The FG5 type absolute gravimeter is at present the most precise and most widely used instrument for absolute gravity measurements. It is a product of the Micro-g Solutions Inc., Erie, Colorado, USA (nowadays Micro-g LaCoste Inc., Lafayette, Colorado, USA, http://www.microglacoste.com). The FG5 No. 215 was acquired in 2001 by the Center of Earth Dynamics Research, Geodetic Observatory Pecný, RIGTC.
FG5 is a high precision, high accuracy, transportable instrument that measures acceleration due to gravity. A concept of operation is simple. A test mass is dropped vertically by a mechanical device inside a vacuum chamber and then allowed to fall a distance of about 20 cm. The FG5 uses a laser interferometer to accurately determine the position of the free-falling test mass accelerated due to gravity. The acceleration of the test mass is calculated directly from the measured trajectory. The laser interferometer generates optical interference fringes as test mass falls. The fringes are counted and timed with an atomic clock to obtain precice time and distance pairs. These data are fit to a parabolic trajectory to get a measured value of g. This method is absolute because the determination is purely metrological and relies on standards of length and time.
Several reductions have to be applied to obtain gravity acceleration from the measured free-fall acceleration, namely the reductions for:
- gravimetric earth tides,
- polar motion,
- air pressure.
The FG5 No. 215 consists of a:
- dropping chamber (including cart/drag-free chamber, test mass, drive mechanism, service ring) with a vacuum
maintained by an ion pump at a nominal pressure of 10-4 Pa,
- interferometer (bulk type), which houses the standard of length, iodine stabilized He-Ne laser, 633 nm, and optics for splitting, directing and recombining the laser beams,
- superspring is an active long-period seismometer (free periods 30-60 sec) used to compensate small ground motions of the meter,
- system controller (computer) allows a flexible user interface, controls the system, acquires the data, analyzes data and stores the result
- electronics provides high accuracy timing (via rubidium atomic clock) and system servo control (superspring, laser, free-fall, etc.).
The FG5 No. 215 is the Czech national standard for gravity acceleration with the following main metrological characteristics:
Gravity Range: 9.77 m×s-2 ... 9.84 m×s-2,
Reproducibility: 1.5×10-8 m×s-2,
Standard Uncertainty: 2.5×10-8 m×s-2.