Abstract
Optical turbulence, as determined by the widely accepted practice of profiling the temperature structure constant, CT2, via the measurement of ambient atmospheric temperature gradients, can be found to differ quite significantly when characterizing such gradients via thermal-couple differential temperature sensors as compared to doing so with acoustic probes such as those commonly used in sonic anemometry. Similar inconsistencies are observed when comparing optical turbulence strength derived via CT2 as compared to those through direct optical or imaging measurements of small fluctuations of the index of refraction of air (i.e., scintillation). These irregularities are especially apparent in stable atmospheric layers and during diurnal quiescent periods. Our research demonstrates that when care is taken to properly remove large-scale index of refraction gradients, the sonic anemometer-derived velocity structure constant, Cv2, coupled with the similarly derived turbulence-driven index of refraction and vertical wind shear gradients, provides a refractive index structure constant, Cn2, that can more closely match the optical turbulence strengths inferred by more direct means such as scintillometers or differential image motion techniques. The research also illustrates the utility and robustness of quantifying Cn2 from CT2 at a point using a single sonic anemometer and establishes a clear set of equations to calculate volumetric Cn2 data using instrumentation that measures wind velocities with more spatial/temporal fidelity than temperature.
Highlights
The index of the refraction structure constant, Cn2, is commonly used to quantify optical turbulence strength, which, in turn, can degrade both active and passive system performance, whether that of a laser central to a free space optical communication uplink, an inter-continental relay mirror high energy laser power-beaming architecture to transport energy to remote sites, or an upward-looking telescope pointed at a distant space object
This paper extends the relationships shown in [5] with evidence demonstrating optical turbulence strength, Cn2, can be derived from a parameter, Cv2, which is more directly tied to the turbulent eddy distribution independent of non-adiabatic temperature gradients that many techniques tend to exploit to their advantage, but that can disappear when turbulence does not
We revisit in detail an alternative approach to extracting Cn2, which capitalizes on another structure constant rarely harvested from sonic anemometers, the velocity structure constant, Cv2
Summary
The index of the refraction structure constant, Cn2 , is commonly used to quantify optical turbulence strength, which, in turn, can degrade both active and passive system performance, whether that of a laser central to a free space optical communication uplink, an inter-continental relay mirror high energy laser power-beaming architecture to transport energy to remote sites, or an upward-looking telescope pointed at a distant space object. Such localized point measurements are especially beneficial to engineers and scientists who develop and apply compensation and correction techniques and associated modalities, as well as those individuals endeavoring to develop, verify, and validate optical turbulence and effects models through carefully instrumented field tests and post-test forensics analysis These high-fidelity flux measurements are of interest to mission planners and system operators seeking to update/nudge an otherwise long range, days-to-weeks advance forecast of system performance to support up-to-the-minute mission briefs [4] and current-mission system settings. The latter advance forecasts are built on a combination of increasingly sophisticated laser propagation and numerical weather prediction models
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