An absolute gravity meter measures the acceleration of gravity directly by observing the free-fall of a reflective corner cube in a vacuum.
Position is measured with an iodine laser and time with an atomic clock.
The value for g is thus determined in terms of atomic constants that are assumed stable over periods of centuries. This is in contrast to a spring-based relative meters which observe gravity differences in time or place. This instrument is available to university investigators on request. It comes with an operator whose salary is paid by an NSF Earth Science facilities grant. FG5 investigators instruction manual Bibliography on absolute-g measurement.