@misc{1156801,
  author = {Donald H Zacharisen},
  title = {World maps of F2 critical frequencies and maximum usable frequency factors:},
  year = {1959},
  month = {1959-01-01 05:01:00},
  publisher = {, National Institute of Standards and Technology, Gaithersburg, MD},
  doi = {https://doi.org/10.6028/NBS.TN.2},
  language = {en},
}