An airborne optical system requiring a large field-of-regard will often use a hemisphere or similarly-shaped "turret" to transmit or receive radiation. The aerodynamic flow, however, creates disturbances about the turret resulting in the formation of turbulent boundary and shear layers, and flow separation. The disturbed flow is characterized by optical phase distortions that vary rapidly in time and degrade system performance.
With the advancement of Computational Fluid Dynamics (CFD) and computing power, the complex flow field around the turret can be accurately modeled, both spatially and temporally. The density field is extracted from the flow field solution and interpolated within the volume defining the transmitted or received beam. By applying the Gladstone-Dale constant to obtain index of refraction, and integrating along the desired line-of-sight, time-dependent OPD (optical path difference) maps are obtained. These are used as complex field phase modifiers in the wave optics system performance analysis.
This methodology was applied to a hemisphere-cylinder turret protruding from a flat plate in Mach 0.3 flow. Time-accurate flow solutions capturing the flow separation and wake oscillations were obtained. Performance of both conformal and flat transmitting windows was assessed as a function of beam angle.
Results are presented for diurnal and seasonal variations in availability for multiple optical ground sites within the Hawaiian Islands. The probability of at least one site available, given one, two and three sites to select from, is modeled. Availability is based on the requirement to simultaneously satisfy constraints on probability of cloud-free line-of-sight and operational thresholds for relative humidity and wind speed. These affect optical coating performance and life, and pointing accuracy, respectively. Modeling techniques are also described for maintaining consistency in the accuracy and fidelity of the atmospheric models in the absence of long-term or site-specific measurements. The results show that availability varies dramatically with time-of-day, and is significantly improved when the number of sites considered increases from only one to two. The analysis also shows that the net effect of wind speed and humidity as a function of time-of-day is not intuitive. The results are applicable to space object imaging, and ground-to-space laser communications and energy projection.
As a corollary to the USAF strategic Airborne Laser program, Boeing has been analyzing the use of the Chemical Oxygen Iodine Laser for tactical scenarios. These include its use in an airborne platform operating against low flying cruise missiles and miscellaneous ground targets, as well as on a mobile ground platform providing support defense against shorter-range rocket attack. Practical design concepts yielding high target lethality at significant ranges have been developed.
Clouds can have a major impact on performance and mission opportunity of ground and airborne laser weapons. Database summaries and modeling techniques are presented for the physical and optical characteristics, frequency of occurrence, and probability of cloud-free line-of-sight (PCFLOS) of mid-level and high-altitude clouds. Emphasis is on subvisual and thin cirrus due to its predominance at mid- latitude. The statistical models presented are for top-level system engagement analyses, and to generate cloud realizations for more accurate simulation. The first portion of the presentation describes the requirements and limitations of current approaches and databases used for modeling. Criteria such as period-of-record, meteorological anomalies, spatial and temporal resolution, and measurement and data collection techniques, are discussed. The next part describes the effects of variations in cloud thickness, altitude, extinction, and scatter ratio on laser weapon operations and availability. This is followed by a practical, validated method for estimating PCFLOS as a function of elevation angle based on commonly-recorded, site-specific meteorological parameters.
The probability distribution function for the mixing scatter ratio is derived and used to model variations in total backscatter coefficient and extinction profiles in the visible and the near IR. The profiles, based on 25 years of lower stratospheric aerosol measurements, are used to estimate signal-to-noise ratio, laser-pulse energy, and wavefront measurement error with respect to backscatter strength, guidestar pulse length and altitude, and read noise. The results show that for a given wavefront error, (1) visible guidestars require less pulse energy for aerosol concentrations near background where molecular backscatter dominates, and (2) for high aerosol loading following a major volcanic event, a NIR guidestar requires a lower pulse energy than for visible wavelength sensing.
Estimates are made of nodding requirements for a secondary mirror used for radiometry of satellites in the 8- to 12-μm band. The first portion of the analysis determines the fundamental nodding frequency based on atmospheric turbulence and wind measurements taken at the Air Force Maui Optical Station. The temporal power spectral density for sky background fluctuations shows that a 50-Hz frequency will satisfy 50-percentile wind and turbulence conditions. The required frequency for a maximum operating wind condition of 40 mph is above 100 Hz. In addition to the 50-Hz fundamental, the analysis indicates that at least one, and preferably two, higher order harmonics of 150 and 250 Hz, respectively, are required to reconstruct a mirror drive waveform with sufficient duty cycle or integration time for background subtraction. The results are shown to be consistent with the performance of existing nodding systems.
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