https://hamradioschool.com/nvis/
http://arrl-ohio.org/SEC/nvis/nvis.pdf
https://hamradioschool.com/d-layer-absorption/
http://www.astrosurf.com/luxorion/qsl-perturbation.htm
http://arrl-ohio.org/SEC/nvis/nvis.pdf
https://hamradioschool.com/d-layer-absorption/
http://www.astrosurf.com/luxorion/qsl-perturbation.htm