multi-GNSS Fusion Research Articles

High-Precision Orbit Determination of the Small TJU-1 Satellite Using GPS, GLONASS, and BDS

Tianjin University-1 (TJU-1) is a small low-Earth-orbiting (LEO) meteorological satellite equipped with a multi-GNSS receiver that provides the dual-frequency pseudorange and carrier-phase measurements from GPS, GLONASS, and BDS, including BDS-2 and BDS-3. In this contribution, the quality of GNSS data during the period of 2022/31~2022/37 is assessed, and the precise orbit determination (POD) based on the onboard GNSS data is conducted. Assessing the pseudorange measurement accuracy, it reveals that the GPS and BDS-3 are comparable, followed by BDS-2, and GLONASS the worst. The POD experiments demonstrate that, the orbit precision of all single-GNSS solutions is almost at an equivalent level of 2.2~3.4 cm, in terms of the overlapped orbit consistency, and the GPS-based solution is slightly worse than the BDS-based one and the GLONASS-based one is worse than other two. According to the orbit differences between different solutions, the orbit accuracy of single-GNSS solutions is inferred to be at the level of about 4.7~6.2 cm. With the multi-GNSS fusion, the precision and inferred accuracy of orbits could be improved significantly to 1.7~2.1 cm and about 1.7~3.7 cm, respectively. Furthermore, the root mean square (RMS) of posteriori carrier-phase residuals for all the IGSO/MEO satellites is at the level of 7~15 mm in different single-GNSS and multi-GNSS fusion solutions. These performances have fully demonstrated that, the centimeter-level POD is easily achievable for small LEO satellites by using the multi-GNSS receiver.

Precise real-time navigation of the small TJU-1 satellite using GPS, GLONASS and BDS

Precise real-time navigation of the small Tianjin University-1 (TJU-1) satellite based on GPS, GLONASS, BDS-2 and BDS-3 is studied in this contribution for the first time. Experimental results showed that (1) the real-time position accuracies could reach the level of 0.6 ∼ 0.8 m, 0.9 ∼ 1.1 m, 2.0 ∼ 3.5 m and 0.18 ∼ 0.22 m, respectively, for the solutions based on single GPS, GLONASS, BDS-2 and BDS-3; (2) The multi-GNSS fusion could significantly improve the accuracy, reliability and availability of real-time navigation compared with the single-GNSS solution especially when the observation conditions of a single-GNSS are poor or over some anomalous orbit arcs where only few or even no observations are available; (3) a remarkable real-time orbit accuracy of 0.187 m for 3D position is achieved based on BDS-2 + BDS-3 fusion. These accuracy performances have fully demonstrated that, precise real-time navigation based on multi-GNSS measurements would be a very attractive technique for broad small LEO satellite applications requiring the high-precision real-time orbits.

Monitoring of Wheat Height Based on Multi-GNSS Reflected Signals

Global Navigation Satellite System interferometric reflectometry (GNSS-IR), a new and inexpensive technique, has become available to the broader scientific community for detecting surface environmental information, such as soil moisture, snow depth and vegetation growth. However, there have been limited experiments focusing on the potential of crop height retrieval, especially the performance evaluation of BeiDou Navigation Satellite System (BDS) with other GNSS. Accuracy and reliability are challenging to achieve with traditional methods utilizing a single GNSS, and few measured verification data. In this study, an improved method that includes segmentation processing and multi-GNSS fusion is proposed based on GPS/GLONASS/Galileo/BDS multi-frequency data. Furthermore, experiments were carried out on a farmland in Fengqiu County, Henan Province, China. The results show that the height retrievals from four GNSS were in good agreement with the in situ observations during the whole growth cycle of the wheat after overwintering. Meanwhile, the retrievals based on the proposed method exhibited greater correspondence than the single frequency results, the correlation coefficient was increased and the root-mean-square error (RMSE) was reduced, respectively. Therefore, this study illustrates the feasibility of the proposed method to precisely estimate wheat height and its potential for use in the early warning of wheat lodging based on GNSS-IR.

Study on PPP Time Comparison Based on BeiDou-3 New Signal

The Chinese BeiDou Navigation Satellite System (BDS-3) has completed the network construction and begun to provide high-precision Positioning, Navigation, and Timing (PNT) services for users all over the world. In this paper, we study the Precision Point Positioning (PPP) time comparison based on the BDS-3 new signal. The data presented in this paper includes the observation data from the National Time Service Center of Chinese Academy of the Sciences (NTSC) and the Electronics Academy of Sciences in Czech Republic (TP), and the precise product data released by the International GNSS Service (IGS), which includes the high-precision clock and orbit products. The results show that the multipath noise of the BDS-3 new signal (B1C and B2a) is less than that of the Asia-Pacific region BDS-2 (B1I and B2I), and the standard deviation (STD) of the multipath noise of B2a is the least. When the BDS-3 PPP is applied to the common clock short-baseline time comparison, the standard deviations of the BDS-3 and BDS-2 signals are 0.0296 ns and 0.0341 ns, respectively, the standard deviation of BDS-3 is smaller than that of BDS-2, and therefore the short-baseline time comparison performance of BDS-3 PPP is better than that of BDS-2, the standard deviation of BDS-3 PPP has increased by 13.19% compared with BDS-2. For long-baseline time comparisons between UTC (NTSC) and UTC (TP), the results based on the BDS-3 single system, or BDS-3 with other navigation system fusion PPP time comparison, follow the same trend as the results released by the International Bureau of Weights and Measures (BIPM). The residual standard deviations relative to the BIPM are 0.7039 ns and 0.2994 ns respectively. The stability of the BDS-3 new signal PPP and multi-GNSS fusion PPP time comparison has the same magnitude as the BIPM. As a result, with the completion of BDS-3, the high-precision time comparison based on BDS-3 expected to offer better services for users. The experimental results demonstrate that BDS-3 could be used to calculate the Coordinated Universal Time (UTC) in the future.

Multi-GNSS Fusion Real-Time Kinematic Algorithm Based on Extended Kalman Filter Correction Model for Medium-Long Baselines

Multi-GNSS Fusion Real-Time Kinematic Algorithm Based on Extended Kalman Filter Correction Model for Medium-Long Baselines

GPS + Galileo + QZSS + BDS tightly combined single-epoch single-frequency RTK positioning

The multi-GNSS fusion makes positioning more reliable and accurate. Considering the signal difference of different systems, GPS + Galileo + QZSS + BDS tightly combined double-difference model (TCDDM), including function and stochastic model, is proposed. The proposed model fully utilizes the overlapping frequency signals of various systems, and thus to enhance positioning model when DISBs are known beforehand. The observations of 3 ultra-short (1~10 m) and 3 short (4~10 km) baselines were processed by self-programming software, and the single-epoch single-frequency RTK performance using different system-combined models was evaluated by ambiguity-fixed correctness rate (ACR) and positioning accuracy. It demonstrated that three- and four-system TCDDM were superior to their corresponding loosely combined double-difference model (LCDDM) for ACR and positioning accuracy especially at high cut-off elevation. Moreover, four-system TCDDM had the best RTK performance obtaining average ACRs of 100% and 97.6% even at 25° cut-off elevation for ultra-short and short baseline, respectively.

Real-time capturing of seismic waveforms using high-rate BDS, GPS and GLONASS observations: the 2017 Mw 6.5 Jiuzhaigou earthquake in China

The rapid development of the BeiDou Satellite Navigation System (BDS) and other Global Navigation Satellite System (multi-GNSS) constellations provides a great opportunity to contribute to earthquake early warning systems in terms of capturing displacement and velocity waveforms for the estimation of magnitude and fault slip inversion. In this study, we demonstrate the capability of BDS and the benefit of multi-GNSS for real-time capturing seismic waveforms using the combined high-rate BDS + GPS + GLONASS data collected during the 2017 Mw 6.5 Jiuzhaigou earthquake. For this event, we found that the displacements, derived from BDS precise point positioning (PPP) are better than that of Global Positioning System-only (GPS) results, especially in the east and vertical components with improvements of 43% and 23%. While the velocity waveforms from BDS present a comparable performance with GPS. the multi-GNSS fusion can significantly improve the accuracy by 47%, 55%, and 28% in the east, north, and vertical components compared with GPS-only results. The BDS and multi-GNSS derived displacement waveforms agree quite well with those obtained from integrating the acceleration, with accuracy at the millimeter level. In addition, the theoretical permanent displacement field calculated from a finite-fault slip model is selected as an independent reference, and the differences between GNSS derived permanent displacements and theoretical permanent displacements are mostly less than 1 mm. Therefore, we conclude that the BDS and multi-GNSS fusion can significantly contribute to the real-time capture of accurate seismic waveforms and that it has the potential to benefit for earthquake early warning and rapid geohazard assessment.

Spatial–temporal characteristic of BDS phase delays and PPP ambiguity resolution with GEO/IGSO/MEO satellites

GPS precise point positioning (PPP) ambiguity resolution (AR) can improve the positioning accuracy and shorten the convergence time. However, for the BeiDou Satellite Navigation System (BDS), the problems of satellite-induced code bias, imperfections in the error models and the inadequate accuracy of orbit products limit the applications of the BDS PPP AR system, which requires more than 6 h to achieve the first ambiguity-fixed solution. In this study, the accuracy of a wide-lane (WL) uncalibrated phase delay (UPD) is improved after careful consideration of the code bias and multipath. Meanwhile, the accuracy of the BDS float ambiguity is also improved by multi-GNSS fusion and improved precise orbit and clock products, which are critical for high-quality narrow-lane (NL) UPD estimations. With three tracking networks of different scales, including Hong Kong, the Crustal Movement Observation Network of China (CMONOC) and the multi-GNSS experiment (MGEX) networks, the spatial–temporal characteristics of WL and NL UPDs for BDS GEO/IGSO/MEO satellites are analyzed, and the PPP AR is performed. Numerous results show that WL and NL UPDs with a standard deviation (STD) of less than 0.15 cycles can be achieved for BDS GEO satellites, while a STD of less than 0.1 cycles can be obtained for IGSO and MEO satellites. With the precise UPD estimation, for the first time, the BDS PPP rapid ambiguity resolution for GEO/IGSO/MEO satellites is achieved. We found that the average time to first fix (TTFF) of the BDS PPP AR is shortened significantly, to approximately 40 min for Hong Kong and the CMONOC, while the TTFF was 57.4 min for the MGEX networks. With ambiguity resolution, the accuracy of the daily BDS PPP in the east, north and vertical directions improves from 1.74 cm, 1.08 cm, and 5.52 cm to 0.72 cm, 0.54 cm, and 3.21 cm for the Hong Kong network, 2.24 cm, 2.31 cm, and 5.64 cm to 1.18 cm, 0.79 cm, and 3.30 cm for the CMONOC, and 2.71 cm, 1.80 cm, and 6.00 cm to 1.58 cm, 1.15 cm, and 4.33 cm for the MGEX networks. Significant improvement is also achieved for kinematic PPP, with improvements of 40.41%, 34.33% and 37.17% in the east, north and vertical directions for the MGEX networks, respectively.

Tropospheric delay parameters from numerical weather models for multi-GNSS precise positioning

Abstract. The recent dramatic development of multi-GNSS (Global Navigation Satellite System) constellations brings great opportunities and potential for more enhanced precise positioning, navigation, timing, and other applications. Significant improvement on positioning accuracy, reliability, as well as convergence time with the multi-GNSS fusion can be observed in comparison with the single-system processing like GPS (Global Positioning System). In this study, we develop a numerical weather model (NWM)-constrained precise point positioning (PPP) processing system to improve the multi-GNSS precise positioning. Tropospheric delay parameters which are derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis are applied to the multi-GNSS PPP, a combination of four systems: GPS, GLONASS, Galileo, and BeiDou. Observations from stations of the IGS (International GNSS Service) Multi-GNSS Experiments (MGEX) network are processed, with both the standard multi-GNSS PPP and the developed NWM-constrained multi-GNSS PPP processing. The high quality and accuracy of the tropospheric delay parameters derived from ECMWF are demonstrated through comparison and validation with the IGS final tropospheric delay products. Compared to the standard PPP solution, the convergence time is shortened by 20.0, 32.0, and 25.0 % for the north, east, and vertical components, respectively, with the NWM-constrained PPP solution. The positioning accuracy also benefits from the NWM-constrained PPP solution, which was improved by 2.5, 12.1, and 18.7 % for the north, east, and vertical components, respectively.