Seasonal effect on vertical positioning by Satellite Laser Ranging and GPS on Absolute Gravity at the OCA geodetic station, Grasse, France

We present a comparison of the vertical displacement monitored by independent techniques at the geodetic observatory of Grasse (France). Both Satellite Laser Ranging and Global Positioning System (GPS) vertical position time-series over the period 1998–2003 show a prominent annual signal with a magnitude of 5–6 mm and reaching a maximum every year in July. Results from 14 absolute gravity measurements are also discussed. We investigate the possible origin of the observed signal by comparing it with predictions from various local and regional contributions. GPS results from a local network indicate that the periodic annual elastic deformation of the ∼1270 m high karstic plateau due to local water storage loading does not exceed 1–2 mm. In contrast, a combination of global model prediction for atmospheric and hydrological loading explains more than 70 per cent of the annual and semi-annual observed signals.