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Approved For Public Release; Distribution Unlimited An Upgrade to TAWS Urban Illumination Model Richard Shirkey Army Research Laboratory Computational & Information Sciences Directorate Battlefield Environment Division WSMR, NM Ph: (575) FAX: (575) WIDA March

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Approved For Public Release; Distribution Unlimited 2 Purpose Beginnings Walker Treanor Objective Garstang’s improvements AIM v2 Spectral Radiance V&V OUTLINE

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Approved For Public Release; Distribution Unlimited To provide urban illumination levels for use in/for Simulations IWARS Target Acquisition TAWS Night Vision Devices Purpose

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Approved For Public Release; Distribution Unlimited 4 Walker took sky brightness measurements of various cities to determine the distance where the artificial sky brightness was 0.1 mag at the zenith. From this he constructed the first Population-Distance relation. Distance (miles) Population x Beginnings: Walker’s Model Garstang reformulated Walker’s P-D relation as B = CPD Known as Walker’s law, it is the simplest model for light pollution. C does not depend on P and D, but depends on factors such as the light emission per head of the population and the reflectivity of the ground

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Approved For Public Release; Distribution Unlimited 5 Beginnings: Treanor’s Model when ' > m scattering is not considered Treanor proposed a simple model for scattering along the direct beam (TZ) by restricting scattering to a small angle m. This restricts scattering to the particles which are contained within the figure of revolution generated by rotating the arc TQZ about TZ. This scattered light then undergoes a final scattering at Z into the observer’s LOS.

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Approved For Public Release; Distribution Unlimited Objective A quick running model that accurately portrays both broadband and spectral illumination received from urban sources by an arbitrarily placed observer point source observer 2.25 x city radius extended source observer 2.25 x city radius internal to the city Easily obtainable inputs Atmospheric variables aerosols clouds : acceptable error D: distance to source R: radius of source E: irradiance L': radiance E ex = πL'R 2 / (R 2 + D 2 ) E pt = πL'R 2 / D 2 = (E pt – E ex )/E ex = R 2 /D 2 D R -½ ( 20% D 2.25 R) criterion

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Approved For Public Release; Distribution Unlimited 7 Garstang’s Model Assumptions for Garstang’s model are: Flat Earth All downward-directed light from fixtures does not undergo scattering or absorption wrt the atmosphere No interaction with buildings or vegetation Aerosol scattering is predominantly in the forward direction © International Dark-Sky Association

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Approved For Public Release; Distribution Unlimited 8 Garstang’s Model Garstang further modified Treanor’s model by introducing an exponential atmosphere and directly relating the atmospheric molecular content to the aerosol content N a a = N m m K e -cH where N is the number density, is the particle cross section, H is the height of the city and K is Garstang’s “clarity factor”.

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Approved For Public Release; Distribution Unlimited 9 Garstang’s Model The city emission function is defined by I = LP/2 {2G (1-F) cos F 4 } where I is the upward intensity, F is the fraction of light that “escapes” the luminaries shielding in an upward direction, G is the surface albedo and is the angle between the upward normal and an upward light ray. Cinzano has found that the shape of the emission function is not important City Emission Function

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Approved For Public Release; Distribution Unlimited 10 “Double Scattering” Final Equations General Scenario Brightness Small Angle Approximation

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Approved For Public Release; Distribution Unlimited 11 Garstang’s Model: Details Broadband Brightness in Garstang’s model is represented by b = N m R e -cH (dx dy / R 2 ) I up S -2 du x 0 EF xq EF qo SAA { e -ch (1 + cos 2 [ + ]) 3/(16 ) + e -ah K f( + )}

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Approved For Public Release; Distribution Unlimited 12 Original model tenets maintained brightness ~ population SSA City population data base included Cloud effects added 8 cloud types reflection Lighting data base added Airglow included AIM v2

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Approved For Public Release; Distribution Unlimited 13 In the absence of moonlight and artificial light, the night sky still is not “dark”. Natural Background Radiation S 10 is a bizarre astronomical unit of surface brightness corresponding to one 10th-magnitude star per square degree

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Approved For Public Release; Distribution Unlimited 14 In general: B nsb = – 2.5 x log 10 (B airglow + B zodiacal + B starlight ), where B airglow, B zodiacal, and B starlight are functions of the solar sunspot cycle, the ecliptic latitude, and the galactic latitude respectively. It was deemed reasonable to set the value of B nsb = 17.6 nL rather than carry out additional computations for only a small additional contribution to the total night sky brightness For Denver at a distance of ~70 km the model predicts that the brightness is ~18 nL. Natural Background Radiation

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Approved For Public Release; Distribution Unlimited 15 Transmission through clouds determined using ILLUMA 8 cloud types in 3 layers High - clear, thin and thick Ci/Cs Medium - clear, As/Ac Low - clear, clear (f/k), Cu/Cb or St/Sc allowance for partial cloudiness Cloud Effects Cloud reflection cloud and angle dependent reflectivity taken from Shaprio simple attenuation in upward and downward paths from lowest layer only

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Approved For Public Release; Distribution Unlimited 16 city name, city radius, city altitude, lumens/head distance from observer to source, observer altitude, zenith angle, azimuth angle clarity factor, surface albedo, fraction of light in upward direction month, day, year, UTC light type percents cloud type & cloud amount for layer i, i = 1,3 AIM Input

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Approved For Public Release; Distribution Unlimited 17 Validation Sky brightness due to Denver as a function of distance for a zenith angle of zero* Sky brightness due to Denver as a function of zenith angle at a distance of 40 km* Clear Skies * Garstang ‘86

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Approved For Public Release; Distribution Unlimited 18 Denver 8.6 km from city center Zenith = 0 Brightness Distribution X Observer

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Approved For Public Release; Distribution Unlimited 19 Boulder Zenith = 65 Azimuth = 180 Distance = 0 km Boulder Zenith = 65 Azimuth = 0 Distance = 0 km Boulder Zenith = 65 Azimuth = 45 Distance = 0 km Boulder Zenith = 65 Azimuth = 45 Distance = 2 km Brightness Distribution X X X X X marks observer position

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Approved For Public Release; Distribution Unlimited 20 Spectral Radiant Energy

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Approved For Public Release; Distribution Unlimited 21 Broadband illumination is broken into spectral components dependant on the City’s light types and amounts Approach Determine 1) Light types 2) Percent contribution 3) Spectral radiance values for each light type Spectral Radiant Energy

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Approved For Public Release; Distribution Unlimited 22 Light Types and their default percentages* Spectral Radiant Energy Light Type Default Value (%) P i Clear Mercury20.2 Low Pressure Sodium0 High Pressure Sodium77.9 Metal Halide1.9 white LED0 Standard Fluorescent0 Incandescent0 Liquid Kerosene0 Pressurized Propane0 * El Paso Electric Co.

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Approved For Public Release; Distribution Unlimited 23 Light Type Spectra (1 nm resolution) Mercury Vapor LPS HPS Metal Halide LED Fluorescent Incandescent Kerosene Propane

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Approved For Public Release; Distribution Unlimited 24 For Light Type i 1)Identify total contribution over wavelength band S 1 to S 2 2)Bin contribution Spectral Radiant Energy Technique HPS wavelength band under consideration HPS S1S1 S2S2

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Approved For Public Release; Distribution Unlimited 25 3)Fractional radiance 4)Final spectral radiance sum over light type Spectral Radiant Energy Technique

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Approved For Public Release; Distribution Unlimited Spectral Radiance Courtesy of Dr. Martin Aubé Spectrum of Los Angles Zenith view

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27 10% Clear Mercury 5% LPS 55% HPS 20% Metal Halide 10% Incandescent Aubé AIM v2 No Airglow

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28 10% Clear Mercury 5% LPS 55% HPS 20% Metal Halide 10% Incandescent SAB + Airglow Aubé AIM v2 Note increase in background level

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Approved For Public Release; Distribution Unlimited 29 Night Illumination Test Evaluation (NITE) A Joint Undertaking with the UK Met Office V&V

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Approved For Public Release; Distribution Unlimited 30 x NITE: Location

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Approved For Public Release; Distribution Unlimited 31 Night Illumination Test Evaluation (NITE)

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Approved For Public Release; Distribution Unlimited 32 x Night Illumination Test Evaluation (NITE)

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Approved For Public Release; Distribution Unlimited 33 x Night Illumination Test Evaluation (NITE) Shuttle Taxiway

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Approved For Public Release; Distribution Unlimited 34 Microtops Sunphotometer 5 channel hand-held sunphotometer for measuring aerosol optical thickness easily, accurately and dependably. NITE: Instrumentation

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Approved For Public Release; Distribution Unlimited 35 LMT Hoffman All-Sky cameraAlbedo stand first day out (brrr) NITE: Instrumentation

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Approved For Public Release; Distribution Unlimited Met data WSMR04 Holloman AFB mesonetwest.utah.edu NITE: Meteorology

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Approved For Public Release; Distribution Unlimited 37 NITE: Satellite Pictures

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Approved For Public Release; Distribution Unlimited 38 What’s Next? Variable turbid atmosphere add assortment of aerosol types Inclusion of horizontal single scattering effects zenith angles > 80 produce dubious results City reflection from partially cloudy skies reflection only allowed from lowest layer Terrain/Vegetation blocking NITE data reduction for V&V

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Approved For Public Release; Distribution Unlimited 39 Acknowledgements Warren Lewis* (for suggesting NITE) Damian Wilson* (for tolerating numerous snafus) Jeff Passner § (for satellite pics) Ed Creegan § (for setup and data acquisition) Will Lee-Rodgers* (for just about everything else) * UKMO § ARL

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Approved For Public Release; Distribution Unlimited 40 ?

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