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.