We describe the design improvements incorporated in a new generation of absolute gravimeters, the FG5. A vertically oriented (in-line) interferometer design is used to remove the influence of floor vibration and tilt on the optical path length. The interferometer uses an iodine-stabilized laser as a primary length standard, with circuitry for automatic peak detection and locking. The seismic isolation system is an active long-period seismometer (Super Spring). The new design has improved passive isolation and thermal drift characteristics over previous systems. Programming flexibility and control of the test mass trajectory have been improved. The computer system has also improved real-time analysis and system capability. The FG5 instrument has a higher level of robustness, reliability and ease of use. These design advances have led to an instrumental uncertainty estimate of 1,1 × 10-8 m s-2 (1,0 μGal). Instrument agreement among nine similar devices is 1,8 μGal and observations under optimal conditions exhibit standard deviations of 5 μGal to 8 μGal.

https://iopscience.iop.org/article/10.1088/0026-1394/32/3/004/pdf

A new generation of absolute gravimeters

[1] Redistribution of air masses due to atmospheric circulation causes loading deformation of the Earth's crust, which can be as large as 20 mm for the vertical component and 3 mm for horizontal components. Rigorous computation of site displacements caused by pressure loading requires knowledge of the surface pressure field over the entire Earth surface. A procedure for computing three-dimensional displacements of geodetic sites of interest using a 6 hourly pressure field from the National Centers for Environmental Prediction numerical weather models and the Ponte and Ray [2002] model of atmospheric tides is presented. We investigated possible error sources and found that the errors of our pressure loading time series are below the 15% level. We validated our model by estimating the admittance factors of the pressure loading time series using a data set of 3.5 million very long baseline interferometry observations from 1980 to 2002. The admittance factors averaged over all sites are 0.95 ± 0.02 for the vertical displacement and 1.00 ± 0.07 for the horizontal displacements. For the first time, horizontal displacements caused by atmospheric pressure loading have been detected. The closeness of these admittance factors to unity allows us to conclude that on average, our model quantitatively agrees with the observations within the error budget of the model. At the same time we found that the model is not accurate for several stations that are near a coast or in mountain regions. We conclude that our model is suitable for routine data reduction of space geodesy observations.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003JB002500

Study of the atmospheric pressure loading signal in very long baseline interferometry observations

: This document is intended to define the standard reference systems realized by the International Earth Rotation Service (IERS) and the models and procedures used for this purpose. It is a continuation of the series of documents begun with the Project MERIT Standards (Melbourne et al., 1983) and continued with the IERS Standards (McCarthy, 1989; McCarthy, 1992) and IERS Conventions (McCarthy, 1996). The current issue of the IERS Conventions is called the IERS Conventions (2003). When referenced in recommendations and articles published in past years, this document may have been referred to as the IERS Conventions (2000). All of the products of the IERS may be considered to be consistent with the description in this document. If contributors to the IERS do not fully comply with these guidelines, they will carefully identify the exceptions. In these cases, the contributor provides an assessment of the elects of the departures from the conventions so that its results can be referred to the IERS Reference Systems. Contributors may use models equivalent to those specified herein. Products obtained with different observing methods have varying sensitivity to the adopted standards and reference systems, but no attempt has been made in this document to assess this sensitivity. The reference systems and procedures of the IERS are based on the resolutions of international scientific unions. The celestial system is based on IAU (International Astronomical Union) Resolution A4 (1991). It was officially initiated and named by IAU Resolution B2 (1997) and its definition was further refined by IAU Resolution B1 (2000). The terrestrial system is based on IUGG Resolution 2 (1991). The transformation between celestial and terrestrial systems is based on IAU Resolution B1 (2000).

/pdf/iers-conventions-2003-34pcce8ynn.pdf

IERS Conventions (2003)

Redistribution of air masses due to atmospheric circulation causes loading deformation of the Earth's crust which can be as large as 20 mm for the vertical component and 3 mm for horizontal components. Rigorous computation of site displacements caused by pressure loading requires knowledge of the surface pressure field over the entire Earth surface. A procedure for computing 3-D displacements of geodetic sites of interest using a 6-hourly pressure field from the NCEP numerical weather models and the Ponte and Ray [2002] model of atmospheric tides is presented. We investigated possible error sources and found that the errors of our pressure loading time series are below the 15% level. We validated our model by estimating the admittance factors of the pressure loading time series using a dataset of 3.5 million VLBI observations from 1980 to 2002. The admittance factors averaged over all sites are 0.95 -+ 0.02 for the vertical displacement and 1.00 -+ 0.07 for the horizontal displacements. For the first time horizontal displacements caused by atmospheric pressure loading have been detected. The closeness of these admittance factors to unity allows us to conclude that on average our model quantitatively agrees with the observations within the error budget of the model. At the same time we found that the model is not accurate for several stations which are near a coast or in mountain regions. We conclude that our model is suitable for routine data reduction of space geodesy observations.

Study of the atmospheric pressure loading signal in VLBI observations

SUMMARY
Gravity Green's functions for a column load of a model atmosphere on a spherical, elastic Earth are presented and they are used to evaluate the contribution of global atmospheric pressure variations to local gravity. The Green's functions are found to be relatively insensitive to the details of the model atmosphere, but they are dependent on the temperature at the base of the column, and on the relative height difference between the base of the column and the gravity station. The total signal that global pressure systems contribute to gravity is about 30 μgal, of which about 90 per cent is produced by the atmosphere within 50 km of the gravity station. A zone between 50 and 1000km from the gravity station contributes a couple of μgal, as does the remainder of the globe. This pattern, the coherence scale of pressure fluctuations, the time and spatial scales appropriate to the hydrostatic approximation, and the distance of the gravity station from the oceans, suggest a division of the globe into local, regional, and global zones. Data requirements, processing details, and the reliability of the computed signal are different in each zone. The local zone is within about 50 km of the gravity station. Within this zone pressure changes rapidly in time, but is spatially coherent, so that hourly observations of pressure and temperature at the gravity site alone are sufficient to compute an accurate correction, except when a front is passing through. The regional zone extends from the edge of the local zone to between several hundred and a thousand kilometres. The signal from this zone is small and is only weakly coherent with the signal from the central zone, so that a rather sparse array of hourly samples of pressure and temperature are required. The gravity signal from the global zone can reach about a μgal. It varies on a time-scale of days, and is influenced by the response of the oceans to pressure variations. Previously reported observations that the admittance between local pressure and gravity residuals depends on epoch, frequency, or site, are most probably due to incorrect modelling. A proper local, regional, temperature, and global correction can adequately account for the gravity signal from the atmosphere to within a few tens of ngal in the diurnal band, and about 100 ngal in the days to seasonal band, except during extreme weather conditions. The application of the local correction lowers the power spectral density of the gravity residuals in every band from seasonal to hourly. The regional, global, and temperature corrections lower the residual noise in the seasonal and synoptic bands, but are not consistently effective at periods less than about half a day.

/pdf/atmospheric-pressure-and-gravity-1nns7xhg19.pdf

Atmospheric pressure and gravity

/pdf/static-deformations-and-gravity-changes-at-the-earth-s-qmir5upzy4.pdf

J. Zschau

Papers

Static deformations and gravity changes at the earth's surface due to atmospheric loading