The TianQin-1 satellite (TQ-1), which is the first technology demonstration satellite for the TianQin project, was launched on 20 December 2019. The first round of experiment had been carried out from 21 December 2019 until 1 April 2020. The residual acceleration of the satellite is found to be about 1 × 10 ⁻¹⁰ m/s ² /Hz ^1/2 at 0.1 Hz and about 5 × 10 ⁻¹¹ m/s ² /Hz ^1/2 at 0.05 Hz, measured by an inertial sensor with a sensitivity of 5 × 10 ⁻¹² m/s ² /Hz ^1/2 at 0.1 Hz. The micro-Newton thrusters has demonstrated a thrust resolution of 0.1 μ N and a thrust noise of 0.3 μ N/Hz ^1/2 at 0.1 Hz. The residual noise of the satellite with drag-free control is 3 × 10 ⁻⁹ m/s ² /Hz ^1/2 at 0.1 Hz. The noise level of the optical readout system is about 30 pm/Hz ^1/2 at 0.1 Hz. The temperature stability at temperature monitoring position is controlled to be about ±3 mK per orbit, and the mismatch between the center-of-mass of the satellite and that of the test mass is measured with a precision of better than 0.1 mm.

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.

TianQin is a Chinese space-based observatory concept with three drag-free spacecraft orbiting the earth to observe gravitational-waves in the millihertz range. Electrical charge is inevitably accumulating on its isolated test masses which are used as the mirrors for interferometers to measure gravity variation. Charge management system based on ultraviolet (UV) light is developed to reduce the charge-induced noises acting on the test mass in space gravitational experiments such as GP-B, LISA, LISA Pathfinder and TianQin mission. The UV LED is a new UV light source with some advantages relative to mercury lamps, such as smaller size, lighter weight, lower power consumption and higher efficiency. For these reasons, ground-based tests for the combination of UV LED and inertial sensor are essential. This paper presents the development of a charge management system based on a UV LED, its application to a torsion pendulum system with a TianQin-like test mass, and assesses the charge-management performance of the system. The experimental results indicate that the isolated test mass can be charged positively or negatively through UV illumination. It demonstrates that the charging and discharging rates can both reach higher than 10 ⁵ e s ⁻¹ , as well as spectral measurement of the pendulum charge resulting in a white noise level equivalent to ∼5 × 10 ⁵ e Hz ^{− 1/2} , satisfying the TianQin requirement.

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.

In spaceborne gravitational-wave detectors, the stringent requirements on the magnetic cleanliness of the test masses (TMs) make the development of ground-based characterization techniques of their magnetic properties important. An electrostatically controlled torsion pendulum has been built to measure the magnetic properties of TianQin-like solid TMs. The remanent magnetization $m_r$ and the magnetic susceptibility $\chi$ of TMs can be measured at the level of 1 nA ${\mathrm{m}}^2$ and $8\times {10}^{-7}$, respectively. The precisions of measurements are mainly limited by the mechanical sensitivity of silicon fiber. The experimental results from a solid tungsten TM as an example show that the apparatus provides a feasible way to investigate the bulk effect of a full-size TM for the TianQin mission with enough resolution.