#!/usr/bin/python
 
#    Functions for calculating the solar radiation hitting the earth
 
#    Copyright 2008 Brandon Stafford
#
#    This file is part of Pysolar.
#
#    Pysolar is free software; you can redistribute it and/or modify
#    it under the terms of the GNU General Public License as published by
#    the Free Software Foundation; either version 3 of the License, or
#    (at your option) any later version.
#
#    Pysolar is distributed in the hope that it will be useful,
#    but WITHOUT ANY WARRANTY; without even the implied warranty of
#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#    GNU General Public License for more details.
#
#    You should have received a copy of the GNU General Public License along
#    with Pysolar. If not, see <http://www.gnu.org/licenses/>.
 
import solar
import math
 
def GetAirMassRatio(altitude_deg):
	# from Masters, p. 412
	# warning: pukes on input of zero
	return (1/math.sin(math.radians(altitude_deg)))
 
def GetApparentExtraterrestrialFlux(day):
	# from Masters, p. 412
	return 1160 + (75 * math.sin((360./365) * (day - 275)))
 
def GetOpticalDepth(day):
	# from Masters, p. 412
	return 0.174 + (0.035 * math.sin((360./365) * (day - 100)))
 
def GetRadiationDirect(utc_datetime, altitude_deg):
	# from Masters, p. 412
 
	if(altitude_deg > 0):
		day = solar.GetDayOfYear(utc_datetime)
		flux = GetApparentExtraterrestrialFlux(day)
		optical_depth = GetOpticalDepth(day)
		air_mass_ratio = GetAirMassRatio(altitude_deg)
		return flux * math.exp(-1 * optical_depth * air_mass_ratio)
	else:
		return 0.0