The FG5 absolute gravimeter

The gravimeter is complex mix of optics, atomic physics, and clockwork, both mechanical and electronic (Niebauer et al. 1995). The heart of the mechanical unit operates within a hard vacuum. A cube-corner reflector drops in free-fall a distance of 20 cm, its position detected both by an iodine stabilized interferometer, and by a photocell that accelerates an elevator ahead of its downward path, removing stray remaining molecules from interfering with its descent. After its fraction-of-a second journey the elevator catches the reflector and transports it back to its starting point from where it is ready for another drop. The cycle has a duration of roughly 1 s. Electronic circuitry sequences every move, timing the zero-crossings of the interference fringes to sub-nano second accuracy, recording the data and undertaking real-time statistical fits to the acceleration curves generated.

The figure shows the principle of operation of the gravimeter. Acceleration is the change of velocity of the falling reflector per second. Velocity is determined by the time taken for each optical fringe to pass the detector. This is, of course, a simple view of the measurement process, but it permits one to understand how the two atomic constants (laser wavelength and atomic time) yield a value for acceleration that is independent of the physical properties of the instrument.

In practice, the mechanical parts and vacuum-seals need attention, the stability of the iodine laser requires must be monitored, the voltage stabilization circuitry, motors, gears, and belts need occasional repairs and replacement. Preventive maintenance of the instrument is routine in that we know what parts should be replaced after every one or two months of field work. A spare-parts kit accompanies all field experiments to anticipate these items.