A new high precision absolute gravimeter

The accurate measurement of local gravitational acceleration (g, normal value 9.81 m/s(2)) is a key approach to the exploration of the gravitational field of the earth, and it has been applied in metrology, geodesy, geodynamics, seismology, and mineral exploration. With the plan of '2000 national gravity network' and 'China crustal motion observation network', highly precision gravity measurement will be increasingly required in the long term. In order to further investigate the possible systematic error of current absolute gravity measurement, and satisfy various requirements for highly accurate gravity observation, the T-1 absolute gravimeter prototype is designed and built at Tsinghua University. The T-I instrument adopts the classic free-fall scheme to measure the g value by tracking the trajectory of a free-fall motion in vacuum with a laser interferometer. The T-1 absolute gravimeter consists of several sub-systems: high vacuum free-fall chamber, compact laser interferometer, low-frequency vertical vibration isolation, high-speed signal acquisition system, instrument control and data processing system. The length and time standards used in g measurement are stabilized He-Ne laser and rubidium atomic clock respectively. It's well known that both of the two standards have an uncertainty of less than 1 x 10(-9). The practical gravity measurement results of T-1 absolute gravimeter give a standard deviation of the mean of less than 1 mu Gal (1 mu Gal = 10(-8) m/s(2)) in a typical observation within 12 h. And the reproducibility of less than 3 mu Gal is verified by repeatedly measuring gravity. The T-1 absolute gravimeter is a promising instrument by which highly accurate gravity measurement at a microgal level can be realized, and it is hopefully to be used in many research and application areas.