A NEW SKY-TO-GROUND RATIO PROGRAM FOR USE IN CASTFOREM AND OTHER WARGAMES

1. A NEW SKY-TO-GROUND RATIO PROGRAM FOR USE IN CASTFOREM AND OTHER WARGAMES Richard Shirkey Sean O'Brien U.S. Army Research Laboratory Computational & Information Sciences Directorate Battlefield Environment Division Contact: US Army Research Lab, Battlefield Environment Division, AMSRL-CI-EE, White Sands Missile Range, NM 88002-5501 Richard Shirkey: Commercial: 505 678-5470; DSN 258-5470; rshirkey@arl.army.mil Sean O'Brien: Commercial: 505 678-1570; DSN 258-1570; sobrien@arl.army.mil

2. Wargame Sensor/Target Environment Reduced target contrast Scattering Emission Smoke/Haze

3. Sky-to-ground ratio (SGR) is a computational shortcut frequently used in wargames to quantify contrast This contrast is degraded by path radiance entering the sensor LOS due to Multiple scattering of ambient light by atmospheric aerosols and/or gases Greybody atmospheric thermal radiance Research grade radiative transfer models can compute path radiance with high accuracy These calculations take a considerable amount of time; the SGR approximation provides a fast answer with moderate accuracy Sky-to-Ground Ratio - Motivation

4. Contrast Component Definitions C(r) = C(0)/{1 + [Ip(r)/Ib(0)] T(r)-1}

5. SGR Definition Limiting path radiance: Ips () =  I(r, ) P(, ) d For a uniform path: Sgr = Ips / Ib(0) = Ip / [Ib(0) (1-T)] For a slant path: <Sgr> = <Ips >/ Ib(0) = Ip / [Ib(0) (1-T)]

6. Why SGR? The SGR provides an approximate, time conservative, method for determining the reduction in contrast due to LOS in-scattering of light Tc = C(r)/C(0) = 1/{1 + [Ip(r)/Ib(0)] T(r)-1} = 1 / {1 + Sgr(1/T - 1)}

7. SGR Model IR Products The SGR model can compute an SGR for the IR bands, using the IR thermal path radiance as a starting point: Ip() = Ib(0) e-/+ (a/) B (1 - e -/) Or, it can calculate a slant path SGR: <Sgr> = Ip() / [Ib(0) (1-T)] Or, the model can compute an IR ΔT by inverting the Planck function for the computed radiances

8. FASCAT Visible band radiances EOSAEL PFNDAT Aerosol phase functions for upgraded FASCAT AFRL MODTRAN Molecular absorption and aerosol attenuation profiles EOSAEL CLTRAN Optical thickness for cloud layers EOSAEL FITTE IR atmospheric path radiance SGR Code Heritage

9. SGR Code Assumptions • General • suitable only for near-earth altitudes ( < 10 km) • narrow absorption features may not well represented • cloud layers will modify the water vapor profile • At Visible wavelengths • delta-Eddington methodology is employed • At IR wavelengths • at 3.0 – 5.0 scattering effects are not included • only thermal radiance is considered • numerical path integration

10. SGR Code – Parameter Domain • Aerosol types • Rural • Maritime • Urban • Tropospheric • Radiation Fog • Advection Fog • Cloud types • Cirrus/cirrostratus • Altostratus/altocumulus • Cumulus • Stratus/stratocumulus • Nimbostratus • Atmospheric profiles • Tropical • Midlatitude Summer (45N, July) • Midlatitude Winter (45N, Jan.) • Subarctic Summer (60N, July) • Subarctic Winter (60N, Jan.) • 1976 US Standard Atmosphere • Geometries • Target height calculated • LOS zenith angle calculated • Target range is calculated

11. N 89.1° W E S Example Scenario Lambertian Ground Plane Upward LOS: helicopter approaches from Sun direction, passes over observer and exits with Sun behind Downward LOS: position of helicopter & observer interchanges

12. Upward Looking in Visual

13. Downward Looking in Visual

14. Upward Looking in Far-IR (10.0 μm)

15. Upward Looking LOS at IR Wavelengths

16. SGR code can be added to wargames without imposing prohibitive run speed penalties SGR IR sky radiance algorithms have been adapted for use in TDA models such as TAWS The new code also treats the problem of upward LOS targets against hillside backgrounds Choice of model atmospheres (and temperature profiles) is limited to AFRL MODTRAN set at present, but can be easily expanded Summary

17. Availability rshirkey@arl.army.mil sobrien@arl.army.mil http://mel.dmso.mil