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.

TianQin is a proposed space-borne gravitational-wave detection mission using circular high Earth orbits. The geocentric concept has raised questions about the disturbing effect of the nearby gravity field of the Earth-Moon system on the highly sensitive intersatellite ranging measurements. Here we examine the issue through high-precision numerical orbit simulations with detailed gravity-field models. By evaluating range accelerations between distant free-falling test masses, the study shows that the majority of the Earth-Moon’s gravity disturbances are not in TianQin’s detection frequency band above ${10}^{-4}\mathrm{Hz}$, and hence present no showstoppers to the mission.

One of the key constraints for the accelerometer of GRACE-type gravity satellites to accurately measure the non-gravitational accelerations acting on the satellite is that the center of mass of the satellite and the proof mass of the accelerometer should maintain a coincidence. In addition, the accuracy requirement is that the center of mass offset (CM-offset) in the three directions is less than 100 microns. Since the center of mass (CoM) of the satellite will change with the consumption of cold-gas fuel in the tanks, it is necessary to regularly carry out the CoM calibration maneuver. Firstly, the observation equations consisting of the accelerometer linear acceleration, angular acceleration, and the CM-offset vector are established in order to estimate the amount of CM-offset. Then, according to the estimated CM-offset, the satellite mass trim mechanisms are used to change the satellite’s CoM, so that the satellite’s CoM always approaches the proof mass of the accelerometer, with an accuracy of 100 μm per axis. The CM-offset of the satellite of GRACE-FO is estimated by using the accelerometer, star camera, magnetic torquer, magnetometer, and the precision orbit data during the GRACE-C CM-offset calibration period on 1 February 2020. Four kinds of CM-offset results are obtained by four different angular accelerations as follows: the angular acceleration based on the attitude dynamics (“MTQ angular acceleration”), the accelerometer angular acceleration calibrated by MTQ, the accelerometer angular acceleration, and the angular acceleration calculated by the star camera. By comparing the four kinds of CM-offset results that are estimated by the four different methods, all four of the results are shown to have the same level of accuracy. Based on the accelerometer (calibrated) angular acceleration, the difference with the JPL result is 0.5 μm, while the difference between the conventional method and the JPL result is 6.0 μm. All four of the methods can achieve the requirement of 50 μm accuracy and using four CM-offset estimation methods simultaneously can improve the integrity of the calibration results. Subsequently, the CM-offset results of GRACE-C since its launch are estimated here. The calibration algorithm that is proposed in this paper can be used as a reference in the calibration of gravity satellites carrying an accelerometer payload.

Non-gravitational force models are critical not only for the applications of satellite orbit determination and prediction, but also for the studies of gravitational reference sensors in space-based gravitational wave detection missions and accelerometers in gravity satellite missions. In this paper, based on the inertial sensor data from the TianQin-1 (TQ-1) mission, a correction has been made in the non-gravitational force models by applying additional terms related to the orbital periods. After taking into account this correction, about 37 hours of TQ-1 inertial sensor data is calibrated in the sensitive axes, i.e. y- and z-axes, by comparing with the simulated non-gravitational accelerations. It is indicated that the peak-to-peak value of the non-gravitational acceleration correction terms are about 2% and 13% of the measured accelerations in the y- and z-axes, respectively. Within the frequency band below 0.01 Hz, the root mean square of calibration residual errors in y- and z-axes are suppressed from 1.03 × 10⁻⁹ and 3.872 × 10⁻⁹ m s⁻² to 8.14 × 10⁻¹⁰ and 1.343 × 10⁻⁹ m s⁻², respectively. The bias and scale factor of the inertial sensor are also obtained from the calibration by the method of least-squares fit. Meanwhile, the inertial sensor measurements are validated and their signal compositions are analyzed.

High-precision electrostatic accelerometer has been widely employed in gravity measurement satellites. Accelerometer can be designed for different resolutions and measurement ranges to adapt to different scientific missions. This paper presents an ultra-high sensitivity 70 g-TM electrostatic accelerometer, designed to achieve an adjustable resolution from 10 − 12 m s − 2 Hz − 1 / 2 to 10 − 10 m s − 2 Hz − 1 / 2 with an optimal resolution of 3.3 × 10 − 12 m s − 2 Hz − 1 / 2 . The in-orbit performance of the 70 g-TM accelerometer was tested on the Tianqin-1 satellite.

The deviation of the center of mass of a gravitational wave detection satellite from its calibrated position can severely impact the accuracy of gravitational wave detection. Therefore, there is an urgent need to develop effective identification methods to achieve precise center of mass identification while ensuring the alignment of the laser link. To address this issue, this paper proposes a method for the center of mass identification of gravitational wave detection spacecraft that ensures the pointing accuracy of the laser link. A study on the relative attitude dynamics modeling of gravitational wave detection spacecraft is conducted, and the conditions for small, periodic spacecraft maneuvers that maintain laser link alignment are analyzed. A high-precision, high-stability center of mass identification method based on dual-model data fusion is proposed and simulated. The results show that this method can maintain the alignment precision of the laser interferometer arms within 10 nrad, while achieving a center of mass identification accuracy of 25 $\mathsf{\mu }$m. Compared to existing methods, this method improves the identification accuracy by an order of magnitude, demonstrating its feasibility for application in gravitational wave detection constellations. It provides theoretical support for the center of mass identification of gravitational wave detection spacecraft.