TianQin is a planned space-based gravitational wave (GW) observatory consisting of three Earth-orbiting satellites with an orbital radius of about $10^5 \, {\rm km}$. The satellites will form an equilateral triangle constellation the plane of which is nearly perpendicular to the ecliptic plane. TianQin aims to detect GWs between $10^{-4} \, {\rm Hz}$ and $1 \, {\rm Hz}$ that can be generated by a wide variety of important astrophysical and cosmological sources, including the inspiral of Galactic ultra-compact binaries, the inspiral of stellar-mass black hole binaries, extreme mass ratio inspirals, the merger of massive black hole binaries, and possibly the energetic processes in the very early universe and exotic sources such as cosmic strings. In order to start science operations around 2035, a roadmap called the 0123 plan is being used to bring the key technologies of TianQin to maturity, supported by the construction of a series of research facilities on the ground. Two major projects of the 0123 plan are being carried out. In this process, the team has created a new-generation $17 \, {\rm cm}$ single-body hollow corner-cube retro-reflector which was launched with the QueQiao satellite on 21 May 2018; a new laser-ranging station equipped with a $1.2 \, {\rm m}$ telescope has been constructed and the station has successfully ranged to all five retro-reflectors on the Moon; and the TianQin-1 experimental satellite was launched on 20 December 2019—the first-round result shows that the satellite has exceeded all of its mission requirements.

The TianQin-1 satellite (TQ-1), which is the first technology demonstration satellite for the TianQin project, was launched on

A novel two-stage electrostatically controlled torsion pendulum has been developed to simultaneously investigate the performance of a translational and a rotational degree of freedom of an electrostatic inertial sensor on ground. The motions of the proof mass (PM) relative to the electrode frame are monitored by a high-precision capacitance transducer, and are synchronously controlled by electrostatic actuators. The parasitic stiffness induced by capacitance transducers and the effect of the magnetic field are measured. Both translational and rotational motions of the PM succeed to be simultaneously controlled, and the cross-coupling effect between both controlled degrees of freedom is also preliminary measured. The experiments show that the scheme obviously suppresses the translational to rotational effect of the PM, and then effectively improves the torque resolution compared with the single-stage torsion pendulum. The noise floors of the controlled torsion pendulum come to 1.2 × 10−11 N Hz−1/2 along the translational degree of freedom, and 1.4 × 10−13 N m Hz−1/2 along the rotational degree of freedom, near 30 mHz, which are mainly limited by the back action of the capacitance transducer below 0.1 Hz and by the horizontal seismic noise disturbance above 0.1 Hz.

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Low noise position measurement is fundamental for space inertial sensors, and at present the capacitive position sensor is widely employed for space inertial sensors. The design for the possible suppression of the front-end electric noises for a capacitive sensor is presented. A prototype capacitive sensor with 2×10 −6 pF/Hz 1/2 at frequency above 0.04 Hz is achieved and further improvements are discussed.