TianQin is a future space-based gravitational wave (GW) observatory targeting the frequency window of 10⁻⁴–1 Hz. A large variety of GW sources are expected in this frequency band, including the merger of massive black hole binaries, the inspiral of extreme/intermediate mass ratio systems, stellar-mass black hole binaries, Galactic compact binaries, and so on. TianQin will consist of three Earth orbiting satellites on nearly identical orbits with orbital radii of about 10⁵ km. The satellites will form a normal triangle constellation whose plane is nearly perpendicular to the ecliptic plane. The TianQin project has been progressing smoothly following the ‘0123’ technology roadmap. In step ‘0’, the TianQin laser ranging station has been constructed and it has successfully ranged to all the five retro-reflectors on the Moon. In step ‘1’, the drag-free control technology has been tested and demonstrated using the TianQin-1 satellite. In step ‘2’, the inter-satellite laser interferometry technology will be tested using the pair of TianQin-2 satellites. The TianQin-2 mission has been officially approved and the satellites will be launched around 2026. In step ‘3’, i.e. the TianQin-3 mission, three identical satellites will be launched around 2035 to form the space-based GW detector, TianQin, and to start GW detection in space.

High-precision electrostatic accelerometers have achieved remarkable success in satellite Earth gravity field recovery missions. Ultralow-noise inertial sensors play important roles in space gravitational wave detection missions such as the Laser Interferometer Space Antenna (LISA) mission, and key technologies have been verified in the LISA Pathfinder mission. Meanwhile, at Huazhong University of Science and Technology (HUST, China), a space accelerometer and inertial sensor based on capacitive sensors and the electrostatic control technique have also been studied and developed independently for more than 16 years. In this paper, we review the operational principle, application, and requirements of the electrostatic accelerometer and inertial sensor in different space missions. The development and progress of a space electrostatic accelerometer at HUST, including ground investigation and space verification are presented.

The way to reach the prelaunch baseline accuracy is still one of crucial issues for Gravity Recovery and Climate Experiment (GRACE) temporal gravity field model determination. In this work, two hybrid processing strategies are developed to improve the accuracy of GRACE temporal solution. One is for kinematic empirical parameters, the other is for arc‐specific parameters. These two hybrid processing strategies are compared with two solely processing strategies including filter predetermined strategy (FPS) and pure predetermined strategy (PPS). Assessing with a closed‐loop simulation study, the results demonstrate the outperformance of our hybrid processing strategies. Using GRACE Level 1B data, the GRACE monthly gravity field models HUST‐FPS‐PPS and HUST‐Grace2019 are, respectively, determined with the hybrid processing strategies, and they are compared with the solely solutions HUST‐FPS and HUST‐PPS. The comparison results demonstrate that (1) as for the kinematic empirical parameter, the hybrid processing strategy can retain the temporal signal but also reduce the noise level. For instance, the annual amplitudes over Amazon River Basin are 16.0 cm for HUST‐PPS, 19.1 cm for HUST‐FPS, and 19.4 cm for HUST‐FPS‐PPS. In contrast, the cumulative geoid difference of HUST‐FPS‐PPS reduces by 11% and 26%, when compared to HUST‐PPS and HUST‐FPS. (2) As for the arc‐specific parameters, the hybrid processing strategy also significantly improves the recovered monthly solution. Compare with Center for Space Research RL05 and HUST‐FPS‐PPS, the cumulative geoid difference of HUST‐Grace2019 reduces by 19% and 12%. The comparison results in spatial domain also support our conclusions. The hybrid processing strategy of this work is likely applicable for GRACE Follow‐On data.

An electrostatic-controlled torsion pendulum combined with a scanning conducting probe is used to measure the temporal and spatial variations of the surface potential on test mass. The apparatus can work in static and scanning modes. Temporal variation of the surface potential can be measured with the static mode, and its voltage variation with a level of $15\mu \mathrm{V}/{\mathrm{Hz}}^{1/2}$ at 0.03 Hz has been achieved for a $5\times 5{\mathrm{mm}}^2$ area. The spatial distribution of the surface potential can be measured with the scanning mode, and the surface potential distribution can be obtained at a level of $330\mu \mathrm{V}$ at 0.125 mm spatial resolution. The experimental results tell us that the apparatus provide a new way to investigate the charge distribution and its variation, which is very useful in helping to understand the physical mechanism of the surface charge distribution of an actual object.

Capacitive sensing electronics are critical to space gravitational wave detection missions, providing the test mass motion information for the inertial sensor’s non-sensitive axis control and the drag-free control of the satellite. For the TianQin mission, the capacitive sensor requirement is 6.9 × 10 − 7 p F H z − 1 / 2 at 6 mHz, corresponding to a displacement noise of 1.7 n m H z − 1 / 2 . This paper presents the system design scheme, noise analysis, and performance testing of the capacitive sensor for the TianQin mission. Based on the previous work of our group, a capacitive sensing scheme based on a transformer bridge and modulation–demodulation is adopted. A transformer with a better high LQ product is fabricated, and the preamplifier current noise is reduced to further improve the performance above mHz. The amplitude stability of the AC carrier and the performance of the lock-in amplifier circuit are further improved to reduce the low-frequency 1 / f noise at m H z and below. Finally, a capacitive sensor prototype is developed and tested, the experimental results show that the capacitance measurement noise is better than 3 × 10 − 7 p F H z − 1 / 2 above 1 mHz and better than 6 × 10 − 7 p F H z − 1 / 2 at about 0.1 mHz , which meets the overall performance requirement of the TianQin mission.