Study of the Time Stability of Tides Using a Long Term (1973-2011) Gravity Record at Strasbourg, France

International audience; The Gravimetric Observatory J9 located near Strasbourg (France), has a long history of recording solid Earth tides. We present here one of the longest available gravity records (1973-2011) from three different instruments operating at the J9 station: a LaCoste-Romberg spring gravimeter (ET005) from 1973 to 1985, a GWR (TT70-T005) superconducting gravimeter from 1987 to 1996 and a GWR compact superconducting gravimeter (C026) since 1996. We give the results of a tidal analysis performed with ETERNA on these data sets. We show the improvement in terms of noise levels from the oldest type to the newest one and check the time stability of the observed tides for every instrument. This stability is also dependent on the stability of the scale factor of the cryogenic gravimeter which is derived from a direct comparison with repeated absolute gravity measurements. The response of the three gravimeters to the atmospheric pressure changes is also checked. With all these results we found that the improvement from a mechanical spring gravimeter to a modern cryogenic instrument is obvious. And also that the improvement from the T005 SG to the C026 SG is less impressive than from L&R to the T005 SG, but still notable.