Absolute gravimeters measure the local strength of Earth's gravitational pull with extreme precision, providing a foundation for geodesy, resource exploration, earthquake studies, and the definition of national and international gravity reference networks. For decades, the field has been dominated by laser-interferometer instruments such as the FG5X and A10, which track a falling object as light bounces off it. These devices are reliable, but their moving parts wear out over time, limiting their use for continuous, unattended monitoring.
Cold atom gravimeters offer an alternative. Instead of dropping a physical test mass, they launch a cloud of laser-cooled rubidium atoms upward in a vacuum chamber and track how the atoms' quantum wave interferes as they fall back down under gravity. Because there is no mechanical free-fall mechanism, the instrument suffers no wear and can, in principle, run unattended for long stretches of time.
The research team at CAS Cold Atom Technology (Wuhan) Co., Ltd. developed the second-generation transportable atom gravimeter, WAG-H5-2. The instrument adopts a highly integrated modular design. Its physical unit measures only 0.50 m × 0.23 m × 1.34 m and integrates an active vibration isolation module and magnetic shielding, while the optical and electronic subsystems are configured as four modular 3U chassis. To verify its performance, the prototype WAG-H5-2#2508 participated in the 2026 CMONOC Absolute Gravimeter Comparison, held in Wuhan from January to March 2026. It was evaluated alongside eight gravimeters from multiple institutions, including five FG5X laser interferometer gravimeters, two A10 laser gravimeters, and one IGG-03B laser absolute gravimeter.
Over two measurement campaigns, the nine instruments occupied five gravity sites at two geodetic observatories on Jiufeng Mountain. A superconducting gravimeter monitored gravity changes at one site around the clock so that all measurements, taken at different times, could be corrected to a common reference epoch. The team also used two relative gravimeters to precisely map how gravity changes with height at each site, allowing every instrument's reading to be transferred to a common reference height for a fair comparison. Two independent statistical adjustments -- one using only the absolute gravity data, and one combining the absolute and relative data -- were then used to calculate a best estimate, or comparison reference value, for each site and a degree of equivalence for each gravimeter.
The results showed that all nine instruments, including the atom gravimeter, agreed with the comparison reference values within their stated uncertainties, with individual deviations ranging from -4.3 to 6.1 microGal (one microGal equals about one billionth of Earth's surface gravity). The WAG-H5-2#2508 itself showed a deviation of just 1.4 microGal, closely matching the roughly 3 microGal accuracy specified by its manufacturer.
Beyond the comparison, the team separately evaluated the instrument's short-term sensitivity, precision, repeatability, and long-term stability. The gravimeter reached a short-term sensitivity of 16.2 microGal per square-root-hertz, close to that of established laser gravimeters, and achieved a measurement precision better than 1.0 microGal after 1000 seconds of averaging. Repeated day-long measurements after restarting the instrument agreed to within 1.5 microGal, and after being transported over 1,125 kilometers from Wuhan to Tianjin, the instrument tracked half a month of continuous tidal gravity variation with a residual scatter of less than 10 microGal, confirming that transportation and restart do not degrade its performance.
The authors note that because atom gravimeters do not need the scale-factor calibration required by relative, spring-based instruments and do not suffer the mechanical wear of drop-based laser gravimeters, they are well suited to calibrating other gravity sensors and to supporting continuous, long-term absolute gravity monitoring in the field. The team suggests that combining atom gravimeters, superconducting gravimeters, and relative gravimeters into an integrated hybrid observation network -- an approach they call "3G hybrid gravimetry" -- represents a promising direction for future gravity reference systems.
See the article:
Performance evaluation of the cold atom gravimeter WAG-H5-2#2508
https://doi.org/10.26464/epp2026058
Earth and Planetary Physics
1-Jul-2026