Absolute Gravity

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The gravity laboratory at Onsala Space Observatory provides two platforms for visiting gravimeters. The Nordic Geodetic Commission coordinates observation campaigns with Absolute gravimeters in order to maintain the reference system for gravity in the Northern countries. A major subject for study and the reason for repeating the campaigns is the glacial isostatic rebound.

The superconducting gravimeter at Onsala contributes an observing site for Absolute Gravity (AG) campaigns where the effects causing variations of gravity over time are well known. Owing to the stability of the superconducting sensor, perturbations can be modelled with well-based parameters and help reduce the AG records. 

A typical AG campaign consists of a number of so-called projects, an installation of the AG metre, between which orientation towards North is changed to South and the instrument is moved between platform points.

The basic measurement of an AG consists of a drop of a prism in free fall. A precision laser together with a Rb clock measures the flight path over time. The parabola-like time-position curve is fitted and the g-value is obtained. The typical reproducibility of a set of drops obtained during, say, 12 hours, is at the 20 nm/s2 level. So the gravity value is obtained with nine significant decimals.  

Below you can see a few illustrating figures.

The Sultans of Absolute Gravity during a two-instrument intercomparison at Onsala end of May, 2014.
From the left: Ludger Timmen, Leibnitz University Hannover, their FG5X-220, Swedish Land Survey's FG5-233 Andreas Engfeldt, Lantmäteriet,
Manuel Schilling (Hannover); the control unit for FG5-233.
An FG5 manufactured by Micro'g LaCoste, Boulder, Colorado Sketch showing the dropping chamber, the "Superspring" suspension and the path of the laser. The laser ray is reflected in the falling prism. An interferometer produces a fast sequence of fringes that is picked up by a photo detector. The superspring is a feedback controlled spring that responds to frame accelerations as if it were infinitely long. It thus counteracts the microseismic noise and thus isolates the dropping chamber mechanically from the monument. The prism that is dropped sits inside a cage that follows the prism in the fall by feedback control in order to avoid drag of the prism against the little remainder of air in the vacuum chamber. When the drop is completed, the cage is used to transport the prism back to its upper position, ready for the next drop. It is feasible to run a long series of drops at four or five seconds interval.