were collected from the Optical RF Communications Adjunct (ORCA) tracking beacon. The receiver system consisted of three telescopes of sizes 51 mm, 137 mm, and 272 mm. Probability of fade, number of fades per second, and mean fade time was computed for various intensity levels for irradiance data collected on all three telescopes. These measured statistics are compared to fading models derived from lognormal and gamma-gamma probability density function (PDF) models. Discussion is centered on the viability of these models under various conditions and on the presence of aero-optic effects. The gamma-gamma and lognormal model are found to be insufficient to model all fading statistics.

Atmospheric channel characterization for ORCA testing at NTTR

The DARPA Optical RF Communications Adjunct (ORCA) program was created to bring high data rate networking to the warfighter via airborne platforms. Recent testing of the ORCA system was conducted by the Northrop Grumman Corporation (NGC) at the Nevada Test and Training Range (NTTR) at the Nellis Air Force Range near Tonopah, NV. The University of Central Florida (UCF) conducted a parallel test to measure path-averaged values of the refractiveindex structure parameter, the inner scale of turbulence, and the outer scale of turbulence along the ORCA propagation path from an airborne platform to the ground at Antelope Peak. In addition, weather instrumentation was set up at ground level on Antelope Peak to measure local conditions on the mountain top. This paper presents background information on expected atmospheric conditions for the channel, models that were used by UCF for the measurements, path-averaged values of the three atmospheric parameters, and a Cn2 profile model as a function of altitude.

Performance of a 10 Gbps FSO system implementing novel beam tracking and a dynamic buffering modem

A 10 Gbps FSO system implements beam tracking, a high dynamic range optical receiver, and a dynamic buffering packet modem. Performance was characterized at the 4.5 km Shuttle Landing Facility at Kennedy Space Center Florida.

Buffer requirements of an optical communication system in atmospheric turbulence

Expressions related to the buffer requirements of an optical communication system in atmospheric turbulence are developed from the channel signal fade time statistics. Laser irradiance data were recorded over the course of one day by a receiving aperture of variable diameter at the Townes Institute Science and Technology Experimentation Facility (TISTEF) 1km laser range located within the Kennedy Space Center at Cape Canaveral, FL. Fade statistics of collected data and scintillometer measurements were compared to the derived model gamma-gamma fade model. Parallel to the laser instrumentation was a commercial scintillometer unit which reported the refractive index structure coefficient, Cn2 and the inner-scale of atmospheric turbulence, l0. The atmospheric parameters inferred from the collected laser data and the commercial instruments were compared. Mean and variance of the fade times were found to agree well with theory for smaller apertures where effects of aperture averaging are not present and in cases where scintillation is weak to moderate. It is suggested that a more appropriate PDF, with a heavier focus on aperture averaging, may be applied in future studies of free space optical communication system fade statistics.