We have presented a detailed numerical simulation of the observations with
GOLF experiment, devoped to check the performances of the instrument which
has not been tested with the Sun before the launch. This simulation contains
a good representation of the instrumental response, including its kinematics
respect to the Sun, and a modelisation of the velocity fields and physical
effects that can contribute to the solar velocity spectrum, mainly, in the
frequency band of interest for GOLF, from 
Hz to 6 mHz.
Two data sets, each one year long, have been generated based on how the Sun's activity looks like in 1986, a year of minimum, and 1991, a year of maximum solar activity. With this choice we cover the two extremal values concerning the magnetic activity of the Sun and could be a representation of what GOLF can be facing during its lifetime.
We have computed the theoretical velocity sensitivity of GOLF over the solar
disk for the blue and red wing measurements. A 
of sensitivity
difference has been found between the two wings, and seems to be nearly
constant for any epoch of the year. In the less favorable case, in April,
there is a reduction of the sensitivity less than a 3%, demonstrating
that the selected working points are ideal.
We have also applied a calibration of the signal in velocity. We compare the
values obtained by this method with the ones for the IRIS instruments
showing a general agreement between the two. The main difference found (more
than 
) has been with the coefficient leading to the calibration of the
velocity signal. This can be explained due to the different slope of the
solar lines, where the working points of the two instruments are located
(GOLF 
IRIS), and to a higher extent due to the fact that GOLF uses the
3 components of the sodium doublet (D1 and D2a,b).
The power spectrum of the two data sets available are presented, as well as the best fitted B.S.V.N.S. obtained from the earth-based instruments. The difference of one order of magnitude between the real and the simulated spectra, in the low frequency range of the spectrum, is well understood. The simulation approaches the lev that we have from space-based observations of integral sunlight with the Sun.
Acknowledgements
We wish to thank F. Hill for sending the Hathaway's program. We are also indebted to P. Boumier and P.L. Pallé for their useful comments. This work has been made possible thanks to the financial support of the Spanish CICYT under grants ESP90-0969 and ESP91-1441.