Sources
Spokas and Forcella (2006)
Paper by Kurt Spokas and Frank Forcella:
https://pubag.nal.usda.gov/catalog/1910
NREL tech report
This link is the info we have for this source, but it is a dead now:
https://rredc.nrel.gov/solar/pubs/spectral/model/section3.html
Git Repo
This repo is the original source code for the python reimplementation (which this tool is based on) of SolarCalc:
https://gitlab.com/appf-anu/pysolarcalc
This repo is the repo which builds this website and the Azure Function Apps that run the backend processing: