Performance and application of the Chinese satellite-to-satellite tracking gravimetry system

The first pair of satellite to satellite tracking gravimetric satellites from China (Chinese gravimetric mission) was successfully launched at the end of 2021. It adopts a hybrid measurement system of high-low satellite to satellite tracking (HL-SST) and low-low satellite-to-satellite tracking (LL-SST) to obtain measurements of the global gravity field and its time-varying information, which can be used to study groundwater changes, glacier ablation, space micro-gravity fluctuations, ocean tides, earthquakes, etc. The data can be used for scientific research and engineering construction in the fields of natural resources, ocean, aerospace, and water conservancy, et al. The Chinese gravimetric satellite platform is designed to be ultra-silent, ultra-stable and ultra-accurate. In-orbit test results show that the working state of the platform is normal and all indicators meet the design requirements with margin. This significantly improves the development level of the satellite platform and the level of space micro-gravity measurement, and will expand the prospect for scientific applications in various fields. Firstly, this paper briefly introduces the Chinese gravimetric satellite and discusses the technical characteristics of the integrated design of the satellite platform. This platform provides an environment with low vibrations, stable temperature, precise pointing and tiny deformation of the structure for the whole satellite, and ensures that the payloads of the inter-satellite ranging system and electrostatic suspension accelerometer work in suitable conditions. Secondly, the characteristics and performance figures of the inter-satellite ranging system, electrostatic suspension accelerometer, BDS/GPS common mode receiver and star sensors are analyzed. The inter-satellite ranging system is based on K/Ka-band ranging (KBR) in the dual-one way ranging configuration with stable performance and ranging noise level being better than 3 mu m/Hz(1/2) in the frequency band of 0.025-0.1 Hz. The accelerometer works with electrostatic suspension mode, which is highly sensitive to the residual thrust of the attitude control thruster, switching of the magnetic torquer and other small vibrations. The spectrum analysis shows its high-sensitive axis noise level is being 3x10(-10) m/(s(2) Hz(1/2)) near the frequency 0.1 Hz, and being 1x10(-9) m/(s(2) Hz(1/2)) for the less-sensitive axis. The three star sensors are designed integrally with the tiny deformation structure. To verify the attitude measurement accuracy, the angles between star sensor boresights arecalculated, then the average of the angles is removed, the standard deviation (STD) of residuals is better than 3 ''. The center of mass detection and adjustment system reaches the accuracy level of 100 mu m. The BDS/GPS common mode receiver observers BDS-3 signals and GPS signals simultaneously. The data from the BDS/GPS receiver are used for satellite precise orbit determination (POD) and time synchronization. The achieved accuracy of the satellite orbits is better than 2 cm determined from a comparison of the kinematic and reduced-dynamic orbits. Comparing the baseline of the KBR system with the POD yields RMS residuals of less than 1 mm. Finally, the time-varying Earth gravity field recovery is performed from April 1, 2022 to August 30, 2022 using the in-orbit satellite data, such as KBR data, attitude and orbit data as well as with measurements of non-gravitational accelerations obtained from accelerometers. The time-varying Earth gravity model with a maximum degree of 60 is named Chinese Earth Gravity Field Model 22-01 (CEGM22S-01), and it is converted into an equivalent water height which can be applied to Amazon terrestrial water storage monitoring. The results show that the model can well characterize global hydrological changes, such as the mass loss of Greenland and the terrestrial water storage changes of the Amazon River, and correlates strongly with the results of GRACE Follow-On with the root mean square (RMS) difference of 2 cm in global equivalent water height. Subsequently, with the accumulation of further satellite telemetry, the continuous time-varying gravity field, space atmospheric density, space horizontal ionosphere, space magnetic field and other products can be produced, which will provide abundant information services for the fields of geophysics, geodesy, earthquake, water resources environment, ocean, national defense, etc. The Chinese gravimetric mission will continue to serve relevant scientific research and engineering applications and continue to reveal more scientific application values.