The first data of a SOHO-GOLF type instrument are presented. The slopes of the solar sodium lines profiles D1 and D2 and their variations with the excursion of the working point are shown. Two methods of determining the oscillation velocity are compared, one usually used with two-point spectrometers and one using the four-point information. It is shown that the second method gives spectra which are significantly less noisy in the very low frequency range. For higher frequencies, terrestrial atmospheric fluctuations alter the measurement, and as far as ground based instruments are concerned, the four-point method loses its efficiency if the magnetic modulation commutation time is not significantly reduced.