Gravity Reference Systems

• Introduction
• Gravimeter
• German Gravity Reference Systems
• European Gravity Reference Systems
• Global Gravity Reference Systems

Gravimeter

Absolute gravimeter

Absolute gravimeters A10 left and FG5 right (click to enlarge)

The free fall of a test mass is recorded with high accuracy. In order to minimize non-gravitational disturbing influences, such as air drag, buoyancy etc. the test mass moves in an evacuated chamber. The "absolute" determination of the gravitational acceleration results from the use of physical standards of highest accuracy: A rubidium standard for time measurement and an iodine-stabilized laser for length measurement. By means of the absolute gravimeter FG5 (manufacturer: Micro-g LaCoste Inc., USA), which represents the currently best possible technical development status, measuring accuracies of ±20…30 nm/s² (±2…3 µGal) can be obtained under favorable ambient conditions. This corresponds to a relative accuracy of ±2…3 x 10^-9. BKG applies the absolute gravimeter A10 with a measuring accuracy of ±100 nm/s² made by the same manufacturer for field measurements.

Absolute gravimeter measurements complete the geodetic reference systems

• by the setup of an absolute gravimetric standard in an earthbound reference system
• and allow the independent detection of vertical point displacements or mass variations, caused by e.g. tectonically induced movements of the earth’s crust, post-glacial uplifts or changes of the sea level.

In this context special emphasis is given to the combination with levelling measurements and geodetic space techniques like VLBI - Very Long Baseline Interferometry, SLR – Satellite Laser Ranging, and GNSS – Global Navigation Satellite System.

Superconducting Gravimeters

Superconducting Gravimeter SG30 in Wettzell fundamental station (click to enlarge)

Gravimeters record temporal changes of the earth’s gravity field. Whereas in conventional relative gravimeters the gravity acceleration is measured by means of a test mass suspended on a spring, in the case of the Superconducting Gravimeter (SG) the very stable magnetic field of a superconducting coil serves as reference force. The test mass is a superconducting hollow sphere the position of which is kept constant in a control system. The electrical current in this control system is continuously measured and digitized with high-resolution. Superconducting Gravimeters are characterized by a very high resolution of the measuring signal (<0.1nm/s²) and a small instrumental zero drift. By using superconducting coils, in which - after an initialization - currents flow continuously and constantly, a long-term stability of 0 and 50 nm/s² per year can be reached.This way, the possibility of recording a broad spectral range of the acceleration changes with high resolution opens up. The spectrum reaches from the seismic eigen-modes of the earth with periods lenghts from 10 minutes via the earth tides with predominantly half-day and full-day periods up to the variation of the centrifugal acceleration as a result of the polar motion with periods of 365 and 435 days (Chandler period) and to long-period tides with a period interval of 18.6 years.  

By the observation of temporal gravity field variations the following objectives are pursued: 

• High-precision acquisition of the tides of the earth body for deriving elastic parameters
• Investigation of attraction and deformation effects as a result of oceanic, atmospheric and hydrological mass displacements
• Acquisition of gravity variations due to polar motion
• Detection of seismically induced oscillations of the earth
• Control of gravity standards by combining and comparing with absolute gravity measurements.

Calibration platform

Spindle block of the Frankfurt Gravimeter Calibration Platform (click to enlarge)

The "Frankfurt Gravimeter Calibration Platform" developed at BKG uses artificially generated accelerations for the calibration of relative gravimeters and was specially designed for the test of Superconducting Gravimeters. 

The calibration system consists of three spindle blocks operated by stepping motors, by means of which the gravimeter to be tested can be synchronously lifted and lowered in a sinusoidal movement. In dependence on the amplitude of motion and its periodicity, artificial accelerations are superimposed to the earth’s normal gravitational force. Since the amount of acceleration and the period interval are known, the calibration factor and the frequency transfer function of the Superconducting Gravimeter can be derived.