M. van Ruymbeke

A calibration system for gravimeters using a sinusoidal acceleration resulting from a vertical periodic movement

An inertial force is induced by the oscillating vertical displacementA sin ωt of a platform supporting a feedback LaCoste Romberg gravimeter.The resulting accelerationdg is proportional to the square of the frequency and allows to perform an absolute calibration of the instrument.The effects of the vertical gradient and of the residual tilt are eliminated by operating at different frequencies.After the experiment with a first prototype, a new platform based on a different mechanical principle has been developed.The relative precision is at least 0.25 % as confirmed by the study of a 6 months tidal record. Some applications are described.

Tidal loading along a profile Europe-East Africa-South Asia-Australia and the Pacific Ocean

Abstract A knowledge of the vertical component of the oceanic tidal load to a precision of at least one microgal is essential for the geophysical exploitation of the high-precision absolute and differential gravity measurements which are being made at ground level and in deep boreholes. On the other hand the ocean load and attraction signal contained in Earth tide gravity measurements can be extracted with a precision which is sufficient to characterize the behaviour of the oceanic tides in different basins and this provides a check of the validity of the presently proposed cotidal maps. The tidal gravity profiles made since 1971 from Europe to Polynesia, through East Africa, Asia and Australia, with correctly intercalibrated gravimeters, comprise information from 91 tidal gravity stations which is used in this paper with this goal in mind. A discussion of all possible sources of error is presented which shows that at the level of 0.5 μgal the observed effects cannot be ascribed to computational or instrumental errors. Cotidal maps which generate computed loads in agreement with the Earth tide gravity measurements over a sufficiently broad area can be used with confidence as a working standard to apply tidal corrections to high-precision measurements made by using new techniques in geodesy, geophysics and geodynamics, satellite altimetry, very long baseline interferometry, Moon and satellite laser ranging and absolute gravity. The recent cotidal maps calculated by Schwiderski for satellite altimetry reductions agree very well with land-based gravimeter observations of the diurnal components of the tides (O1, K1 and P1 waves) but his semi-diurnal component maps (M2, S2 and N2 waves) strangely appear less satisfactory in some large areas. The maps of Hendershott and Parke give good results in several large areas but not everywhere. More detailed investigations are needed not only for several coastal stations but mainly in the Himalayas.

Transcontinental tidal transect: European Atlantic coast-Southern Siberia-Russian Pacific coast

The paper presents results of measurements with digital tidal LaCoste-Romberg gravimeters on the European Atlantic coast-Southern Siberia-Russian Pacific coast transect in 1995–2005. The transect includes four West European (Chizé, Ménesplet, Mordelles, and Wikle), two South Siberian (Klyuchi and Talaya), and two Far Eastern (Zabakalskoe and Yuzhno-Sakhalinsk) stations. Gravimetric measurements at the Talaya station (SW Baikal rift zone) are supplemented by long-term laser extensometer observations. The position of the stations within the rectangle (45°–55°N, 0.4°–142°E) allows one to assess existing tidal strain models (WD93 and DDW99) and various ocean tide models (SCW80, CSR3, FES95, ORI96, CSR4, FES02, GOT00, NAO99, and TPX06). Data of intracontinental stations (with a small ocean effect at distances of 2000–3000 km) agree well with the DDW99 tidal strain model (with regard to the mantle viscosity). The uncertainty of digital tidal gravity measurements is 0.25%. Results of laser extensometer measurements are at the same accuracy level. Then, the Love and Shida numbers calculated at midlatitudes of the intracontinental zone of Eurasia from combined data are h = 0.6077 ± 0.0008, k = 0.3014 ± 0.0001, and l = 0.0839 ± 0.0001. The analysis of results of Pacific and Atlantic stations located at distances of 30–300 km from the ocean showed that the FES02, CSR4, GOT00, NAO99, and TPX06 ocean tide models are preferable.

Main results of the PICARD mission

PICARD is a mission devoted to solar variability observations through imagery and radiometric measurements. The main goal is to provide data for scientific investigation first in the area of solar physics, and second in the assessment of the influence of the solar variability on the Earth climate variability. PICARD contains a double program with in-space and on-ground measurements. The PICARD spacecraft was launched on June 15, 2010, commissioned in-flight in October of the same year and was retired in April 2014. The PICARD ground-based observatory is operational since May 2011. We shall give a short overview of the PICARD instrumentation. New estimates of the absolute values of the total solar irradiance, of the solar spectral irradiance at typical wavelengths, and of the solar oblateness will be given. We will also report about helioseismic studies. Finally, we will present our current results about solar radius variations after six years of solar observation.

TIDAL GRAVITY MEASUREMENTS IN CHINA

The results of tidal gravity measurements obtained at nine places in China, Beijing, Wuhan, Shanghai, Kunming, Lanzhou, Urumqi, Guangzhou, Shenyang and Qingdao, are discussed and compared to an earth model with liquid core, covered with oceans (using Schwiderski and different near shore cotidal maps). Special attention is given to the calibration problems. As a conclusion, we show that the tidal gravity effects are successfully modelized with a precision of 0.6 μgal.