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Concept and History of the FG5
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1. Concept and History of the FG5
1.1. The FG5 Absolute Gravimeter
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 20cm. 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 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 the FG5 can be
used without the loop reductions and drift corrections normally required when
using relative instrumentation.
1.2. HISTORY
The FG5 is a new generation of absolute gravimeter based on technology
developed over the last thirty years by Dr. James Faller of the National Institute
of Standards and Technology (NIST), and his colleagues. Beginning with a
white-light-fringe interferometric system built in 1962, Faller and coworkers have
continuously improved the designs of the instruments. The most recent
predecessors of the FG5 was the series of six JILAg gravimeters, built in 1985 at
the Joint Institute of Laboratory Astrophysics (JILA), with support from NIST,
the Defense Mapping Agency (DMA), the National Oceanographic and
Atmospheric Administration (NOAA), the Canadian Geophysical Survey (GSC),
the University of Hanover Institute for Earth Measurement, Germany, the
Finnish Geodetic Institute, Finland, and the University of Vienna Institute for
Metrology and Geophysics, Austria.
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