Charmaine Gilbreath

Humidity contribution to the refractive index structure function C2n.

Humidity and C2n data collected from the Chesapeake Bay area during the 2003/2004 period have been analyzed. We demonstrate that there is an unequivocal correlation between the data during the same time periods, in the absence of solar insolation. This correlation manifests itself as an inverse relationship. We suggest that C2n in the infrared region is also function of humidity, in addition to temperature and pressure.

Applying the Hilbert-Huang decomposition to horizontal light propagation C n 2 data

The Hilbert Huang Transform is a new technique for the analysis of non-stationary signals. It comprises two distinct parts: Empirical Mode Decomposition (EMD) and the Hilbert Transform of each of the modes found from the first step to produce a Hilbert Spectrum. The EMD is an adaptive decomposition of the data, which results in the extraction of Intrinsic Mode Functions (IMFs). We discuss the application of the EMD to the calibration of two optical scintillometers that have been used to measure Cn2 over horizontal paths on a building rooftop, and discuss the advantage of using the Marginal Hilbert Spectrum over the traditional Fourier Power Spectrum.

A new marine atmospheric spectrum for laser propagation

Current mathematical models describing laser propagation through the atmosphere were developed for terrestrial environments. An atmospheric index of refraction power spectrum specifically tailored to the marine environment has been created and applied to scintillation theory. Optical measurements of a diverge laser beam propagating in a marine environment, in combination with scintillation theory and a numerical scheme, were used to infer the refractive index structure parameter, Cn2, along the propagation paths. The analysis was repeated for both marine and terrestrial theoretical scintillation expressions, each resulting in one set of inferred Cn2-values. In the moderate-to-strong fluctuation regime, the inferred Cn2-values based on marine theory were about 20% smaller than those based on terrestrial theory, but a minimal difference was observed in the weak fluctuation regime.

Initial Measurements of Atmospheric Parameters in a Marine Environment

April 2005, a laser propagation experiment was conducted over a 470m horizontal maritime path. Scintillation measurements of a divergent Gaussian beam wave were taken simultaneously for different receiver aperture sizes. Terrestrial scintillation theory combined with a numerical algorithm was used to infer the atmospheric parameters Cn2 and lo from the optical maritime scintillation measurements. This paper presents the initial results.

Tripod Operators for Efficient Search of Point Cloud Data for Known Surface Shapes

We address the problem of searching large amounts of 3D point set data for specific objects of interest, as characterized by their surface shape. Motivating applications include the detection of ambush weapons from a convoy and the search for objects of interest on the ground from an aircraft. Such data can occur in the form of relatively unstructured point sets or range images, and can be derived from a variety of sensors. We study here the performance of Tripod Operators (TOs) on synthetic range image data containing the shape of an oil drum; a cylinder with planar top. Tripod Operators are an efficient method of extracting coordinate invariant shape information from surface shape representations using discrete samples extracted in a specially constrained manner. They can be used in a variety of ways as components of a system which performs detection, recognition and localization of objects based on their surface shape. We present experimental results which characterize the approximate accuracy of detection of the test shape as a function of the accuracy of the surface shape data. This is motivated by the need for an estimate of the required accuracy of 3D surveillance data to enable detection of specific shapes.

Mitigation of scintillation noise in a 32-km maritime path

We present a technique to correct for atmospheric scintillation noise in free space optical communications. Co-channel noise is removed by common mode rejection, where each channel is transmitted on separate, but closely spaced, wavelengths.

On the relationship between C n 2 and humidity

We have recently shown the refractive index structure constant C2n in the visible and near infrared to be a strong function of humidity in the absence of solar insolation effects, in stark contrast to the commonly held assumption that the humidity contribution can be ignored in that waveband. We expand our analysis of the effects of humidity on C2n as measured across a 100-m long horizontal beam path to include temperature. Also we present a new technique for extracting information on changes in the parameter space of C2n and local weather variables, which we term Hilbert Phase Analysis (HPA). This methodology, based on extracting the phase of the analytic signal via Hilbert transforms, reveals a wealth of detail that conventional analysis techniques cannot determine. The HPA provides additional confirmation that C2n is strongly influenced by local humidity in the visible region. We have also found that HPA provides a clear demonstration that humidity competes with temperature in affecting the value of C2n.

Early type stars as calibrators for ground-based interferometry

Visibility measurements with Michelson interferometers, particularly the measurement of fringe contrast, are affected by various atmospheric and instrumental effects, all of which reduce the measured contrast. To compensate for this, stars with known or predictable diameters (calibrators) are observed so that the overall reduction in the visibility can be measured. Objects with the smallest possible diameters are preferred as calibrators, since the predicted visibilities become less sensitive to any uncertainties. Therefore, unreddened, early type stars are usually chosen if they are available because they are relatively bright for a given angular diameter. However, early type stars bring additional complications. Rapid rotation, common with these stars can cause variations in the visibility amplitudes due to oblateness and surface brightness asymmetries that are larger than implied by the usual error estimates. In addition, rotation can introduce significant phase offsets. Using Roche models, von Zeipel theory, and the observed constraints of V, B-V, and v sin i, it is possible to put limits on the size of these effects and even estimate the distribution of possible visibilities. To make this easily available to the community, we are in the process of creating a catalog of possible calibrators, including histograms of the visibilities, calculated for configurations used at a number of observatories. We show the examples of several early type stars which are potential calibrators using parameters appropriate for the Navy Prototype Optical Interferometer.

Study on the implementation of spatial light modulator liquid crystal device atmospheric simulator for short wavelength infrared applications

The use of Spatial Light Modulators (SLM), Liquid Crystal Devices for atmospheric turbulence simulation in optical system has increased in the recent years. These devices allow the development of test-beds that can be used to simulate, analyze and improve optical components or systems in a controlled laboratory environment before further implementation on the eld. Most research has been performed at visible wavelengths with the use of a vast array of atmospheric turbulence simulation algorithms. We present preliminary work on an atmospheric simulation test bed which uses an algorithm developed at NRL with a transmissive high denition Liquid Crystal Device SLMs for applications in the short-wavelength infrared, with the main focus of interested at 1550nm. Preliminary results are shown for the application to a high denition re ective Liquid Crystal Device SLM for the same wavelength.

Applying the Hilbert Phase Analysis to the Study of Atmospheric Turbulence Data

Hilbert Phase Analysis is a new technique that combined with the Hilbert-Huang Decomposition method allows analysis of a non-stationary data. We apply this method for the study of the Cn2 parameter and weather data.