The GOLF experiment on the SOHO mission will study the internal structure of the sun by measuring the spectrum of global oscillations in the frequency range 10-7 to 6 10-3 Hz. Both p and g mode oscillations will be investigated, with the emphasis on the low order long period waves which penetrate the solar core. The instrument employs an extension to space of the proven ground-based technique for measuring the mean line-of-sight velocity of the viewed solar surface. A sodium vapour resonance filter is used in a longitudinal magnetic field to sample the two wings of the solar absorption line. The addition of a small modulating field component enables the slope of the wings to be measured. This provides not only an internal calibration of the instrument sensitivity, but also opens the possibility to recognise, and correct for, the solar background signal produced by the effects of solar magnetically active regions. The use of an additional rotating polariser enables also measurement of the mean solar line-of sight magnetic field. (see final Section)