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The FG5 Measurement System

Schematic Diagram

Principles of Operation

The FG5 operates by using the free-fall method. An object is dropped inside a vacuum chamber (called the dropping chamber). The descent of the freely-falling object is monitored very accurately using a laser interferometer. Click here for some diagrams of the process.

The free-fall trajectory of the dropped object is referenced to a very stable active-spring system called a Superspring. The Superspring provides seismic-isolation for the reference optic to improve the noise performance of the FG5

The optical fringes generated in the interferometer provide a very accurate distance measuremen t system that can be traced to absolute wavelength standards. Very accurate and precise timing of the occurence of these optical fringes is done using an atomic rubidium clock that is also referenced to absolute standards.

The fact that the measurement is directly tied to absolute standards kept at all of the national and international laboratories around the world is the reason why the FG5 is an ABSOLUTE GRAVIMETER. Absolute standards of length and time provide the means to achieve a calibrated gravity value that does not drift over time.