INTRACOMPARISON TESTS OF THE FG5 ABSOLUTE GRAVITY METERS

The FG5 series absolute gravimeters have an estimated instrumental acc uracy of 1-2 muGal (1-2 x 10(-8) ms-2). A number of instrument compari sons were conducted with six FG5 instruments over a period of 9 months ; the predecessor series JILA4 instrument was also used. The standard deviation of mean g values (averaged over 24-48 h), as observed by dif ferent instruments, is 1.8 muGal. Observations taken within 48 h typic ally agree within 2 muGal, and the maximum observed disagreement is 6 muGal for two observations taken 37 days apart. Individual data sets o f 100-250 measurements (16-42 min duration) show minimum standard devi ations of 7-8 muGal, placing an upper bound on instrument noise. The d ata are not corrected for local water-table effects or regional atmosp heric loading, and thus the observed repeatability is probably an unde restimate of the instrument accuracy. The observed instrument agreemen t is consistent with the instrumental accuracy estimate. Ignoring defi ciencies in modeling environmental gravity signals, this accuracy leve l should allow resolution of vertical motions at the sub-centimeter le vel.