With the rapid development and gradual perfection of GNSS in recent years, improving the real-time service performance of GNSS has become a research hotspot. In GNSS single-point positioning, broadcast ephemeris is used to provide a space–time reference. However, the orbit parameters of broadcast ephemeris have meter-level errors, and no mathematical model can simulate the variation of this, which restricts the real-time positioning accuracy of GNSS. Based on this research background, this paper uses a BP (Back Propagation) neural network and a PSO (Particle Swarm Optimization)–BP neural network to model the variation in the orbit error of GPS and BDS broadcast ephemeris to improve the accuracy of broadcast ephemeris. The experimental results showed that the two neural network models in GPS can model the broadcast ephemeris orbit errors, and the results of the two models were roughly the same. The one-day and three-day improvement rates of RMS(3D) were 30–50%, but the PSO–BP neural network model was better able to model the trend of errors and effectively improve the broadcast ephemeris orbit accuracy. In BDS, both of the neural network models were able to model the broadcast ephemeris orbit errors; however, the PSO–BP neural network model results were better than those of the BP neural network. In the GEO satellite outcome of the PSO–BP neural network, the STD and RMS of the orbit error in three directions were reduced by 20–70%, with a 20–30% improvement over the BP neural network results. The IGSO satellite results showed that the PSO–BP neural network model output accuracy of the along- and radial-track directions experienced a 70–80% improvement in one and three days. The one- and three-day RMS(3D) of the MEO satellites showed that the PSO–BP neural network has a greater ability to resist gross errors than that of the BP neural network for modeling the changing trend of the broadcast ephemeris orbit errors. These results demonstrate that using neural networks to model the orbit error of broadcast ephemeris is of great significance to improving the orbit accuracy of broadcast ephemeris.

The broadcast ionospheric model is one of the main methods for eliminating ionospheric delay errors for the Global Navigation Satellite Systems (GNSS) single-frequency users. GPS Klobuchar model (GPSK8) is the widely used broadcast ionospheric model for GPS, while BDS usually implements the BDS Klobuchar model (BDSK8) and BeiDou Global Broadcast Ionospheric Delay Correction Model (BDGIM). Geomagnetic storms may cause interference within the ionosphere and near-Earth space, compromising the accuracy of ionospheric models and adversely affecting the navigation satellite systems. This paper analyzes the static Standard Point Positioning (SPP) accuracy of GPS and BDS by implementing the broadcast ionospheric models and then investigates the impact of strong geomagnetic storms occurring in 2021 on positioning accuracy. The results show that the global 3D positioning accuracy (95%) of GPS + GPSK8, BDS + BDSK8, and BDS + BDGIM are 3.92 m, 4.63 m, and 3.50 m respectively. BDS has a better positioning accuracy in the northern hemisphere than that of the southern hemisphere, while the opposite is valid for GPS. In the mid-latitude region of the northern hemisphere, BDS + BDSK8 and BDS + BDGIM have similar positioning accuracy and are both better than GPS + GPSK8. The positioning accuracy after applying those three broadcast ionospheric models shows the superior performances of winter and summer over spring and autumn (based on the northern hemisphere seasons). With the exception of during winter, nighttime accuracy is better than that of daytime. The strong geomagnetic storm that occurred on the day of year (DOY) 132, 2021 has an impact on the positioning accuracy for only a small number of stations; however, the global average positioning accuracy is not significantly affected. The strong geomagnetic storms that occurred in DOY 307 and DOY 308 have a significant impact on the positioning accuracy of dozens of stations, and the global average positioning accuracy is affected to a certain extent, with some stations experiencing a serious loss of accuracy. Decreased degrees in positioning accuracy is proportional to the intensity of the geomagnetic storm. Of the 33 IGS Multi-GNSS Experiment (MGEX) stations worldwide, those located in the low and mid-latitudes are more significantly affected by the geomagnetic storms compared with higher latitudes. Evident fluctuations of the positioning errors existed during the strong geomagnetic storms, with an increase in extreme values, particularly in the up direction.

With the development of Internet technology, it is easier to obtain vector data, and it is necessary to ensure the security of data storage and transmission. This paper presents a method of vector data coordinate encryption in Shapefile format using domestic password SM4 algorithm, which can ensure data security without changing the vector data structure. The system clock is used as the random seed to generate the encryption key, and the coordinate sequence of each element of vector data is read. Firstly, the coordinates are preprocessed according to the ID of the element, and then the encryption key is used to encrypt the coordinate information to obtain the ciphertext data. The experiments verify the feasibility of the domestic cipher SM4 algorithm to encrypt vector data. The encryption efficiency can reach 90M/S, and the decryption efficiency can reach 92MB/S. The encryption method can protect the storage security of vector data, improve the theoretical system of vector data encryption, and has certain research significance.

The rapid growth of space-based activities has greatly heightened the need for accurate, real-time tracking of low Earth orbit (LEO) satellite positions. Currently, the real-time precise orbit determination (RTPOD) for LEO satellites typically relies on ambiguity-float solutions derived from onboard Global Navigation Satellite System (GNSS) observations. This research aims to perform RTPOD with undifferenced ambiguity resolution for the LuTan-1A satellite based on the square root information filter. The influence of the GPS/BDS-3 combination on ambiguity resolution and LEO RTPOD performance is also investigated. For the purpose of enabling the ambiguity resolution of the LuTan-1A satellite, the proposed method adopts a globally distributed network to conduct the epoch-by-epoch estimation of the GPS/BDS-3 real-time satellite clock and phase bias. The RTPOD results of the LuTan-1A satellite reveal that the three schemes, namely GPS-only, BDS-3-only, and GPS+BDS-3, have an ambiguity fixing rate of 96.7%, 89.7%, and 99.8%, and the time to first fix of 3.7, 11.0, and 1.0 min, respectively. The dual-system combination of GPS and BDS-3 brings about more available GNSS satellites, thereby resulting in an evident improvement in ambiguity resolution performance. Furthermore, the application of undifferenced ambiguity resolution enables more accurate and robust RTPOD results for the LuTan-1A satellite. With regard to convergence time, thanks to the ambiguity fixing, a reduction of more than 60% is accomplished for the GPS-only and GPS+BDS-3, and 23% for BDS-3-only. Additionally, the LEO three-dimensional orbit accuracy for the ambiguity-fixed solution of the GPS-only, BDS-3-only, and GPS+BDS-3 schemes are 2.6, 2.8, and 1.7 cm, respectively, which are 38.1%, 35.7%, and 49.0% better than the corresponding ambiguity-float solutions.

In autonomous navigation mode, the navigation constellation needs to establish an independent space-based time scale to provide a time-frequency reference. The BeiDou Global Navigation Satellite System (BDS-3) is equipped with inter-satellite link (ISL) payloads and high-precision atomic clocks, which makes it possible to verify the establishment of autonomous time scales. We present the autonomous clock synchronization and timekeeping method of BDS-3 constellation using in-orbit ISL data, and evaluate the autonomous timekeeping performance under different schemes. By processing 60 days of ISL data, the obtained clock results of 29 BDS-3 satellites are evaluated and compared to precise clock products obtained from the Deutsche GeoForschungsZentrum (GFZ). The average RMS of detrended residuals of ISL clock offsets for all satellites is 0.12 ns, which is superior to the 0.22 ns for GFZ clocks. We also discuss and analyze the important factors that affect autonomous timekeeping performance, including satellite clock noise and the clock ensemble involved. When timekeeping is performed using broadcast clock parameters, the time drift of satellite clocks can reach up to 1000 ns over 60 days. However, applying the method we provide, and when the clock ensemble contains eight better performing PHM clocks, with noise levels determined according to Hadamard deviation, the time drift of the established time scale is reduced to 2.6 ns over 60 days, and the frequency stability at 1-day interval is 8.4E-16. This study first demonstrates the performance of 60-day autonomous timekeeping for BDS-3 constellation using real ISL measurements, providing a practical reference for building space-based time scales in the future global navigation satellite system.

The global navigation satellite system (GNSS) is developing rapidly, and the related market applications and scientific research are increasing. Studies based on large low Earth orbit (LEO) satellite constellations have become research hotspots. The global coverage of the LEO constellation can reduce the dependence of navigation satellites on ground-monitoring stations and improve the precise orbit determination (POD) accuracy of navigation satellites. In this paper, we simulate various LEO satellite constellations (with 12, 30, and 60 satellites), along with ground stations’ observation data, to examine the impact of LEO satellites on the precision of the BeiDou-3 Global Navigation Satellite System (BDS-3) in terms of its POD accuracy. Using the simulated observation data of both LEO satellites and ground-monitoring stations, we analyze the integrated orbit determination for the LEO and BDS-3 satellites. The findings reveal that the 3D orbital accuracy of BDS-3 is 9.51 dm by using only seven ground-monitoring stations, and it is improved to a centimeter level after adding the LEO constellations. As the number of LEO constellation satellites increases, the impact on improving accuracy gradually diminishes. In terms of time synchronization accuracy in the BDS-3, compared to the results of clock offset using only ground stations, the addition of 12 LEO satellites resulted in an improvement of 49% for RMS1(root mean square) and 52% for RMS2 (standard deviation), the addition of 30 LEO satellites resulted in an improvement of 66% for RMS1 and 70% for RMS2, and the addition of 60 LEO satellites resulted in an improvement of 87% for RMS1 and 90% for RMS2. The integrated orbit determination of the LEO and BDS-3 satellites constellation greatly improves the accuracy of time synchronization. In addition, we also use simulated inter-satellite link (ISL) data to perform enhanced BDS-3 satellites POD and time synchronization experiments. The experiments showed that the orbit determination accuracy of the seven sta (seven stations) and ISL scheme is comparable to that of the seven sta and LEO12 scheme, and that the time synchronization accuracy of the seven sta and ISL scheme is slightly worse. The preliminary experiments showed that the LEO satellite could enhance the orbit determination accuracy of BDS-3 and obtain a higher time synchronization accuracy.