Improvements of the MPG-2 transportable absolute ballistic gravimeter

Abstract

The MPG-2 (Max-Planck-Gravimeter) is a transportable absolute gravimeter built on a classical free-fall scheme to measure the local gravity value. With significant improvements and further investigations in recent years, the standard deviation of the mean for a typical measurement over 12 h to 24 h is 1.0 µGal to 3.0 µGal (1 µGal = 10−8 m s−2), and the combined standard uncertainty is estimated to be less than 10 µGal. The major improvements include the new interferometer design and alignment, longer drop length, reduced recoil effects and demagnetization of the falling body. The revised uncertainty budget and new measurement results of MPG-2 are reported. The results of observations at the reference gravity station Bad Homburg confirmed the revised uncertainty budget.

Frequency-domain analysis of absolute gravimeters

“…The floor recoil appears at the start of free-fall in the corner-cube absolute gravimeters. It is widely discussed in the literature [10,11,[13][14][15][16][17][18][19]. Such a perturbation produces a systematic measurement error.…”

“…One of the dominant sources of measurement error in absolute gravimetry is ground vibrations, which affect the reference reflector, alter the measured distance intervals and distort the computed g value. Starting from the earliest measurements of g based on optical interferometry [4][5][6][7][8], vibration perturbations have always been considered [9][10][11][12][13][14][15][16][17][18][19]. The problem is also unavoidable in atom absolute gravimeters, which use an atom interferometer to measure the free-fall acceleration of the atom cloud [20][21][22][23][24].…”

Comparison of three digital fringe signal processing methods in a ballistic free-fall absolute gravimeter

“…Figure 9 shows typical least-squares residuals in an individual drop in this measurement session. Residual oscillations up to ±1 nm are caused by the recoil effect in the MPG-2 gravimeter [15,32,33], which is the common / Figure 10. Comparison of three methods in a one-day measurement set.…”

“…Finally, all the methods will agree to within 0.5 µGal, if only the same data location (EST) is considered. Since the result of any method could be in principle reported, an associated standard uncertainty of 0.5 µGal is now included in the revised uncertainty budget of the MPG-2 gravimeter as a contribution from the digitization of the fringe signal [33]. For the same time, the resampling procedure gives the statistically significant shift of the calculated gravity value by about 1 µGal (figure 10).…”

“…Such bias could be even larger for the shorter drop durations (figure 7, bottom part). This difference between the two results obtained by the same method of fringe signal processing is now covered in the revised uncertainty budget of the MPG-2 gravimeter by the contribution from the recoil effect, which is usually from 3 µGal to 5 µGal (both droplength and site-dependent) [33]. Therefore, a combination of the different methods of fringe signal processing in the same instrument is a useful tool to compose a more complete uncertainty budget.…”