On the determination of verticality and Eotvos effects in absolute gravimetry

We describe a new method for the accurate alignment of verticality in absolute gravimeters with macroscopic objects that allows simultaneous determination of the Eotvos effect. The method is based on measurements of lateral motion of freely falling objects by a position sensitive detector, and has been used experimentally for one FG5 and one FG5X gravimeter. As for the verticality effect, we show that the current method of verticality alignment by using the reflection off the geopotential surface placed outside the vacuum dropping chamber is insufficient, and that systematic errors exceeding 1 mu Gal might be expected-as demonstrated for FG5X-251. On the other hand, the new method of verticality alignment, based on minimizing the measured lateral accelerations, allows the reduction of systematic errors to about 0.1 mu Gal. Moreover, the measurements underlying the alignment can be used for determination of the Eotvos effect, which depends on the lateral velocity of the falling object in the east-west direction. The results for two FG5(X) gravimeters are presented, with Eotvos effects below 1 mu Gal that, however, cause biased gravity values so that relevant corrections should be applied. The method we are presenting does not allow determination of the verticality and Eotvos effects for particular drops. To do so, optical paths in the interferometer should have to be re-designed. Nevertheless, it is possible to determine both effects with a standard uncertainty better than 0.2 mu Gal using less than 50 drops carried out in a non-measurement mode.