The TianQin space-borne gravitational wave detector will orbit at an altitude of 1.0 x 105 km with an arm length of 1.7 x 105 km, structured in an equilateral triangular satellite formation. The TianQin project delineates detailed requirements for orbit determination during both the launch phase and subsequent scientific experimentation. This paper pioneers the investigation of Precise Orbit Determination (POD) for TianQin utilizing the third generation of BeiDou global navigation satellite system (BDS-3) Inter-Satellite Link (ISL) through simulation. By analyzing the visibility, the feasibility of using BDS-3 ISLs for TianQin POD is explored. Furthermore, a refined Solar Radiation Pressure (SRP) model is developed, and the POD accuracy of TianQin is assessed, considering factors such as arc length, ranging intervals, and error sources. The results indicate the following: (A) Visibility between TianQin and BDS-3 satellites is intermittent, with an average of 10.6 BDS-3 satellites visible to TianQin, and the average Root Mean Square (RMS)value of Position Dilution of Precision (PDOP) for TianQin is 48.13. (B) POD accuracy improves with shorter ranging intervals. (C) The error in ephemeris is the dominant factor affecting the POD accuracy. (D) The establishment of three links between TianQin and the BDS-3 satellites satisfies the POD requirements of TianQin. With a ranging interval of 600 s and over a 7-day arc, the average three-Dimensional (3D) position accuracy of the three TianQin satellites is 1.35 m, while the 3D velocity accuracy is 0.08 mm/s. (c) 2024 Published by Elsevier Ltd on behalf of Chinese Society of Aeronautics and Astronautics. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Earth-Moon libration point navigation constellation has a merit of fully covering the cislunar space and providing navigation services with merely a small number of satellites. Furthermore, this type of constellation itself can achieve orbit determination by using only its Inter-satellite links (ISL) owing to the well-known gravitational asymmetry near the libration points. This paper adopts a representative four-satellite navigation constellation including three satellites located near the libration point L3, L4, L5, and an extra one in Distant Retrograde Orbit (DRO), and studies the orbit determination accuracy of the constellation under two conditions: establishing ISLs within itself (called Solely libration point satellites) and establishing additional ISLs with BeiDou Navigation Satellite System (BDS) (called Libration + BeiDou satellites). The simulation results indicate that, for the Solely libration point satellites scenario with the 1 m range error and solar radiation pressure (SRP) error with the level of 10% deviation between ball model and macro model, the final orbit accuracies for L3, L4, L5, and DRO satellites can respectively achieve 11.3 m, 12.7 m, 12.6 m, and 7.3 m. Reducing the link interval can significantly shorten the arc length to achieve final accuracy, whileas it has less impact on the final orbital accuracy. When the link interval is set as 240, 60, 10, and 2 minutes, the arc lengths at the final accuracy of better than 15 m are about 35, 29, 23, and 20 days, respectively. To balance the communication burden and the orbit determination arc length, the recommended ISL building interval is 10 minutes. Errors coming from range measurement model and dynamic model, in addition, are key factors in orbit determination, which can result in meters to tens of meters of orbital accuracy. For the Libration + BeiDou satellites scenario, the augmented ISLs to BDS can substantially improve constellation orbital accuracy and shorten the orbit determination arc length. When the DRO satellite attached to the Geostationary Earth Orbit (GEO), Inclined Geosynchronous Orbit (IGSO), and Medium Earth Orbit (MEO) satellites of BDS respectively, the improved accuracy are 37.1%, 61.2%, and 42.2%, while the shortened arc length are 20.1%, 28.3%, and 28.1%, respectively. The research findings presented in this paper can serve as a reference for the construction and assessment of navigation constellations in the Earth-Moon system.

TianQin project, a Chinese initiative in space gravitational wave detection, demands high precision in satellite orbit during both entry and scientific operations. As means of developmental maturation, ground-based measurements play a vital role in ensuring the smooth execution of TianQin satellite's detection mission. This paper conducts a simulation analysis by utilizing various ground-based measurement data, including the China Deep Space Network (CDSN), S/Ka-band ranging system, and Satellite Laser Ranging (SLR). The main focus is to explore the distinctions in Precise Orbit Determination (POD) capabilities among different methods and to enhance POD accuracy through the integration of multiple techniques for TianQin satellites. The results indicate: (1) Leveraging a strategically positioned station distribution, CDSN stations offer extended observation time, averaging 17.3 h per satellite daily, compared to S/Ka's 10.5 h. (2) In single-measurement POD scenarios, S/Ka proves superior, achieving accuracy better than 10 m and 0.4 mm s-1 for TianQin satellites with a 7-day orbit arc length. This superiority is attributed to its exceptional observational accuracy, outperforming CDSN's 40 m and 2.2 mm s-1 for POD accuracy. (3) By integrating high-precision SLR data on the foundation of CDSN or S/Ka observations, the POD accuracy of TianQin satellites is further enhanced, despite the limited SLR data quantity.

Over the past 20 years, the Gravity Recovery and Climate Experiment (GRACE), and its successor mission, GRACE‐Follow On (GRACE‐FO) have made significant contributions to time‐variable gravity field modeling. A Chinese low‐low satellite‐to‐satellite tracking gravimetry mission (i.e., Chinese future gravimetry mission) has been confirmed to be selected as the polar‐orbiting satellite gravimetry mission for China, because of the capability to collect gravity data globally. However, the analysis of potential contributions to geosciences from GRACE‐FO coupling with the Chinese future gravimetry mission is still limited. This study combines GRACE‐FO and Chinese future gravimetry missions as the Dual GRACE‐like Polar satellite Constellation (DGPC). By carefully choosing the initial orbit parameters of the Chinese future gravimetry mission with the differential evolution algorithm, the DGPC is expected to mitigate the temporal aliasing effects by improving the temporal resolution of time‐variable gravity solutions (i.e., 1‐day and 3‐day solutions). Regarding the spectral‐domain evaluation, zonal, tesseral, and sectorial coefficients estimated by the DGPC show approximately 6.01%–13.42% noise reductions compared with GRACE‐FO. Regarding the spatial‐domain evaluation, the DGPC can suppress noises of about 39.44% and 31.12% in annual amplitude and long‐term trend, respectively. On this basis, this paper analyzes the effectiveness of the DGPC in potential contributions to geosciences (e.g., hydrology, glaciology, and seismology). Specifically, the DGPC can improve accuracy by about 36.96%, 25.85%, and 33.16% with respect to GRACE‐FO for signals in the subhumid basin, signals of ice‐sheet mass balance over Greenland, and coseismic displacement of the fault zone, respectively. In general, the potential capability for high‐frequency signals recovery of the DGPC would facilitate contributions of satellite gravimetry to geosciences.

Satellite laser ranging (SLR) is the space geodetic technique with the highest degree of range, measuring precision and distances right down to the millimeter level. Thanks to the improvement of SLR station layouts and the advance of SLR technology, in recent years, more research has been conducted to determine Global Navigation Satellite System (GNSS) satellite orbits using SLR data. The primary goal of this contribution is to investigate the accuracy of BeiDou Navigation-3 (BDS-3) Satellite precise orbit determination (POD) using solely SLR data, as well as explore the impact of various factors on that accuracy. Firstly, we used actual SLR data to make the POD for BDS-3 satellites, and the POD accuracy was positively connected with the orbital arc lengths. The 9-day median root mean square (RMS) in radial (R), along-track (T), and cross-track (N) directions were estimated at 4.7–8.2, 22.1–35.2, and 27.4–43.8 cm, respectively, for comparison with WUM precise orbits. Then, we explored the impact of SLR observations and stations on POD accuracy. For 9-day orbital arc lengths, five station or 20 observation arcs may offer an orbit with a 1 m precision. Six to eight stations or 30–35 observation arcs allow an improved orbit accuracy up to approximately 0.5 m. Furthermore, we examined how measurement errors and orbit modeling errors affect the SLR-only POD accuracy using simulated SLR data. For orbital arc lengths of 9 days, each cm of random error leads to a 9.3–11.0 cm decrease in orbit accuracy. The accuracy of an orbit is reduced by 10.1–15.0 cm for every 1 cm of systematic error. Moreover, for solar radiation pressure (SRP) errors, the effect of POD accuracy is 20.5–45.1 cm, respectively.

The BeiDou Navigation Satellite System with global coverage (BDS-3) was officially launched on July 31, 2020. In addition to the legacy B1I and B3I signals from the regional system (BDS-2), BDS-3 provides several new signals, i.e., B1C, B2a, B2b and B2a + b, with advanced technologies and global availability, which brings new opportunities for ground-based BDS multipath reflectometry (BDS-MR) altimetry. Previous studies focused on the MR altimetry performance of the legacy signals using signal-to-noise ratio (SNR) observations, but the new BDS signals have not been investigated. In this contribution, all types of BDS-3 SNR data collected from seven International GNSS Service (IGS) stations are analyzed signal by signal in terms of SNR quality and altimetry performance, while the legacy BDS B2I and Global Positioning System (GPS) L2C signals are also selected for comparison. Seven globally distributed IGS stations are first investigated for the quality of SNR observations, and then two optimally selected land stations are used to assess BDS-MR based altimetry performance, while a coastal station is used to assess BDS-MR for sea level retrieval performance. It is found that the quality of SNR observations varied site by site due to the differences in environment and equipment, but in general, the new signals except B1C show a higher level of SNR. Then, no detectable biases among each signal are found for land altimetry, while new signals could yield higher amplitude when performing spectral analysis. In coastal sea level altimetry, only two new signals B1C and B2a are available at the selected IGS station, and it is observed that the retrieval performance of BDS B2a is comparable to that of B2I, B3I and GPS L2C with root-mean-square errors (RMSEs) of about 21 cm, while that for B1I and B1C is slightly poorer with RMSEs of 29.68 cm and 26.66 cm respectively. (c) 2021 COSPAR. Published by Elsevier B.V. All rights reserved.

Abstract The use of low earth orbit (LEO) satellites to enhance the performance of global navigation satellite system navigation and positioning services has become a popular research topic. In this study, NSGA‐III optimisation algorithm was used to design two hybrid configurations of 177 and 186 LEO constellations for enhancing the BeiDou Satellite Navigation System (BDS). Under the enhanced effect of optimisation constellation, the global average geometric dilution of precision (GDOP) of BDS was reduced to 0.8 ± 0.1, and the maximum GDOP was reduced from 2.4 to less than 1.1 (54.2% reduction). In order to verify the contribution of the two constellations to the convergence time and positioning accuracy of BDS precise point positioning (PPP), a LEO enhanced BDS PPP simulation experiment was carried out using International GNSS Service data from five stations. The results show that after 10 min of static positioning, both LEO constellations improved the positioning accuracy of BDS from the decimetre level to less than 5 cm. The maximum improvement for 177 and 186 LEO was 95.0% and 96.9%, respectively. Additionally, the convergence time for 177 and 186 LEO reduced to less than 3.5 and 3 min, and the maximum improvement was 93.5% and 95.2%, respectively. Overall, both constellations can improve the positioning accuracy and convergence time of BDS PPP.

Global Navigation Satellite System interferometric reflectometry (GNSS-IR) is a technique that utilizes multipath effects to sense the near-surface environment. Previous studies have focused on the retrieval performance of using geodetic GNSS equipment. Fortunately, Google announced in 2016 that smart devices running Android Nougat (7.0) operating system could provide raw GNSS measurements, which makes it possible to use a low-cost Android device to monitor the surrounding environment. In this study, we assessed the altimetry performance by using signal-to-noise ratio (SNR) data from a Huawei P30 smartphone, while a u-blox F9P receiver that is equipped with a low-cost antenna was placed next to the smartphone for comparison. Twenty days of SNR data from multiple GNSS constellations and signals were collected and processed. First, the characteristics of SNR data of Huawei P30 and u-blox F9P were compared and analyzed. For Huawei P30, it is observed that the SNR data have longer and clearer oscillations, which indicates that a higher range of elevation angles can be utilized. We further assessed the altimetry performance of Huawei P30 in different antenna orientations and different elevation angle ranges. It is shown that the effects of antenna orientation do exist. Meanwhile, the SNR oscillations become irregular when elevation angles exceed 60°, which suggests that SNR data with elevation angles below 60° can be used for Huawei P30. Finally, the altimetry performance of each signal was also assessed, and it is found that GPS L1, GLONASS G1, and BDS B1I could provide precise and stable height retrievals, while the performance of GPS L5 and Galileo E5a is slightly poorer. Overall, the Android smartphone possesses special cost and data availability superior to conventional GNSS equipment, making it a promising solution for GNSS-IR applications.

The regional navigation constellation can provide the navigation ability for the users of the targeted area with the balanced cost and efficiency, and the gcosynchronous orbits arc some of the important orbit types to construct non-polar regional navigation constellation. A configuration method was proposed for regional navigation constellation bascd on GEOs (gcosynchronous orbits) and IGSOs (inclined gcosynchronous orbits). Firstly, a set of SSP (sub-satellite point) involved design parameters were presented and depicted especially for the symmetric constellation. And the long-term perturbations due to the earth's oblateness were induced to recalculate the mean orbital elements of the GEOs and IGSOs and propagate their orbits. Then the statistical GDOP (geometric dilution precision) of the grid points of the specific navigation service area was used as the objective function for the DE (differential evolution) optimization algorithm. Finally, taking the Indian constellation of IRNSS as an example, the presented design and optimization algorithm was verified by the constellation simulation, and various configurations and orbit types of IRNSS constellation were discussed in detail. The results show that for India and surrounding region, the 7-satellite constellation can best consider the navigation performance and construction cost ; the elliptical orbit is unable to take into account the navigation performance of the northern and southern hemispheres, which is difficult to expand to global navigation constellation. This method can improve the global optimization efficiency of GEO/IGSO hybrid regional navigation constellation, and provide the number of satellites and configuration design quickly for the subsequent asymmetric constellation.