The KHFFILT program can be used to perform Karous-Hjelt and Fraser filtering of geophysical VLF (very-low-frequency) data. In VLF method two orthogonal components of the magnetic field are measured, and normally the tilt angle, a, and ellipticity, e, of the vertical magnetic polarization ellipse are derived. Real (in-phase) and imaginary (quadrature) components, however, are traditionally used in Scandinavia and also in KHFFILT program. These components are based on the tilt angle and ellipticity as: Re= tan(a)x100 % and Im= ex100 %.