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r.sun is a solar irradiance and irradiation model for GRASS GIS, which is a free, open-source geographical information system (GIS) capable of handling raster, topological vector, image processing, and graphic data.

r.sun computes direct ( beam), diffuse and reflected solar irradiation raster maps for given day, latitude, surface and atmospheric conditions. This is useful in several areas of solar energy including solar thermal heating and photovoltaics. Several solar parameters are saved in the map history file including:

  1. sunrise,
  2. sunset times,
  3. declination,
  4. extraterrestrial irradiance,
  5. daylight length

Alternatively, r.sun enables users to specify a local time to compute solar incidence angle and/or irradiance raster maps, and the shadowing effect of the topography. It has recently begun to be used with GRASS for estimating potential solar photovoltaic yields.