TY  - RPRT
TI  - Letter Circular 575: the ionosphere and radio transmission conditions, with special reference to the observing and reporting service of the National Bureau of Standards (replaces Letter Circular 499)
PY  - 1939
PB  - National Institute of Standards and Technology
CY  - Gaithersburg, MD
SN  - LCIRC 575
DO  - 10.6028/NBS.LCIRC.575
ER  -