Satellite measurements of the HF radio environment in the topside ionosphere

Measurements made on board the orbiting Air Force meteorological satellite equipped with a high‐frequency (HF) receiver have been studied in order to describe the HF radio environment at the satellite height. The satellite is in a sun‐synchronous dawn‐dusk orbit at a height of nearly 500 nmi (860 km) above the surface on the earth. The receiver sweeps repeatedly through the band 1.2–13.9 MHz in 100‐kHz steps once every 32 s, providing observations at each frequency approximately every 200 km along the satellite orbit. Measurements from a 3‐month period early in the satellite lifetime—September, October, and November, 1977—have been grouped and analyzed by frequency to yield results that are consistent with the hypothesis that the primary source of the observations is emissions from terrestrial transmitters. The relative differences in the received signals vary with frequency in a manner that agrees with the allocation of the HF spectrum.