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Potential Impacts of Elevated Aerosol Layers on High Energy Laser Aerial Defense Engagements
Abstract
This study quantifies the impacts on high energy laser (HEL) air defense
performance due to atmospheric effects in the marine boundary layer
driven by varying elevated aerosol layers. The simulations are run using
several different engagement geometries to more completely show the
effects of aerosols. High adaptive optics are applied to reduce the
turbulence effects. The atmospheric effects are defined using the
worldwide probabilistic climatic database available in the High Energy
Laser End-to-End Operational Simulation (HELEEOS) model. The anticipated
effects on HEL propagation performance is assessed at 1.0642 μm
across the world's oceans, mapped on a 1° × 1° grid, and
at 573 land sites. The scenarios evaluated are primarily near-surface
and horizontal over ranges up to 10000 meters. Seasonal and boundary
layer variations (summer and winter) for a range of relative humidity
percentile conditions are considered. In addition to realistic vertical
profiles of molecular and aerosol absorption and scattering, correlated
optical turbulence profiles in probabilistic (percentile) format are
used. Results indicate profound effects of elevated aerosol layers on
HEL engagements as compared to standard scenarios without elevated
layers. Also, results suggest changing optical properties to have
additional significant effects. HELEEOS includes a fast-calculating,
first principles, worldwide surface to 100 km, atmospheric propagation
and characterization package. This package enables the creation of
profiles of temperature, pressure, water vapor content, optical
turbulence, atmospheric particulates and hydrometeors as they relate to
line-by-line layer transmission, path and background radiance at
wavelengths from the ultraviolet to radio frequencies. Physics-based
cloud and precipitation characterizations are coupled with a probability
of cloud free line of sight (CFLOS) algorithm for air-to-air,
air-tosurface, and surface-to-air (or space) look angles. HELEEOS
characterizes aerosol environments using the Advanced Navy Aerosol Model
(ANAM) or various representations of maritime particulates from the
Global Aerosol Dataset (GADS). In the lowest 50 m, HELEEOS defines
optical turbulence with the Navy Surface Layer Optical Turbulence
(NSLOT) model. HELEEOS was developed under the sponsorship of the High
Energy Laser Joint Technology Office.
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Infrared scintillation measurements were obtained along a 7-km path over San Diego Bay concurrently with meteorological measurements obtained from a buoy at the midpoint of the path. Bulk estimates of the refractive index structure parameter cb were computed from the buoy data and compared with scintillation-derived C-n(2) values. The bulk C-n(2) estimates agreed well with the scintillation measurements in unstable conditions. In stable conditions the bulk C-n(2) estimates became increasingly higher than the scintillation values as the air-sea temperature difference increased. This disagreement may be due to enhanced wave-induced mixing of the lower atmosphere that decreases the vertical temperature and humidity gradients in stable conditions from the assumed Monin-Obukhov similarity (MOS) theory forms, resulting in bulk C-n(2) values that are too high. The bulk C-n(2) estimates decrease rapidly when the absolute air-sea temperature difference approaches small positive values. These predicted decreases in C-n(2) were nor observed in either the path-averaged scintillation measurements or in single-point turbulence measurements, indicating that bulk models for estimating scalar structure parameters based on mean air-sea scaler differences are not valid when the mean air-sea difference approaches zero. The authors believe that the most promising means toward improving the bulk C-n(2) model is to obtain a better understanding of the MOS functions over the ocean for a wide stability range, and particularly of the role of ocean waves in modifying near-surface vertical gradients and turbulence characteristics.
The Air Force Institute of Technology's Center for Directed Energy has developed a software model, the High Energy Laser End-to-End Operational Simulation (HELEEOS), under the sponsorship of the High Energy Laser Joint Technology Office (JTO), to facilitate worldwide comparisons across a broad range of expected engagement scenarios of expected performance of a diverse range of weight-constrained high energy laser system types. HELEEOS has been designed to meet JTO's goals of supporting a broad range of analyses applicable to the operational requirements of all the military services, constraining weapon effectiveness through accurate engineering performance assessments allowing its use as an investment strategy tool, and the establishment of trust among military leaders. HELEEOS is anchored to respected wave optics codes and all significant degradation effects, including thermal blooming and optical turbulence, are represented in the model. The model features operationally oriented performance metrics, e.g. dwell time required to achieve a prescribed probability of kill and effective range. Key features of HELEEOS include estimation of the level of uncertainty in the calculated Pk and generation of interactive nomographs to allow the user to further explore a desired parameter space. Worldwide analyses are enabled at five wavelengths via recently available databases capturing climatological, seasonal, diurnal, and geographical spatial-temporal variability in atmospheric parameters including molecular and aerosol absorption and scattering profiles and optical turbulence strength. Examples are provided of the impact of uncertainty in weight-power relationships, coupled with operating condition variability, on results of performance comparisons between chemical and solid state lasers.
This study quantifies the potential impacts on ship-defense high-energy-laser (HEL) performance due to
atmospheric effects in the marine boundary layer driven by recent observations and analysis of worldwide sea
surface temperatures (SSTs). The atmospheric effects are defined using the worldwide probabilistic climatic
database available in the High Energy Laser End-to-End Operational Simulation (HELEEOS) model, which
includes an SST database for the period 1854–1997. A more recent worldwide sea surface temperature database was provided by the Naval Postgraduate School for the period 1990–2008. Mean differences and trends between the two SST databases are used to deduce possible climate change impacts on simulated maritime HEL engagements. The anticipated effects on HEL propagation performance are assessed at an operating wavelength of 1.0642 mm across the world’s oceans and mapped onto a 18 3 18 grid. The scenario evaluated is near surface and nearly horizontal over a range of 5000 m in which anticipated clear-air maritime aerosols occur. Summer and winter scenarios are considered. In addition to realistic vertical profiles of molecular and aerosol absorption and scattering, correlated optical turbulence profiles in probabilistic (percentile) format are used.
Scattering of electromagnetic radiation from a sphere, so-called Mie scattering, requires calculations that can become lengthy and even impossible for those with limited resources. At the same time, such calculations are required for the widest variety of optical applications, extending from the shortest UV to the longest microwave and radar wavelengths. This paper briefly describes new and thoroughly documented Mie scattering algorithms that result in considerable improvements in speed by employing more efficient formulations and vector structure. The algorithms are particularly fast on the Cray-1 and similar vector-processing computers.
This paper describes the status of the 2008 edition of the HITRAN molecular spectroscopic database. The new edition is the first official public release since the 2004 edition, although a number of crucial updates had been made available online since 2004. The HITRAN compilation consists of several components that serve as input for radiative-transfer calculation codes: individual line parameters for the microwave through visible spectra of molecules in the gas phase; absorption cross-sections for molecules having dense spectral features, i.e. spectra in which the individual lines are not resolved; individual line parameters and absorption cross-sections for bands in the ultraviolet; refractive indices of aerosols, tables and files of general properties associated with the database; and database management software. The line-by-line portion of the database contains spectroscopic parameters for 42 molecules including many of their isotopologues.
This paper discusses techniques to describe the aerosol and the electro optical properties of the marine atmosphere from 15 meters down to the tops of the highest wave. Emphasis is placed on the experimental Rotorod technique to measure the concentrations of giant sea salt droplets. Data from these devices are parameterized using a lognormal function that is in turn related statistically to parameters such as wind speed, atmospheric stability and height above the surface. The new lognormal function is combined with the Navy Aerosol Model (NAM) to develop a first version of the Advanced Navy Aerosol Model (ANAM). Thus, ANAM allows the construction of an aerosol size distribution at any level from the wave tops to 15 meters an the assessment of electro optical parameters from this distribution using Mie theory. The first results of the ANAM model are compared to experimental data.