FG5 Absolute Gravity Meter
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Measurement of absolute gravity is conceptually simple (recall the anecdote of Newton discovering the law of gravitation by observing a falling apple). However, accurate free-fall measurement of the acceleration of gravity is technologically challenging.
An absolute gravity meter is essentially an optical laser interferometer which measures the free-fall acceleration of a retroreflector in a vacuum. (See Schematic of absolute meter.) The measurement is directly referenced to atomic standards of length and time. The laser is stabilized to hyperfine optical absorption peaks in an iodide absorption cell and the time base is locked to hyperfine microwave transitions in a rubidium vapor cell.
Data obtained from FG5 instruments (See figure) confirm both the improved repeatability of the FG5 compared to earlier types of absolute gravimeters and the utility of the instrument at measuring very long period signals like polar motion. The dynamic range and absence of drift in an absolute gravimeter compared to relative gravimeters with similar precision, permit geophysical studies with no degradation in accuracy in experiments that have durations of many decades, spatial scales greater than 1000 km, or elevation ranges exceeding 3 km.
The current instrumental accuracy estimate for the FG5 gravity meter is 1.1 microGal, or about one part per billion of the gravitational acceleration (g) at the Earth's surface. This is approximately the change in g that would be expected from a 3 mm change in vertical position.