Ground wave is important for radio signals below 30 MHz, but is generally insignificant at higher frequencies where line-of-sight propagation dominates.
AM and longwave broadcasting, navigation systems such as LORAN, low-frequency time signals, non-directional beacons, and short-range HF communications all make use of it.
Propagation predictions thus require knowing the electrical properties of subsurface layers, which are best measured from groundwave attenuation.
The increased effectiveness of groundwave at lower frequencies gives AM radio stations more coverage at the low end of the band.
Early commercial and professional radio services relied exclusively on long wave, low frequencies and ground-wave propagation.
To prevent interference with these services, amateur and experimental transmitters were restricted to the high frequencies (HF), felt to be useless since their ground-wave range was limited.
In the 1930s, Alfred Norton was the first author to accurately describe groundwave mathematically, deriving an equation for field strength over a flat earth.
Later work focused on paths with variable conductivity, the effects of terrain and objects on the ground, and computer modeling.