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.\" RCSid $Id$
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.TH "GENSSKY" "1" "8/31/24" "RADIANCE" ""
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.SH "NAME"
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genssky - generates a RADIANCE description of the spectral daylight sources
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.SH "SYNOPSIS"
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\fBgenssky month day hour [-y year] [-d AOD] [-c cloud_cover] [-l file] [-g grefl] [-n nproc] [-r res] [-f out]\fR
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.SH "DESCRIPTION"
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\fIGenssky\fR produces a RADIANCE scene description of spectral sky
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and ground using the precomputed atmospheric scattering method
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(Bruneton et al). For a given atmospheric condition, a set of
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Radiance data files are pre-computed and reused for other solar
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positions. Precomputation can be sped up using multithreading with
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the \fI-n\fR flag. Extraterestrial solar radiation are spectrally
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resolved from 380-780nm at 20nm interval and constant regardless
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of location and time. The resulting sky source is in the same spectra
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range. The output are three files defaults to {prefix}.rad,
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{prefix}_sky.hsr, {prefix}_ground.hsr. The default prefix is "out"
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and can be changed using \fI-f\fR flag. The two .hsr files stores
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the fisheye spectral image of the sky and ground, with a default
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resolution of 128x128, which can be changed using \fI-r\fR flag.
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.PP
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Non-clear conditions are modeled by linearly interpolating between
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clear sky and CIE overcast sky. The resulting sky is the weight
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(cloud cover) average of clear and overcast sky. The overcast sky
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has a CCT of 6415K, from which the daylight spectra was computed.
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.PP
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Genssky models the Mie scattering differently from the original
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implementation. Instead of assuming exponential decay uniformly
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across the spectral range, the Mie scattering coefficients, as a
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functinon of the altitude, are computed into a lookup table using
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libradtran radiative transfer solver and interpolated directly
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during precomputation. The default Mie scattering profile is OPAC
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continental average, and can be overridden using the \fI-l\fR flag.
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The overall aerosol profile can also be scaled linearly by applying
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a broadband aerosol depth value using the \fI-d\fR flag.
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.PP
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Rayleigh scattering as a function of altitude is assumed to be
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exponential decay. The Rayleigh scattering coefficients at sea level
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are taken from Anderson et al (1986). There are a total of five
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sets of Rayleigh scattering data: mid-latitude summer, mid-latitude
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winter, sub-arctic summer, sub-arctic winter, and tropical. Latitudes
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are divided at 23.5 and 62.5 deg and summer is from month [4-9].
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The scale-height of each Rayleigh scattering profile are tuned to
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fit the corresponding results from a libradtran calculation.
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.PP
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\fIgenssky\fR can be used with the following input parameters.
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.PP
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\fB-a Latitude in degree, north positive. Used along with datetime to determine solar angle and Rayleigh scattering profile.
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.br
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\fB-c Total cloud cover, value ranging from 0(clear)-1(overcast), default=0.
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.br
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\fB-d Broadband aerosol optical depth, default: 0.115. This parameter linearlly scales the overall aerosol scattering, default=0.115.
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.br
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\fB-f Output file name, default="out"
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.br
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\fB-g Average ground reflectance, default=0.2
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.br
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\fB-l Custom Mie scattering profile file
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.br
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\fB-m Standard meridian, west positive. For example, UTC-8:00 = 120. Used to calculated solar angle.
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.br
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\fB-n Number of threads for precomputation, no benefits beyond 16 threads
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.br
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\fB-o Longitude in degree, west positive. Used to calculated solar angle.
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.br
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\fB-r Sky and ground hyperspectral image resolution. default=128. Increase this value to reduce artifacts at horizon.
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.br
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.SH "EXAMPLES"
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To generate a clear sky for March 2th at 3:15pm standard time at a site latitude of 42 degrees, 108 degrees west longitude, and a 110 degrees standard meridian, using 8 threads:
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.br
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genssky 3 2 15.25 -a 42 -o 108 -m 110 -n 8
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.br
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For the same sky but with 100 percent total cloud cover:
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.br
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genssky 3 2 15.25 -a 42 -o 108 -m 110 -n 8 -c 1
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.br
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.SH "FILES"
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/usr/local/lib/ray/mie_ca.dat
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.br
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.SH "AUTHOR"
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Taoning Wang
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.br
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.SH "SEE ALSO"
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gensky(1), gensky(1), rcomb(1), rcontrib(1), rpict(1), rtpict(1),
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rtrace(1), xform(1)
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.br
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