TY  - RPRT
TI  - Supplementary world maps of F2 critical frequencies and maximum usable frequency factors 
AU  - Zacharisen, Donald H
PY  - 1959
PB  - National Bureau of Standards
CY  - Gaithersburg, MD
SN  - NBS TN 2-2
DO  - 10.6028/NBS.TN.2-2
ER  -