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Absolute Gravimeter FG5

The FG5 absolute gravimeter is a high precision, high accuracy, transportable instrument that measures the vertical acceleration of gravity (g).
The operation of the FG5 is simple in concept. 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 as it accelerates due to gravity. The acceleration of the test mass is calculated directly from the measured trajectory.

The laser interferometer generates optical interference fringes as the test mass falls. The fringes are counted and timed with an atomic clock to obtain precise time and distance pairs. These data are fit to a parabolic trajectory to give a measured value for g.

This method of measuring gravity is called "absolute" because the determination is purely metrological and relies on standards of length and time.The distance scale is given by a frequency stabilized helium neon (HeNe) laser used in the interferometer.

A rubidium atomic time-base provides the time scale used for the accurate timing. The value of gravity obtained with FG5 can be used without the loop reductions and drift corrections normally required when using relative instrumentation.

Collecting data since 2007, the ASI/CGS takes part to several gravity measurements campaigns.


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