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Progress and performance evaluation of BeiDou global navigation satellite system: Data analysis based on BDS-3 demonstration system
The first two Medium Earth Orbit (MEO) satellites of the third generation of BeiDou satellite navigation System (BDS-3) were successfully launched on November 5, 2017. This historical launch starts the new era of the global navigation satellite system of BeiDou. Before the first two satellites of BDS-3, a demonstration system for BDS-3 with five satellites, including two Inclined Geosynchronous Orbit satellites (IGSO) and three MEO satellites, was established between 2015 and 2016 for testing the new payloads, new designed signals and new techniques. In the demonstration system, the new S frequency signal and satellite hydrogen clock as well as inter-satellite link (ISL) based on Ka-band signals with time-division multiple addresses (TDMA) were tested. This paper mainly analyzes the performances of the demonstration system, including the signalto- noise ratios, pseudorange errors and the multipath errors of the civilian signals of BDS-3. The qualities of signals in space, time synchronization and timing precision were tested as well. Most of the performances were compared with those of the regional BeiDou satellite navigation system (BDS-2). At last, the performances of positioning, navigation and timing (PNT) of the future BeiDou global system (BDS-3) were evaluated based on the signal quality of the present demonstration satellite system.
Journal Article
Characteristics of receiver-related biases between BDS-3 and BDS-2 for five frequencies including inter-system biases, differential code biases, and differential phase biases
It is foreseeable that the BeiDou navigation satellite system with global coverage (BDS-3) and the BeiDou navigation satellite (regional) system (BDS-2) will coexist in the next decade. Care should be taken to minimize the adverse impact of the receiver-related biases, including inter-system biases (ISBs), differential code biases (DCB), and differential phase biases (DPB) on the positioning, navigation, and timing (PNT) provided by global navigation satellite systems (GNSS). Therefore, it is important to ascertain the intrinsic characteristics of receiver-related biases, especially in the context of the combination of BDS-3 and BDS-2, which have some differences in their signal level. We present a method that enables time-wise retrieval of between-receiver ISBs, DCB, and DPB from multi-frequency multi-GNSS observations. With this method, the time-wise estimates of the receiver-related biases between BDS-3 and BDS-2 are determined using all five frequencies available in different receiver pairs. Three major findings are suggested based on our test results. First, code ISBs are significant on the two overlapping frequencies B1II and B2b/B2I between BDS-3 and BDS-2 for a baseline with non-identical receiver pairs, which disrupts the compatibility of the two constellations. Second, epoch-wise DCB estimates of the same type in BDS-3 and BDS-2 can show noticeable differences. Thus, it is unreasonable to treat them as one constellation in PNT applications. Third, the DPB of BDS-3 and BDS-2 may have significant short-term variations, which can be attributed to, on the one hand, receivers composing baselines, and on the other hand, frequencies.
Journal Article
Dual-frequency to five-frequency real-time precise point positioning using new BDS-3 PPP-B2b service
BeiDou global navigation satellite system (BDS-3), a developed GNSS by China, has the ability to support five different signals, including B1I, B3I, B1C, B2a, and B2b. Meanwhile, BDS-3 has officially provided the satellite-based precise point positioning (PPP) service through the B2b signal (PPP-B2b) since 2021. It’s necessary to conduct a comprehensive analysis on multi-frequency PPP with PPP-B2b corrections. In this study, a multi-frequency undifferenced and uncombined PPP model (UDUC) using PPP-B2b corrections was employed to investigate dual-frequency to five-frequency real-time PPP performance. The results show that compared with the conventional dual-frequency solutions, multi-frequency solutions can improve both the convergence performances and positioning accuracy of PPP-B2b service, especially during the convergence stage. The quad-frequency and five-frequency solutions can achieve the best positioning performance. The static solutions of multi-frequency PPP models reach the centimeter-level accuracy after convergence. In kinematic mode, the convergence time of the five-frequency PPP results is reduced by 23.5% compared with the dual-frequency results. The root mean square (RMS) errors of the five-frequency PPP in the E, N, and U components are 7.1 cm, 4.8 cm, and 12.4 cm, which are improved by 6.8%, 11.5%, and 5.5%, respectively.
Graphical Abstract
Journal Article
Orbit monitoring and space environment simulations of the satellite M26 of BDS
No. 3 BeiDou navigation satellite system (No. 3 BDS) has been open to worldwide users since 2020. Its navigation service benefits more and more people from different countries. The satellite M26 is one of the standby satellites of BDS, which was sent into its operation orbit by the end of the year 2023. Its functions include stabilizing BDS, replacing the nearly retired satellite, and carrying out assessments of the new concept of GNSS. Due to the importance of satellite M26, authors have gathered the key orbit elements information; the orbit altitude, eccentricity and orbit inclination have been completely monitored and analyzed since its launch into working orbit. Based on the orbit elements, space environment effects for the first 271-day stay in orbit of M26 are simulated and space collision probability between M26 and other objects is studied. This work will partially support M26 to quickly join in the routine operation of BDS.
Journal Article
Investigating the Effects of Ionospheric Scintillation on Multi‐Frequency BDS‐2/BDS‐3 Signals at Low Latitudes
Ionospheric scintillation could seriously disrupt the signal tracking of the global navigation satellite systems (GNSS), further causing positioning accuracy degradation or unavailability. BeiDou navigation satellite system (BDS), a newly developed GNSS by China, has begun to provide global positioning, navigation, and timing service. The objective of the present study is to investigate the effects of ionospheric scintillation on BDS‐2 and BDS‐3 multi‐frequency signals. Ionospheric scintillation monitor receiver data from four monitors in Brazil were collected from October 2021 to May 2022. The results illustrate that S4(B2) and S4(B3) linearly increase with S4(B1) for S4(B1) ≤ 0.6, which is consistent with weak scattering theories, and average experimental ratios of S4(B2)/S4(B1), S4(B3)/S4(B1), and S4(B2)/S4(B3) are less than corresponding theoretical ones by 6.1%, 4.4%, and 1.9%, respectively. Meanwhile, as S4 values increase, lower‐frequency scintillation saturates earlier than higher ones, and the probability of ionospheric scintillation events on B2 and B3 signals is approximately twice (S4 ≥ 0.7) as B1 signals in the equatorial ionization anomaly (EIA) regions. To alleviate the undesirable effects of missing data on GNSS positioning, we first investigate the inter‐frequency relationship and distribution probability of two significant spectral parameters, that is, T (the spectral strength of the phase noise at 1 Hz) and p (the spectral slope of the phase power spectral density) in the tracking jitter model among three BDS frequencies. Results show that the performances among B1, B2, and B3 frequencies have a higher correlation respectively, and their values for B2 and B3 signals are more susceptible to be impacted by ionospheric scintillation.
Journal Article
Assessment of BeiDou differential code bias variations from multi-GNSS network observations
The differential code bias (DCB) of global navigation satellite systems (GNSSs) affects precise ionospheric modeling and applications. In this paper, daily DCBs of the BeiDou Navigation Satellite System (BDS) are estimated and investigated from 2-year multi-GNSS network observations (2013–2014) based on global ionospheric maps (GIMs) from the Center for Orbit Determination in Europe (CODE), which are compared with Global Positioning System (GPS) results. The DCB of BDS satellites is a little less stable than GPS solutions, especially for geostationary Earth orbit (GEO) satellites. The BDS GEO observations decrease the precision of inclined geosynchronous satellite orbit (IGSO) and medium Earth orbit (MEO) DCB estimations. The RMS of BDS satellites DCB decreases to about 0.2 ns when we remove BDS GEO observations. Zero-mean condition effects are not the dominant factor for the higher RMS of BDS satellites DCB. Although there are no obvious secular variations in the DCB time series, sub-nanosecond variations are visible for both BDS and GPS satellites DCBs during 2013–2014. For satellites in the same orbital plane, their DCB variations have similar characteristics. In addition, variations in receivers DCB in the same region are found with a similar pattern between BDS and GPS. These variations in both GPS and BDS DCBs are mainly related to the estimated error from ionospheric variability, while the BDS DCB intrinsic variation is in sub-nanoseconds.
Journal Article
Continuous ground monitoring of vegetation optical depth and water content with GPS signals
Satellite microwave remote sensing techniques can be used
to monitor vegetation optical depth (VOD), a metric which is directly linked
to vegetation biomass and water content. However, these large-scale
measurements are still difficult to reference against either rare or not
directly comparable field observations. So far, in situ estimates of canopy
biomass or water status often rely on infrequent and time-consuming
destructive samples, which are not necessarily representative of the canopy
scale. Here, we present a simple technique based on Global Navigation
Satellite Systems (GNSS) with the potential to bridge this persisting scale
gap. Because GNSS microwave signals are attenuated and scattered by
vegetation and liquid water, placing a GNSS sensor under a vegetated canopy
and measuring changes in signal strength over time can provide continuous
information about VOD and thus on vegetation biomass and water content. We
test this technique at a forested site in southern California for a period
of 8 months. We show that variations in GNSS signal-to-noise ratios reflect
the overall distribution of biomass density in the canopy and can be
monitored continuously. For the first time, we show that this technique can
resolve diurnal variations in VOD and canopy water content at hourly to
sub-hourly time steps. Using a model of canopy transmissivity to assess
these diurnal signals, we find that temperature effects on the vegetation
dielectric constant, and thus on VOD, may be non-negligible at the diurnal
scale or during extreme events like heat waves. Sensitivity to rainfall and
dew deposition events also suggests that canopy water interception can be
monitored with this approach. The technique presented here has the potential
to resolve two important knowledge gaps, namely the lack of ground truth
observations for satellite-based VOD and the need for a reliable
proxy to extrapolate isolated and labor-intensive in situ measurements of
biomass, canopy water content, or leaf water potential. We provide
recommendations for deploying such off-the-shelf and easy-to-use systems at
existing ecohydrological monitoring networks such as FluxNet or SapfluxNet.
Journal Article
Analyzing the Precise Point Positioning Performance of Different Dual-Frequency Ionospheric-Free Combinations with BDS-3 and Galileo
The BeiDou global navigation satellite system (BDS-3) and Galileo systems both broadcast satellite signals on five frequencies, which can form many observation combinations with dual-frequency ionospheric-free (DFIF) precise point positioning (PPP). This study analyzes the PPP static and kinematic performance of a total of eight different DFIF combinations, including BDS-3’s B1C/B2a, B1C/B3I, B1I/B2b, and B1I/B3I and Galileo’s E1/E5, E1/E6, E1/E5a, and E1/E5b combinations. A 10-day dataset from 60 Multi-GNSS Experiment (MGEX) stations was adopted. The root mean square error (RMSE) of the PPP was tested in the north, east, and up (NEU), horizontal (H), and three-dimensional (3D) components. The PPP accuracy of BDS-3 was comparable with that of Galileo. Both BDS-3 and Galileo signals allow for independent PPP processing both in static and kinematic modes. When the 3D error was used as the evaluation criterion, the order of the combinations in which the positioning accuracy gradually deteriorated was as follows: E1/E5, B1C/B3I, B1I/B2b, E1/E6, B1I/B3I, E1/E5b, E1/E5a, and B1C/B2a; The 3D RMSE values for the best combination, E1/E5, and the worst combination, B1C/B2a, were 1.06 cm and 1.43 cm, respectively; the positioning accuracies of all combinations remained at the level of 1 cm in static mode. In kinematic mode, the order of the combinations in which the PPP accuracy gradually deteriorated was as follows: E1/E5, E1/E5a, E1/E5b, B1I/B2b, B1I/B3I, B1C/B2a, B1C/B3I, and E1/E6. The 3D RMSE values for the best combination, E1/E5, and the worst combination, B1C/B2a, were 3.89 cm and 1.95 cm, respectively. The best results could be achieved with the E1/E5 combination, which outperforms the worst combination, E1/E6, by about 1 cm.
Journal Article
Multi-Global Navigation Satellite System (GNSS) real-time tropospheric delay retrieval based on state-space representation (SSR) products from different analysis centers
The troposphere plays an important role in a range of weather and various climate changes. With the development of the Global Navigation Satellite System (GNSS), the zenith tropospheric delay (ZTD) retrieval using GNSS technology has become a popular method. Research on ZTD accuracies of state-space representation (SSR) corrections from different analysis centers derived from real-time precise point positioning (RT-PPP) is important for Earth observation correction, meteorological disaster forecasting, and warning with the increasing abundance of state-space representation (SSR) products obtained by the International GNSS Service (IGS) analysis center. Therefore, accuracies and availability of real-time orbits and clock errors obtained by the Chinese Academy of Sciences (CAS), GMV Aerospace and Defense (GMV), Centre National d'Etudes Spatiales (CNE), and Wuhan University (WHU) are evaluated, and the RT positioning performance and ZTD accuracies are analyzed for Global Positioning System (GPS), Galileo (GAL), and BeiDou Navigation Satellite System-3 (BDS3) satellites. The results indicate that CAS has the higher satellite availability, providing SSR corrections for 82 GPS, Galileo, and BDS3 satellites. The accuracies of GPS, Galileo, and BDS3 orbits are best at WHU, CAS, and WHU with values of 5.57, 5.91, and 11.77 cm, respectively; the standard deviations (SDs) of clock error are all better than 0.22, 0.19, and 0.55 ns, and the root mean square errors (RMSEs) are better than 0.54, 0.32, and 1.46 ns. CAS has the best signal-in-space ranging errors (SISREs) followed by WHU, while CNE and GMV are worse. In the RT-PPP test, convergence times for CAS and WHU are 14.9 and 14.4 min, respectively, with 3D positioning accuracy for both of around 3.3 cm, which is better than for CNE and GMV. Among them, WHU SSR has the higher accuracy of RT-PPP-derived ZTD, with an RMSE of 6.06 mm and desirable availability with a completeness rate of 89 %.
Journal Article