Advances and Prospects in Gravity Potential Determination Based on the GNSS Frequency Shift Approach

The precise determination of the gravity potential (geopotential) field is one of the foundational tasks in geodesy. With the rapid advancements in time and frequency science, a novel relativistic geodetic approach has garnered widespread attention in geodesy and geophysics. This method, grounded in the principles of general relativity, employs high-precision atomic clocks alongside advanced time and frequency transfer techniques to accurately measure both the geopotential difference and the orthometric height (OH) difference between two arbitrary stations on the ground. Notably, the global navigation satellite system (GNSS) time and frequency transfer technique, boasts advantages such as high precision, all-weather capability, flexible networking, and economic feasibility, gradually forming an intriguing research field of geopotential determination by a GNSS frequency shift approach. First, the principle of the relativistic geodetic approach is outlined, and the mathematical model conducive to centimeter-level geopotential difference measurement is analyzed. Second, the essential characteristics of both the cable connection and GNSS frequency shift approaches are summarized. Third, a focused review is conducted on the research progress of the GNSS frequency shift approach. The two typical GNSS time and frequency transfer methods, the common view and the precise point positioning methods, are investigated in detail. Representative experiments associated with these two methods are reviewed and discussed, followed by an analysis and summary of the development of the GNSS frequency shift approach. Afterward, the critical challenges that necessitate urgent resolution within this field are delineated, including but not limited to the modeling of the GNSS receiver clock offset, the error process in GNSS time and frequency transfer, and the data process for the receiver clock offset series. Considering the vital role of high-performance clocks in the GNSS frequency shift approach, a detailed overview of the development of both microwave atomic clocks and optical atomic clocks is presented. Finally, potential applications of the GNSS frequency shift approach are explored, including high-precision geopotential difference or OH difference measurement, the establishment of a unified world height datum with high accuracy, and the simultaneous measurement of three-dimensional geometric position along with their corresponding geopotential values. This paper aims to serve as a reference for further research on geopotential determination through the GNSS frequency shift approach and related studies.

On-Ground Test Method, Technology and Application of Space Electrostatic Accelerometer

Satellite gravity measurement is the only direct means to obtain the global gravity field data. As the core payload of satellite gravity measurement, electrostatic accelerometer has the characteristics of high resolution and small measurement range. The lack of performance detection and evaluation means directly limits the development level and application efficiency of space electrostatic accelerometer in China.

Research Progress on Dynamic Measurement of Cold Atom Gravimeter

The dynamic measurement technique based on cold atom gravimeter is distinguished from the application of relative gravimeter dynamic measurement of gravity value method, which has the outstanding advantages of high accuracy, no drift, long-term stable measurement, etc. However, the gravimeter from the static bench experiment to the dynamic measurement in the field is still affected by multiple factors such as the volume, the dead time, the noise, and so on. This paper summarizes the research progress of the dynamic measurement technology of cold atom gravimeter at home and abroad. First, it discusses the basic principle of cold atom gravimeter, analyzes the miniaturization and inertial stabilization platform, vibration isolation and damping, “dead time” compensation, combined measurement and other dynamic error suppression techniques. Second,it details some representative work carried out by the cold atom gravimeter in the integration of dynamic measurements in the land, sea, sky and space.Finally, it looks forward to the new methods and the military and other applications of cold atomic gravimeter measurement techniques.

Review on the Study of Numerical Simulation of Crustal Deformation Dynamic Models in Sichuan-Yunnan Region

Sichuan-Yunnan region is located in the southeast margin of the Tibetan Plateau, and its crustal deformation characteristics have been one of the hot issues in the research, but the driving mechanism of the dynamic model of the deformation is still controversial. First,the theoretical results and research progress of the structural deformation model in Sichuan-Yunnan region based on the finite element numerical simulation method in recent years are reviewed, aiming to explore the influence of four tectonic models: block extrusion, middle-lower crust flow, gravity diffusion and mantle convection. The block extrusion model focuses on the crustal deformation controlled by the activity of the block boundary faults, but there are controversies in the selection of the secondary faults. The middle-lower crust flow model holds that there are weak layers with low viscosity and flow ability in deep crust, but its scale and size are still unclear. The gravity diffusion model plays an obvious role in the high altitude difference of Sichuan-Yunnan region, but it is uncertain whether the middle-lower crust flow are involved in this process. The influence of the drag force exerted by mantle convection on the bottom boundary of the lithosphere on the crustal movement has been gradually paid attention to. Second, the existing problems and challenges of finite element numerical simulation methods of dynamic models in Sichuan-Yunnan region are discussed. It is necessary to comprehensively consider all 4 types of dynamic models and reasonably construct finite element geometric and physical models in order to clarify the contribution of various external and internal driving forces to crustal deformation. Finally, the development direction of numerical simulation of dynamic model in Sichuan-Yunnan region is pointed out. Considering the coupling of multiple physical fields and the comprehensive application of multi-disciplinary data, the improvment of algorithm efficiency and stability on the basis of high performance computing technology may be the future trend of dynamic finite element numerical simulation research.

Construction of a System for Full Tensor Gravity Gradient Forward Modeling and Visualization

To address the inflexibility and inadequacies of existing visualization tools for full-tensor gravity gradient (FTG) forward modeling, and to achieve accurate identification, classification, and interpretation of gravity gradients,a method for forward modeling FTG and constructing a visualization system based on Mathematica is proposed.

Analysis and Reflections on the Development of Underwater Gravity-Aided Inertial Navigation Technology in the United States and Russia

The development of underwater gravity-aided inertial navigation technology in China is coming to a critical period and faces many difficulties and challenges. There are both cognitive problems in research ideas and “stuck neck” problems in key technologies, which require us to concentrate our wisdom and make creative thinking, and to find ways to solve these problems.

Concept Analysis of Map Projection and Its Applications Based on Manifold Mapping Principle

After a concise statement of the historical origin of topological manifold and Gauss-Krüger projection, this paper expoundes and analyzes the principle of manifold mapping of cartographic projection.

A Visual Global Positioning Method Based on Smartphone with High Usability in Large Indoor Spaces

The visual global positioning based on smartphone is a research hotspot in location community services. The existing methods suffer from the problems of poor reliability and low computing efficiency especially when they are used for large indoor environments such as airport and shopping mall.

A Model for Quantitatively Calculating and Qualitatively Describing Direction Relationships Between Object Groups

The modeling of direction relationships between object groups is one of the important research topics in the theory of spatial relationships. However, existing models fail to reasonably classify the direction relationships between object groups, and ignore the impacts of distribution shapes of object groups and distribution density of their sub-objects on direction relationships, making it difficult to accurately determine direction relationships.

Application of GNSS in the Study of Earth Surface Processes

The earth surface system is the part of the earth system that is most closely related to human beings. The study of surface processes is becoming more and more important in earth system research. The innovative application of geophysics and geodesy to surface processes has gradually become a new interdisciplinary development direction. Global navigation satellite system (GNSS) observation technology is widely used in the study of surface processes because of its characteristics of high accuracy, all-weather, large range and quasi-real-time. In this paper, the application of GNSS technology in the study of surface processes is briefly introduced from the aspects of long-term crustal deformation, coseismic and post-seismic deformation, atmospheric precipitable water, load response, magma and volcanic activity, landslide monitoring and reflection measurement, and then the future development is discussed. The advantages of GNSS in observation technology highlight the importance of GNSS in the earth surface processes research.

Onboard Ultra-Stable Oscillator Long-Term Drift Calibration for Tianwen-1 Mission

The stability of ultra-stable oscillator (USO) which provides frequency standard and transmits downlink signal affects the accuracy of one-way Doppler measurements directly. It is a necessity to estimate the frequency bias during orbit determination with one-way Doppler measurements.

Urban Sensing Systems

Urban sensing, as the first layer of urban computing, is the foundation of intelligent cities, gene‑rating crucial data representing city dynamics for digital applications. Aiming to solve the challenges that urban sensing is facing, this paper proposes an urban sensing system that is comprised of a theoretical framework, a technical platform and an operational model. The theoretical framework consists of six categories of content to be sensed, four sensing paradigms, and four technical challenges. The technical platform provides digital tools for managing sensors and collecting data, and supplies upper-layer applications with interfaces for using urban sensing services. Urban sensing systems can reduce redundant sensor deployment and further operational workloads. It improves the capability of urban sensing service providers in solving problems and the synergy among each other. It also generates continuous economic benefits, such as income and employment, ensuring urban sensing functions stable and sustainable.

High-Precision Inter-Satellite Baseline Determination Method for Lutan-1 Based on BDS-3

Lutan-1(LT-1) is the first formation-flying mission of China, enabling interfero‑metry synthetic aperture radar (InSAR) in the L-band. High-precision inter-satellite baseline is crucial for InSAR processing and application.

Application of InSAR Monitoring Large Deformation of Landslides Using Lutan-1 Constellation

Lutan-1 (LT-1) satellites are the first scientific research satellite constellation with interferometric synthetic aperture radar (InSAR) measurement of surface deformation as its main features and focusing on long-term natural disaster monitoring and warning in complex areas. It consists of two L-band synthetic aperture radar (SAR） satellites with the same hardware parameters, and has high resolution, high revisit, and full polarization imaging capabilities. The constellation uses the bi-satellites “single-pass in close formation” mode to map terrain, and the bi-satellites “repeat-track” mode to measure surface deformation. Taking the Xieliupo landslide in Zhouqu County, Gansu Province, China as a demonstration example, the ability to monitor large deformation was tested using 4 d and 8 d high revisit observation data.

Application of Lutan -1 SAR Satellite Constellation to Earthquake Industry and Its Prospect

Lutan-1 (LT-1), as the first constellation of civil L-band interferometric synthetic aperture radar (InSAR) satellites in China, primarily focuses on surface deformation measurement by leveraging the differential interferometric, while also possessing height measurement and fully polarimetric imaging capabilities. With its dual-satellite formation imaging mode, it can achieve a strict revisit period of four days, which is highly beneficial for surface deformation monitoring and emergency response to natural disasters such as earthquakes. From the perspective of the LT-1 SAR satellite's imaging capabilities and the demands of the seismic industry, we primarily highlight the preliminary applications of the satellite system during the testing phase. The main objective is to evaluate the satellite data's capabilities in co-seismic deformation monitoring, emergency observations, and remote sensing interpretation of active fault zones. The ultimate purpose is to provide a reference for the future commercialization of seismic applications, based on the potential applications of the satellite data in these fields.