Cloud Free Line Of Sight Research Articles

Altitude-dependent probability of vertical, cloud-free line-of-sight from European Centre for Medium Range Weather Forecasting Re-Analysis-interim cloud cover

A method to calculate the altitude-dependent, vertical, cloud-free line-of-sight (CFLOS) using the fractional cloud cover from the European Centre for Medium Range Weather Forecasting Re-Analysis Interim (ERA-I) dataset has been developed. This method enables users of airborne and satellite collections of optical ground data to understand the statistical coverage limitations of these collection systems by informing them of global probabilities of CFLOS versus altitude as well as time of year. This method is accurate for regions between ±60 deg of latitude; it should not be applied to polar regions due to limitations in the underlying ERA-I data. Our CFLOS calculations have been compared to the results of CloudSAT/Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation analysis and with Moderate Resolution Imaging Spectroradiometer (MODIS) total cloud cover data. It is shown that the ERA-I reports on average less cloud cover by about 7.5% (absolute) for regions within ± 60 deg of the equator relative to MODIS cloud cover retrievals. Our CFLOS calculation leverages the resolution and diversity of ERA-I that enables spatial coverage as well as frequency of occurrence CFLOS calculations for nearly all non-polar regions on earth.

Implications of four-dimensional weather cubes for improved cloud-free line-of-sight assessments of free-space optical communications link performance

We advance the benefits of previously reported four-dimensional (4-D) weather cubes toward the creation of high-fidelity cloud-free line-of-sight (CFLOS) beam propagation for realistic assessment of autotracked/dynamically routed free-space optical (FSO) communication datalink concepts. The weather cubes accrue parameterization of optical effects and custom atmospheric resolution through implementation of numerical weather prediction data in the Laser Environmental Effects Definition and Reference atmospheric characterization and radiative transfer code. 4-D weather cube analyses have recently been expanded to accurately assess system performance (probabilistic climatologies and performance forecasts) at any wavelength/frequency or spectral band in the absence of field tests and employment data. The 4-D weather cubes initialize an engineering propagation code, which provides the basis for comparative percentile performance binning of FSO communication bit error rates (BERs) as a function of wide-ranging azimuth/elevation, earth-to-space uplinks. The aggregated, comparative BER binning analyzes for different regions, times of day, and seasons applying a full year of 4-D weather cubes data provided numerous occasions of clouds, fogs, and precipitation events. The analysis demonstrated the utility of 4-D weather cubes for adroit management of CFLOS opportunities to enhance performance analyses of point-to-point as well as evolving multilayer wireless network concepts.

Optimizing the Ground Network of Optical MEO Satellite Communication Systems

This contribution is focused on MEO satellite communication systems operating at optical wavelengths, and especially on the optimum dimensioning of their optical ground station (OGS) network (OGSN). The objective of the optimization is to minimize the size of the network, while still ensuring the target link availabilities due to cloud blockage. Two new optimization algorithms for this optimum selection of OGSs are proposed. The algorithms take into account the monthly variability of cloud coverage, take advantage of locations in different hemispheres, and select OGSs, which are within the visibility area of the satellite for longer time. The methodology for calculating the single and joint cloud free line of sight considering the integrated liquid water content statistics, the time dependent elevation angles and the MEO satellite movement are presented. Representative numerical results are presented considering either a single MEO satellite or a full MEO constellation.

Medium earth orbit optical satellite communication networks: Ground terminals selection optimization based on the cloud‐free line‐of‐sight statistics

SummaryIn this paper, a methodology for the generation of cloud coverage time series correlated on both temporal and spatial domains and the estimation of cloud‐free line‐of‐sight (CFLOS) probability for a single optical ground station (OGS) and a network of OGSs (multiple OGS) for medium earth orbit (MEO) constellation satellite communication systems is presented. Spatial diversity is employed as a mitigation technique against cloud blockage to increase the availability of an OGS network (OGSN). In the second part of the paper, an effective algorithm based on the ant colony optimization (ACO) algorithm is proposed for the optimum selection of OGSs forming an OGSN. The proposed algorithm takes into consideration the spatial correlation between the OGSs. In the simulation results section, a MEO constellation system with 12 MEO satellites has been assumed. The integrated liquid water content (ILWC) statistical parameters that are necessary for the CFLOS time series are taken from ECMWF Re‐Analysis (ERA)‐Interim database, European Centre for Medium‐Range Weather Forecasts (ECMWF). Finally, numerical results for the optimum selection of OGSs using the proposed methodology are exhibited and commented. Interesting conclusions are drawn, and future work with technical challenges is briefly described.

Cloud Free Line of Sight Prediction Modeling for Optical Satellite Communication Networks

Analytical models for the prediction of cloud-free line-of-sight (CFLOS) probability for a single optical satellite link and for multiple spatially separated optical satellite links are presented. The presence of clouds along the slant paths causes their blockage. As a result, an ON/OFF channel with the presence of clouds is assumed. The proposed model for the calculation of CFLOS considers the elevation angle of the optical satellite links, the altitude of ground stations, and the spatial variability of clouds. The methodology is based on the assumption that integrated liquid water content (ILWC) follows lognormal distribution and a well-defined relationship between ILWC and liquid water density. Moreover, joint CFLOS statistics are evaluated and CFLOS probability for simultaneously available optical links for the application of spatial multiplexing transmission techniques is estimated. The models are validated through simulated results and finally, the numerical results are devoted to optical satellite network examples with a view to achieving specific availability percentage.

Worldwide Mission Planning Tool for Tactical Laser Systems

The directed energy modeling and simulation community can make important direct contributions to the joint warfighting community by providing the capability to estimate expected performance of high energy laser systems on a worldwide basis over both land and ocean regions accounting for variability in system performance arising from spatial, spectral and temporal variations in operating conditions. Key recently introduced features of theAir Force Institute of Technology Center for Directed Energy’s high energy laser end-toend operational simulation parametric one-on-one engagement level model allow it to meet modeling and simulation needs and function as a near term mission planning tool. These features include the capability to derive vertical profiles of atmospheric effects based probabilisticclimatology,historicalweatherreanalysisgrids,orreal-timeforecastmodelsavailable on-line. Each atmospheric gas or particulate is evaluated based on its wavelength-dependent forward and off-axis scattering characteristics and absorption effects on electromagnetic energy delivered at any wavelength from 0.4 μm to 8.6m. High energy laser end-to-end operational simulation can produce profiles, including correlated optical turbulence profiles in percentile format, from probabilistic climatology for over 400 land sites worldwide for all times of day and for a 1 ◦ × 1 ◦ grid over all ocean locations. In addition, probability of cloud free line of sight for hundreds of land sites worldwide is available in the model. Target surface orientation is defined in three-dimensional space, supporting accurate assessment of the effectiveness of a particular engagement geometry. Effects of thin layers of fog, several types of rain and several types of water droplet and ice clouds can also be considered. In the current study, performance predictions at several wavelengths for a number of geographically diverse land and sea locations are made using numerical weather reanalysis data and are compared with results derived from probabilistic climatology. Use of web-based

Worldwide assessments of laser radar tactical scenario performance variability for diverse low altitude atmospheric conditions at 1.0642 μm and 1.557 μm

Spatial, spectral and temporal variations in operating conditions are major contributors to the expected variability/uncertainty in system performance. The ratio of signal-to-noise ratio (SNR) based on climatological data to a standard atmosphere is the primary performance metric used, with results presented in the form of histograms and maps of worldwide LADAR performance variation. This metric is assessed at 2 wavelengths, 1.0642 µm and 1.557 µm, for a number of widely dispersed land and maritime locations worldwide over oblique and vertical air to surface paths in which anticipated clear air aerosols and location specific heavy rain and 150 m thick fog occur. Seasonal, boundary layer, and time of day variations for a range of relative humidity percentiles are also considered. In addition to realistic vertical profiles of molecular and aerosol extinction, air-to-ground cloud free line of sight (CFLOS) probabilities as a function of location for this geometry are computed. Observations from the current study strongly indicate that use of the standard atmosphere to predict performance will produce overly optimistic, in many cases extremely so, estimates of expected performance. Locally heavy rain, when present, severely limits LADAR system performance at these wavelengths. Some operational capability exists for vertical looks through fog.

Cloud-Free Line-of-Sight Calculations

A method for computing the probability of a cloud-free line-of-sight (CFLOS) between a given point in the atmosphere and a given point on the earth, was developed. The method is based on 3-hr synoptic weather reports of clouds, and whole-sky photographs taken during daylight hours over a period of three years at Columbia, Mo. The computational procedure is an effort to eliminate an apparent oversimplification in previously published methods that results from the use of mean cloud cover, an unsatisfactory vertical cloud distribution, and sunshine data. Present results are at variance with those earlier estimates, but compatible with recent observations actually taken from aircraft. Although the CFLOS estimates obtained are, by nature, uncertain, the range of uncertainty was estimated by using published empirical data and a quantitative error analysis.