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20 (FG5) or 33(FG5-X) centimetres. The dropping chamber is
maintained at vacuum by use of an ion pump and sits upon its own
tripod, to isolate it from the other components of the FG5, most
importantly from the super-spring component, as the vibrations

created in the movement of the cart within the chamber are significant,
but even so the recoil from the floor of from the tripod can be easily
detected in the individual drop residuals.

Below the dropping chamber and central to the design of the FG-
5 is a modified Mach-Zender interferometer. A collimated continuous
wave 633nm laser beam enters into the side of the interferometer and
is split into two optical arms, one arm straight through the beam

splitter to generate a reference arm whilst the other is reflected up into
the dropping chamber to reflect off of the corner cube before being
reflected down to another corner cube within the ‘super-spring’

assembly which is then recombined inside the interferometer with the
reference arm of light. These recombined arms of laser light generate
the interference fringes which form the basis of the determination of

gravity.
To provide an inertial reference frame and vibration isolation
from the high frequency natural micro-seismic activity of the planet,

the FG-5 (Figure 13.2.) has a unique super-spring component (or
assembly) which is coupled to the interferometer above it. The free-
fall trajectory of the dropped object is referenced to a very stable
activespring system called a “Superspring”. The Superspring provides

seismic-isolation for the reference optic to improve the noise
performance of the FG5.

Figure 13.2. Image courtesy of Micro-G LaCoste
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