The greatest
effect on ADF accuracy is created by Skywave
interference with the surface wave signal, which only occurs at night, the
greatest interference occurring at dawn and dusk.
Owing
to the attenuation of the signal by the lower ionospheric
D and E layers during the day no daytime Skywave is
produced. However, at night the D and E Layers disappear and the F1 and
F2 layers combine into one higher and weaker F layer which doesn’t attenuate
the signal totally but refracts it back to earth at greater ranges than the
surface wave.