sun = sun_position(time, location)
This function computes the sun position (zenith and azimuth angle at the observer location) as a function of the observer local time and position.
It is an implementation of the algorithm presented by Reda et Andreas in:
Reda, I., Andreas, A. (2003) Solar position algorithm for solar radiation application. National Renewable Energy Laboratory (NREL) Technical report NREL/TP-560-34302, Revised January 2008.
This document is avalaible at http://rredc.nrel.gov/solar/codesandalgorithms/spa/ and is included in the .zip file
This algorithm is based on numerical approximation of the exact equations. The authors of the original paper state that this algorithm should be precise at +/- 0.0003 degrees. I have compared it to NOAA solar table (http://www.srrb.noaa.gov/highlights/sunrise/azel.html) and to USNO solar table (http://aa.usno.navy.mil/data/docs/AltAz.html) and found very good correspondance (up to the precision of those tables), except for large zenith angle, where the refraction by the atmosphere is significant (difference of about 1 degree). Note that in this code the correction for refraction in the atmosphere has been implemented for a temperature of 10C (283 kelvins) and a pressure of 1010 mbar. See the subfunction «sun_topocentric_zenith_angle_calculation» for possible modification to explicitely model the effects of temperature and pressure as describe in Reda & Andreas (2003).
time: a structure that specify the time when the sun position is calculated.
time.year: year. Valid for [-2000, 6000]
time.month: month [1-12]
time.day: calendar day [1-31]
time.hour: local hour [0-23]
time.min: minute [0-59]
time.sec: second [0-59]
time.UTC: offset hour from UTC. Local time = Greenwich time + time.UTC
This input can also be passed using the Matlab time format ('dd-mmm-yyyy HH:MM:SS'). In that case, the time has to be specified as UTC time (time.UTC = 0)
location: a structure that specify the location of the observer
location.latitude: latitude (in degrees, north of equator is positive)
location.longitude: longitude (in degrees, positive for east of Greenwich) location.altitude: altitude above mean sea level (in meters)
sun: a structure with the calculated sun position
sun.zenith = zenith angle in degrees (angle from the vertical)
sun.azimuth = azimuth angle in degrees, eastward from the north.
Only the sun zenith and azimuth angles are returned as output, but a lot of other parameters are calculated that could also be extracted as output of this function. See the documentation in the code.
Exemple of use
location.longitude = -105.1786;
location.latitude = 39.742476;
location.altitude = 1830.14;
time.year = 2003;
time.month = 10;
time.day = 17;
time.hour = 12;
time.min = 30;
time.sec = 30;
time.UTC = -7;
sun = sun_position(time, location);