A NEW-GENERATION OF ABSOLUTE GRAVIMETERS

We describe the design improvements incorporated in a new generation o f absolute gravimeters, the FG5. A vertically oriented (in-line) inter ferometer design is used to remove the influence of floor vibration an d tilt on the optical path length. The interferometer uses an iodine-s tabilized laser as a primary length standard, with circuitry for autom atic peak detection and locking. The seismic isolation system is an ac tive long-period seismometer (Super Spring). The new design has improv ed passive isolation and thermal drift characteristics over previous s ystems, Programming flexibility and control of the test mass trajector y have been improved. The computer system has also improved real-time analysis and system capability. The FG5 instrument has a higher level of robustness, reliability and ease of use. These design advances have led to an instrumental uncertainty estimate of 1,1 x 10(-8) m.s(-2) ( 1,0 mu Gal). Instrument agreement among nine similar devices is 1,8 mu Gal and observations under optimal conditions exhibit standard deviat ions of 5 mu Gal to 8 mu Gal.