Any single Positioning, Navigation and Timing (PNT) technology has its vulnerability and limits, even the powerful Global Navigation Satellite System (GNSS) is no exception. To provide continuous and reliable PNT information to users, the theory and technique of comprehensive PNT information system and resilient PNT application system have attracted great attention from Chinese scholars. We try to summarize the progress and development of the synthetic PNT system, including the proposal, the modification and the improvement of the comprehensive PNT, as well as the follow-up resilient PNT. The frame of China’s comprehensive PNT system consisted of comprehensive PNT infrastructure and comprehensive PNT application system is initially described; the achievements on some main PNT technologies are introduced; the conceptual models of resilient PNT are given; besides, existing researches on resilient function models and stochastic models are summarized according to different user scenarios.

The high-precision terrestrial reference frame, as the spatial benchmark for geodesy, is an important national infrastructure. However, due to the influence of nonlinear factors related to geophysical phenomena, the overall maintenance accuracy of the ITRF framework is still at the centimeter level. Therefore, accurately characterizing the true trajectories of linear motion, nonlinear motion, and geocentric motion of the reference station is the key to achieve the construction and maintenance technology of a millimeter level terrestrial reference framework. Based on long-term global and regional GNSS observation data, more Chinese geodesy scientists devoted much efforts to the maintenance of millimeter-level geodetic reference framework. The main contributions of this work included the followings: ①Dynamic maintenance of millimeter-level terrestrial reference frame;②Research progress on the method of maintenance of regional reference frame based on GNSS; ③The progress of CGCS2000 frame maintenance in millimeter level accuracy; ④Reprocessing and reanalysis of two-decade GNSS observation in continental China; ⑤Research on current GNSS velocity field model and deformation in Chinese mainland; ⑥The preliminary realization and evaluation of CTRF2020.

Gravity field is the most basic physical field generated by the material properties of the Earth system. It reflects the spatial distribution, movement and change of materials determined by the interaction and dynamic process inside the Earth. Over the years, a variety of technical means have been used to detect the Earth’s gravity field and supported numerous studies on the global change, resource detection, geological structure movement, water resources change and other related fields of research. Here is part of the progress in surface and marine gravimetry obtained by Chinese geodesy scientists from 2019 to 2023 from the following aspects, including: ① Continuous gravity network in Chinese mainland; ② Application of superconducting gravity measurement; ③ Network adjustment for continental-scale gravity survey campaign and data quality control; ④ Regional time-variable gravity field and its application; ⑤ Research progress on novel technologies for gravity inversion; ⑥ Research progress on marine gravity field determination; ⑦ Application research on marine gravity field.

Geodetic functional models, stochastic models, and model parameter estimation theory are fundamental for geodetic data processing. In the past five years, through the unremitting efforts of Chinese scholars in the field of geodetic data processing, according to the application and practice of geodesy, they have made significant contributions in the fields of hypothesis testing theory, un-modeled error, outlier detection, and robust estimation, variance component estimation, complex least squares, and ill-posed problems treatment. Many functional models such as the nonlinear adjustment model, EIV model, and mixed additive and multiplicative random error model are also constructed and improved. Geodetic data inversion is an important part of geodetic data processing, and Chinese scholars have done a lot of work in geodetic data inversion in the past five years, such as seismic slide distribution inversion, intelligent inversion algorithm, multi-source data joint inversion, water reserve change and satellite gravity inversion. This paper introduces the achievements of Chinese scholars in the field of geodetic data processing in the past five years, analyzes the methods used by scholars and the problems solved, and looks forward to the unsolved problems in geodetic data processing and the direction that needs further research in the future.

As one of the Analysis Centers (AC) of the International GNSS Service (IGS), Wuhan University (WHU) has been contributing to the IGS by providing ultra-rapid as well as rapid orbit and clock solutions for the established GPS and GLONASS since 2012. In the same year, the IGS initiated the Multi-GNSS Experiment (MGEX) to support the analysis of the emerging GNSS systems and prepare the IGS for Multi-GNSS, which includes GPS, GLONASS, the European Galileo system, the Chinese Beidou Navigation Satellite System (BDS), the Japanese Quasi-Zenith Satellite System (QZSS) and the Indian Regional Navigation Satellite System (IRNSS/NaVIC). The major products, i.e., orbits, Earth Orientation Parameters (EOPs), satellite clock as well as attitude have also been provided by WHU since 2012. More recently, WHU has engaged the third reprocessing of IGS for generating the highly accurate station coordinates as inputs for establishment of the International Terrestrial Reference Frame (ITRF) 2020 during 2019—2020. This article presents the recent major advancements of the IGS AC at Wuhan University, including precise products, real-time products, bias products, antenna phase center calibration, and the non-linear motion modeling for GNSS Reference Stations.

The ocean accounts for approximately 71% of the total area of the Earth. Whether it is studying the shape of the Earth itself through geodesy or the future development of earth system science, strengthening the construction of ocean geodesy disciplines and innovating ocean geodetic observation technologies have evident theoretical and practical significance. In recent years, the discipline of ocean geodesy in China has been continuously developing and growing, and notable breakthroughs have been made in ocean satellite geodesy and seafloor geodetic observation technology. Research on ocean geodetic observation models and algorithms has also made great progress.

The satellite gravimetry technology effectively recovers the global Earth’s gravity field. Since 2000s, HL-SST satellite CHAMP, LL-SST satellite GRACE, Gravity Gradient Measurement (GGM) satellite GOCE have been launched successfully, producing some Earth’s gravity models solely from satellites data. However, the space and time resolution of the Earth’s gravity fields do not adequately satisfy scientific objectives. The main reason is that the gravimetry satellites are not enough and observation data insufficient. The paper outlines the current and future status of Chinese gravity satellite missions. The Chinese gravimetry satellite system, named Chinese Gravimetry augment and Mass change exploring mission (ChiGaM), successfully launched in Dec. 2021 after four years of production and over a year of calibration and valiation. The accelerometer, K-band ranging system and the three stellar sensors, among others, were precisely calibrated and trimmed. The satellite mass center was determined and coordinated with the proof center of accelerometer with an accuracy 100μm. The inter-satellite ranging system and BDS/GPS receiver operate together seamlessly. The range and range rate noise is less than 3μm/Hz^1/2 and 1μm/s/Hz^1/2, respectively, in band of 0.025~0.1Hz. The electrostatic suspension accelerometer is working well. Its high-sensitive axis noise level is 3×10^-10 m/s²/Hz^1/2 near the frequency 0.1Hz, and 1×10^-9 m/s²/Hz^1/2 for the less-sensitive axis. Meanwhile the BDS/GPS receiver is used to achieve data for precise orbit determination, yielding an orbit result with accuracy better than 2cm. When compared with KBR observations, the RMS of the bias is less than 1mm. Besides above mission, next gravimetric satellite is being developed now. TQ-2 mission is designed as a totally experimental satellite for gravitational wave detection at low Earth orbit, which can detect the Earth’s gravity field simultaneously. The Bender-type mission is considered the most promising configuration for TQ-2 and consists of two polar satellites and two inclined satellites. Due to the extra observations at the east-west direction derived from the inclined satellite pair, significant improvements has been made in detecting temporal signals with higher accuracy and spatial-temporal resolution. To achieve the scientific goal, the ACC MBW can shift from 0.001~0.1Hz to 0.004~0.1Hz for ACC, and the LRI MBW can shift from 0.01~1Hz to 0.1~1Hz. For future research, a gravimetric potential survey using cold-atomic-clock based on the general relativity theory, cold atom gradiometer should be pursued. Gravimetric technologies should be mined and researched, and the gravimetry satellite constellation should be developed, so as to improve the time resolution and space resolution for meeting the requirements of geophysics, geodesy, earthquake, water resources environment, oceanography, etc.

The contribution presents the representative research progress on global static gravity field modeling, regional geoid/quasigeoid determination, vertical datum study, as well as the theory, algorithm and software for gravity field study in China from 2019 to 2023, which are the highlights of the chapter 6 “Progress in Earth Gravity Model and Vertical Datum” in the “2019—2023 China National Report on Geodesy” that submitted to the International Association of Geodesy(IAG). In addition, suggestions are proposed to promote the research in the fields of earth gravity field, geoid/quasigeoid and vertical datumin China according to trends of international geodesy and related disciplines.

As an essential component of future comprehensive Positioning, Navigation, and Timing (PNT) system, indoor positioning technology has extensive application demands, making it a focal point of attention in both academia and industry. This article comprehensively reviews the research status of indoor positioning technology in China, with a focus on highlighting representative achievements and application validations from major research institutions in recent years. It addresses the challenges and issues faced in promotion and application of large-scale, high-precision indoor positioning. Furthermore, a universal and seamless indoor-outdoor positioning system architecture is proposed, along with a technical roadmap and key technologies to achieve this architecture. Finally, an analysis and outlook on future technological trends are presented.

Global Navigation Satellite System (GNSS) can provide all-weather, all-time, high-precision positioning, navigation and timing services, which plays an important role in national security, national economy, public life and other aspects. However, in environments with limited satellite signals such as urban canyons, tunnels, and indoor spaces, it is difficult to provide accurate and reliable positioning services only by satellite navigation. Multi-source sensor integrated navigation can effectively overcome the limitations of single-sensor navigation through the fusion of different types of sensor data such as Inertial Measurement Unit (IMU), vision sensor, and LiDAR, and provide more accurate, stable and robust navigation information in complex environments. We summarizes the research status of multi-source sensor integrated navigation technology, and focuses on the representative innovations and applications of integrated navigation and positioning technology by major domestic scientific research institutions in China during 2019—2023.

The ionosphere is the ionized part of the upper atmosphere of the Earth, which plays an important role in atmospheric electricity and forms the inner edge of the magnetosphere. It influences radio propagation significantly, such as the Global Navigation Satellite System (GNSS). Meanwhile, the GNSS is also an essential technique for sensing the variation of ionosphere. During the years of 2019—2023, a large number of Chinese geodesy scientists devoted much efforts to the geodesy related ionosphere. Due to the very limited length, the achievements are carried out from the following six aspects, including: ① The ionospheric correction models for BDS and BDSBAS; ② Real-time global ionospheric monitoring and modeling; ③ The ionospheric 2D and 3D modeling based on GNSS and LEO satellites; ④ The ionospheric prediction based on artificial intelligence; ⑤ The monitoring and mitigation of ionospheric disturbances for GNSS users; ⑥ The ionospheric related data products and classical applications.

Modern geodetic technologies, including high-precision ground-based gravity measurements, satellite gravimetry, satellite altimetry, Global Navigation Satellite Systems (GNSS), and Interferometric Synthetic Aperture Radar(InSAR), offer a wealth of observations for monitoring global hydrological processes with exceptional accuracy and spatio-temporal resolutions. Mass redistribution and Earth’s surface deformation over land related to global and regional water cycling can be inferred from modern gravimetry, altimetry, GNSS, and InSAR techniques. Hydrogeodesy becomes an emerging field of geodesy aiming to analyze the changes of water in the Earth system. The paper introduces the China’s advances in hydrogeodesy in recent years. It brings together multiple geodetic teams’ work from China, showcasing the application of modern geodetic technologies in the field of hydrology, including research on terrestrial water storage, groundwater storage, glaciers/ice sheets, and reservoir water storage.

With the continued development of multiple Global Navigation Satellite Systems (GNSS) and the emergence of various frequencies, UnDifferenced and UnCombined (UDUC) data processing has become an increasingly attractive option. In this contribution, we provide an overview of the current status of UDUC GNSS data processing activities in China. These activities encompass the formulation of Precise Point Positioning (PPP) models and PPP-Real-Time Kinematic (PPP-RTK) models for processing single-station and multi-station GNSS data, respectively. Regarding single-station data processing, we discuss the advancements in PPP models, particularly the extension from a single system to multiple systems, and from dual frequencies to single and multiple frequencies. Additionally, we introduce the modified PPP model, which accounts for the time variation of receiver code biases, a departure from the conventional PPP model that typically assumes these biases to be time-constant. In the realm of multi-station PPP-RTK data processing, we introduce the ionosphere-weighted PPP-RTK model, which enhances the model strength by considering the spatial correlation of ionospheric delays. We also review the phase-only PPP-RTK model, designed to mitigate the impact of unmodelled code-related errors. Furthermore, we explore GLONASS PPP-RTK, achieved through the application of the integer-estimable model. For large-scale network data processing, we introduce the all-in-view PPP-RTK model, which alleviates the strict common-view requirement at all receivers. Moreover, we present the decentralized PPP-RTK data processing strategy, designed to improve computational efficiency. Overall, this work highlights the various advancements in UDUC GNSS data processing, providing insights into the state-of-the-art techniques employed in China to achieve precise GNSS applications.